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Red colouration on a signaller's body may be an informative signal in many animals. For species that inhabit architecture (e.g., burrows, nests, or other structures), certain parts of the body are more exposed than others, potentially serving as superior platforms for signalling via colouration. Yet if animals differentially advertise red colouration on body parts that are more versus less exposed from their architecture remains to be tested. Here we systematically quantified red coloration in social hermit crabs (Coenobita compressus). These crabs inhabit architecturally remodelled shells and have claws that visibly block the shell entrance, like doors to their home. We hypothesised that red colouration of claws may be a signal of resource-holding potential (RHP). Consistent with this RHP signalling hypothesis, we found that within the same individuals’ bodies, exposed claws showed significantly greater red colouration than unexposed carapaces. Furthermore, larger body size predicted greater red colouration of claws. Competing hypotheses (e.g., interspecific signalling, camouflage, and UV protection), while not explicitly tested, nevertheless appear unlikely based on natural history. Red claw coloration may therefore function as a signal to conspecifics, and experiments are now needed to test recipient responses. Broadly, relative to surrounding architecture, exposed body surfaces offer rich potential as signalling platforms for colouration.

Animals’ cognitive abilities can be tested by allowing them to choose between alternatives, with only one alternative offering the correct solution to a novel problem. Hermit crabs are evolutionarily specialized to navigate while carrying a shell, with alternative shells representing different forms of ‘extended architecture’, which effectively change the extent of physical space an individual occupies in the world. It is unknown whether individuals can choose such architecture to solve novel navigational problems. Here, we designed an experiment in which social hermit crabs (Coenobita compressus) had to choose between two alternative shells to solve a novel problem: escaping solitary confinement. Using X-ray microtomography and 3D-printing, we copied preferred shell types and then made artificial alterations to their inner or outer shell architecture, designing only some shells to have the correct architectural fit for escaping the opening of an isolated crab’s enclosure. In our ‘escape artist’ experimental design, crabs had to choose an otherwise less preferred shell, since only this shell had the right external architecture to allow the crab to free itself from isolation. Across multiple experiments, crabs were willing to forgo preferred shells and choose less preferred shells that enabled them to escape, suggesting these animals can solve novel navigational problems with extended architecture. Yet, it remains unclear if individuals solved this problem through trial-and-error or were aware of the deeper connection between escape and exterior shell architecture. Our experiments offer a foundation for further explorations of physical, social, and spatial cognition within the context of extended architecture. Electronic supplementary material The online version of this article (10.1007/s10071-020-01419-7) contains supplementary material, which is available to authorized users.

Collective movement may emerge if coordinating one's movement with others produces a greater benefit to oneself than can be achieved alone. Experimentally, the capacity to manoeuvre simulated groups in the wild could enable powerful tests of the impact of collective movement on individual decisions. Yet such experiments are currently lacking due to the inherent difficulty of controlling whole collectives. Here we used a novel technique of experimentally simulating the movement of collectives of social hermit crabs (Coenobita compressus) in the wild. Using large architectural arrays of shells dragged across the beach, we generated synchronous collective movement and systematically varied the simulated collective's travel direction as well as the context (i.e., danger level). With drone video from above, we then tested whether focal individuals were biased in their movement by the collective. We found that, despite considerable engagement with the collective, individuals' direction was not significantly biased. Instead, individuals expressed substantial variability across all stimulus directions and contexts. Notably, individuals typically achieved shorter displacements in the presence of the collective versus in the presence of the control stimulus, suggesting an impact of traffic. The absence of a directional bias in individual movement due to the collective suggests that social hermit crabs are individualists, which move with a high level of opportunistic independence, likely thanks to the personal architecture and armour they carry in the form of a protective shell. Future studies can manipulate this level of armour to test its role in autonomy of movement, including the consequences of shell architecture for social decisions. Our novel experimental approach can be used to ask many further questions about how and why collective and individual movement interact.

Many animals shape and modify their physical environment, thereby creating a diversity of structures, from underground burrows to constructed nests to towering above-ground edifices, all of which are referred to as 'animal architecture'. Examples of animal architecture are found everywhere on Earth: beneath the sea and on land, below and above ground, and hanging into the air off trees and precipices. Fossils suggest that animals have been acting as architects by constructing shelters and other built structures for hundreds of millions of years. Animal architects are widespread taxonomically, spanning invertebrates (Figure 1) and vertebrates (Figure 2). Their architectural creations are diverse, including: the fortress-like mounds of termites, the housing markets of architecturally remodeled shells of social hermit crabs, the subterranean tunnel systems of naked mole rats, the intricately decorated bowers of bowerbirds or the engineered dams of beavers. Even the tallest of human architecture is rivaled by animal architecture: termite mounds exceed skyscrapers in their size relative to that of the architects. Animal architecture raises many fascinating questions at the interface of behavior, ecology and evolution: How is this architecture built? What instinctive 'blueprints' or cognitive mechanisms underlie its creation? What functions does the architecture serve? And why did it evolve? Notably, because architecture changes the world, it may have far-reaching impacts on collective behavior and social life, interactions among communities of species and whole ecosystems. Architecture may even have altered the very course of evolution.

All living organisms must eventually die, though in some cases their death can bring life‐giving opportunities. Few studies, however, have experimentally tested how animals capitalize on conspecific death and why this specialization would evolve. Here, we conducted experiments on the phylogenetically most closely‐related marine and terrestrial hermit crabs to investigate the evolution of responses to death during the sea‐to‐land transition. In the sea, death of both conspecifics and heterospecifics generates unremodeled shells needed by marine hermit crabs. In contrast, on land, terrestrial hermit crabs are specialized to live in architecturally remodeled shells, and the sole opportunity to acquire these essential resources is conspecific death. We experimentally tested these different species’ responsiveness to the scent of conspecific versus heterospecific death, predicting that conspecific death would have special attractive value for the terrestrial species. We found the terrestrial species was overwhelmingly attracted to conspecific death, rapidly approaching and forming social groupings around conspecific death sites that dwarfed those around heterospecific death sites. This differential responsiveness to conspecific versus heterospecific death was absent in marine species. Our results thus reveal that on land a reliance on resources associated exclusively with conspecifics has favored the evolution of an extreme collective attraction to conspecific death.

Complex environments present substantial spatio-temporal uncertainty in where and when rare ecological resources become available. How animals navigate this uncertainty to turn the seemingly unpredictable into the predictable is a fundamental question in evolutionary ecology. Here we use subtidal hermit crabs (Pagurus acadianus) as a model system to experimentally test in the field how animals resolve spatio-temporal uncertainty in resource availability. Quadrat sampling within the subtidal zone revealed that hermit crabs face an extreme ecological challenge, based on the rarity of empty shells across space and time. We show how this spatio-temporal uncertainty is ultimately resolved using long-distance chemical cues, which are associated with non-destructive shell predation on living gastropods, the original source of shells. By experimentally releasing cues that simulated the chemical by-products of predation, we reveal that certain flesh cues provide fine-grained information about the precise spatial and temporal window of new shell availability. These cues were most attractive to individuals with the greatest existing resource needs, and in the absence of this information individuals were highly constrained in their ability to discover newly available resources. Broadly, these experiments reveal that exploiting simple cues from heterospecific predators can provide a solution to the general ecological challenge of finding resources that are rare in space and time.

Animals have been specialized by natural selection to perceive features of their environment that strongly impact their reproductive success. For many social animals, the social world of conspecifics provides the most pertinent information, ultimately enabling individuals to adaptively anticipate future events, like time-sensitive opportunities to acquire rare resources. Here we investigated whether ‘social timing’—joining others at the right time for resource acquisition—ultimately drives the perception of different social structures among highly social terrestrial hermit crabs, Coenobita compressus. These crabs are specialized to live in architecturally remodelled homes, which can only be acquired through coordinated social interactions among conspecifics. We experimentally simulated these social interactions using static arrays of shells that mimicked the temporary social structures formed at each stage in the social shell-acquisition process. Free-wandering crabs in the wild were then allowed to choose among these different social structures. We found that crabs were most attracted to social structures representing early stages of the social shell-acquisition process, which predict forthcoming opportunities and hence allow individuals to join in time to take priority spots in ensuing social formations. In contrast, social structures representing late stages of the social shell-acquisition process were less attractive. When crabs joined such late-stage social structures they did not stay long, assessing they had arrived too late to insert themselves into the existing social arrangement. Broadly, these results suggest that strong selective pressures exist for sensory specializations that are in tune with the temporal and spatial patterning of opportunities in the social world.

Architecture, like nests, burrows, and other types of fortresses, may have played an important role in the evolution of social life on land. However, few studies have examined architecture in organisms that transitioned from sea to land to test how and why architectural and morphological changes might have jointly impacted social evolution. Here I contrasted the shell architecture and body morphology of two of the phylogenetically most closely-related land versus sea species of hermit crab (the terrestrial hermit crab, Coenobita compressus, and the marine hermit crab, Calcinus obscurus), as well as the original builder of their shells (the gastropod, Nerita scabricosta). In contrast to the shells of gastropods and marine hermit crabs, only the shells of terrestrial hermit crabs were architecturally remodeled, with no columella inside for the occupants to grip upon to resist eviction. The bodies of terrestrial hermit crabs were also significantly more exposed outside the enlarged openings of their remodeled shells, whereas the substantially smaller-bodied marine hermit crabs were safeguarded deep within the recesses of their unremodeled shells. Ultimately, these changes in shell architecture and body morphology likely had consequences for social evolution on land, making conspecifics not only more dependent upon one another for homes, but also potentially easier to evict. Further changes in claw shape on land (with the claws of terrestrial hermit crabs becoming shorter, wider, and thicker) may have evolved to help offset their heightened danger of social eviction, acting as a more effective door against conspecifics.

Organisms architecturally modify environments and these modifications may persist across generations, potentially strongly shaping social behavior. However, few experiments have directly tested the impact of architectural modifications from earlier generations on social behavior in later generations. Here, I report experiments using extremely durable resources, shells, which endure for decades to centuries in stable form. Terrestrial hermit crabs (Coenobita compressus) architecturally remodel shells and pass these modified shelters to subsequent generations, which reuse them long after the original architect's death. I conducted controlled field experiments in a population of these crabs in which shells have been individually marked and tracked for a decade. I examined the impact of architectural modifications by contrasting social behavior around introduced shells, either remodeled shells (whose internal architecture was modified by earlier generations) or unremodeled shells (whose architecture had never been modified). Remodeled shells generated radically different social dynamics than unremodeled shells, catalyzing vacancy chains in which shells were socially redistributed across the population. Social groups that formed around remodeled shells consisted of size-ordered queues, with precise timing and social coordination required if individuals were to acquire superior shells. Interestingly, comparative experiments in two non-architect species (Clibanarius albidigitus and Calcinus obscurus) failed to show any impact of architectural modifications on social behavior; such impacts were only found in the architect species (C. compressus). Broadly, architecture from earlier generations can thus play a major role in driving social dynamics among later generations.

Complex environments may place constraints on animal sensory perception. However, little is known about how ecological constraints impact the formation of animal social groups, which ultimately necessitate that conspecifics detect one another. Here we studied the fission–fusion social groups of highly social terrestrial hermit crabs (Coenobita compressus), which frequently split and recombine, requiring individuals to actively sense the location of conspecifics. We manipulated the environment by relocating abundant natural fallen leaves on the beach, testing how and why this debris may constrain social group formation. Our experiments revealed that fallen leaves impose a fundamental limit on social grouping, with experimentally simulated groups attracting significantly fewer conspecifics (75% less) if they were surrounded by leaves. This constraint on social grouping was not the result of leaves acting as a physical barrier to movement. Furthermore, leaves only hindered visual perception of conspecifics and did not hinder other modalities besides vision. By experimentally moving leaves above the horizon, such that they no longer blocked animals’ field of view, we found that the impact of these visual constraints on grouping could be effectively abolished. Broadly, these experiments elucidate how complex environments impose sensory constraints on social animals’ ability to navigate toward groups. Social animals must be able to detect and orient toward conspecifics if they are to form social groups. Features of the environment, however, may impose constraints on sensory perception that interfere with the detection of conspecifics. Here we studied social hermit crabs, which form social groupings within a complex habitat that contains abundant fallen leaves along the beach–forest interface. By experimentally manipulating leaves, we show that these seemingly insignificant ecological materials create visual constraints that block the cues free-wandering individuals normally use to orient toward social groups. Our study thus reveals how common aspects of the environment can exert major constraints on sensory perception, thereby severely limiting social group formation and ultimately sociality.

The sensing of chemical information allows aquatic animals to interpret their surroundings over long distances. If the use of chemosensory information is impacted, the ability to find resources may be impaired. Anthropogenic activities directly contacting the seabed produce sediment vibrations which have the potential to affect behaviors guided by not only the vibrational modality but also other modalities such as chemical. Here, in subtidal field conditions, free-ranging hermit crabs (Pagurus acadianus) were exposed to an impulsive source directly contacting the substrate. A specific chemical cue, indicative of the availability of a gastropod shell (i.e. a new home), was used to attract crabs. Numbers of crabs arriving at the bait were monitored in noise versus control conditions. Fewer crabs congregated at the chemical cue after noise exposure; there was a significant effect of treatment (control/noise), time (before/after) and the interaction between the two. The results indicate that noise acted cross-modally by affecting a chemicallyguided shell searching behavior. The findings are collated with other studies here which also indicate crossmodal affects of sound and noise. Overall there is increasing evidence to suggest that the impacts of noise pollution should be tested across sensory modalities rather than uni-modally.

Burrows represent a prominent example of animal architecture that fundamentally alters the surrounding physical environment, often with important consequences for social life. Crustaceans, in particular, offer a model system for understanding the adaptive functions of burrows, their ecological costs and benefits, and their long-term evolutionary impacts on sociality. In general, burrows are central to the life history of many species, functioning as protective dwellings against predators and environmental extremes. Within the refuge of a burrow, one or multiple inhabitants can feed, molt, grow, mate, and raise offspring in relative safety. Depending on the substratum, substantial construction costs can be incurred to excavate a burrow de novo or enlarge a preexisting natural crevice. This investment has been evolutionarily favored because the benefits afforded by the burrow outweigh these costs, making the burrow an “extended phenotype” of the architect itself. Yet even after a burrow is fully constructed, the architect must incur continued costs over its life history, both in maintenance and defense, if it is to reap further benefits of its burrow. Indeed, because burrows accumulate value based on the work involved in their construction, they can attract conspecific intruders who seek to shortcut the cost of construction by evicting an existing occupant and usurping its burrow. Consequently, a burrowing lifestyle can lead to escalating social competition, with many crustaceans evolving elaborate weapons and territorial signals to resolve conflicts over burrow ownership. Some burrows even outlast the original architect as an “ecological inheritance,” serving as a legacy that impacts social evolution among subsequent generations of kin and nonkin. Comparative studies, using cutting-edge technology to dig deeper into the natural history of crustacean burrows, can provide powerful tests of general theoretical models of animal architecture and social evolution, especially the extended phenotype and niche construction.

Mark Laidre introduces the coconut crab (Birgus latro), the world's largest terrestrial invertebrate.

While marine hermit crabs are well known for being omnivorous filter feeders, less is known about the role they may play as active carrion scavengers in intertidal ecosystems. Prior studies have revealed that intertidal hermit crabs can be attracted to chemical cues from predated gastropods. Yet their attraction is usually assumed to be driven primarily by the availability of new shells rather than by food. We conducted field experiments to assess hermit crabs’ potential role as generalist carrion scavengers on the California Coast, examining their speed of attraction and the size of the aggregations they formed in response to chemical cues from freshly smashed gastropods and mussels, both of which indicated available carrion. Compared to all other marine species, hermit crabs (Pagurus samuelis, P. hirsutiusculus, and P. granosimanus) were the fastest to arrive at the provisioning sites. Hermit crabs also dominated the provisioning sites, accumulating in the largest numbers, with aggregations of up to 20 individuals, which outcompeted all other scavengers for carrion. Notably, hermit crabs arrived equally quickly for both smashed gastropod and mussel, even though the latter does not offer suitable shells for hermit crabs and even though the former only yields shell-related chemical cues over time frames longer than our experiments. These results thus suggest that shell availability is not the only, or even the primary, reason marine intertidal hermit crabs aggregate at carrion sites; they also aggregate to forage, thereby playing an important role as active carrion scavengers in intertidal ecosystems.

In his autobiography, Charles Darwin wrote: “As a little boy I was much given to inventing deliberate falsehoods” (pp. 1958:23). In an example Darwin relates: “I remember stealing apples from the orchard, for the sake of giving them away to some boys and young men who lived in a cottage not far off, but before I gave them the fruit I showed off how quickly I could run and it is wonderful that I did not perceive that the surprise and admiration which they expressed at my powers of running, was given for the sake of the apples. But I well remember that I was delighted at them declaring that they had never seen a boy run so fast!” (p. 1958:24). Poignant as Darwin’s youthful self-deception was—handing out apples to the very people cheering his running skills —it also raises a fundamental scientific question: to what extent are humans subject to self-deception? Studies suggest self-deception may be far from uncommon. For instance, most people suffer from the so-called end of history illusion: when reflecting back on how much we have changed, we falsely believe we’ve reached the apex of our personal development, underestimating how much we will actually change in the future (Quoidbach et al. 2013). Yet if humans are so subject to self-deception, failing to see the world as it is, why would natural selection have favored this? Is self-deception adaptive? Robert Trivers—a luminary among evolutionary biologists, one of the foremost theoretical experts on social evolution, conflict, and cooperation, and also a 2007 Hum Nat (2014) 25:285–297 DOI 10.1007/s12110-014-9201-8

Gastropod shells represent an essential resource for hermit crabs (Decapoda, Anomura). In many cases, hermit crabs acquire used shells from conspecifics who previously occupied the shell. Terrestrial hermit crabs (Coenobita compressus H. Milne Edwards, 1836) strongly prefer used shells. Here we test whether marine hermit crabs (Pagurus samuelis (Stimpson, 1857)) also exhibit a preference for used shells by providing them with matched pairs of (1) a used shell (previously occupied by either the choosing crab itself or by a conspecific) versus (2) a brand new shell (freshly derived from a predated gastropod). Unlike terrestrial hermit crabs, marine hermit crabs showed no preference for used shells (either their original shell or a shell from a conspecific). We suggest the divergent shell preferences of marine and terrestrial hermit crabs relate to the contrasting natural selection pressures in the sea versus on land. In particular, the used shells of terrestrial hermit crabs are architecturally remodeled by prior occupants and these remodeled shells represent a superior resource on land. In contrast, marine hermit crabs never remodel shells, and for them a used shell may be less protective than a new shell against the many specialized shell predators in the ocean.

Animals may use a variety of sensory modalities to assess ownership and resource-holding potential (RHP). However, few studies have experimentally tested whether animals can assess these key variables through a purely vibrational modality, exclusively involving substrate-borne vibrations. Here we studied social terrestrial hermit crabs ( Coenobita compressus ), where competitors assess homeowners by climbing on top of a solid external structure—an architecturally remodelled shell home, inside of which the owner then produces vibrations. In the field, we used a miniature vibratory device, hidden within an empty shell, to experimentally simulate a ‘phantom owner’, with variable amplitudes of vibration representing different levels of homeowner strength. We found that assessors could use these vibrations to deduce the owner's RHP: for strong vibrations (indicative of a high RHP owner) assessors were least likely to escalate the conflict; for weak vibrations (indicative of a low RHP owner) assessors showed intermediate escalation; and in the absence of vibration (indicative of an extremely weak or absent owner) assessors were most likely to escalate. These results reveal that animals can assess homeowner strength based solely on substrate vibrations, thereby making important decisions about whether to escalate social conflicts over property.

SummaryThe study of animal signals began in earnest with the publication in 1872 of Charles Darwin’s The Expressions of the Emotions in Man and Animals, which laid the basis for a comparative study of signals across all animals, including humans. Yet even before Darwin, the exceptional diversity of animal signals has gripped the attention of natural historians and laymen alike, as these signals represent some of the most striking features of the natural world. Structures such as the long ornamented tail of the peacock, the roaring sounds of howler monkeys, audible kilometers away, and the pheromone trails laid by ants to guide their nestmates to resources are each examples of animal signals (Figure 1). Indeed, because signals evolved for the purpose of communicating (Box 1), their prominence can be hard for even a casual observer to overlook. Animal signals therefore raise many scientific questions: What are their functions? What information do they transmit? How are they produced? And why did they evolve?

Information from others can be unreliable. Humans nevertheless act on such information, including gossip, to make various social calculations, thus raising the question of whether individuals can sort through social information to identify what is, in fact, true. Inspired by empirical literature on people's decision-making when considering gossip, we built an agent-based simulation model to examine how well simple decision rules could make sense of information as it propagated through a network. Our simulations revealed that a minimalistic decision-rule 'Bit-wise mode' - which compared information from multiple sources and then sought a consensus majority for each component bit within the message - was consistently the most successful at converging upon the truth. This decision rule attained high relative fitness even in maximally noisy networks, composed entirely of nodes that distorted the message. The rule was also superior to other decision rules regardless of its frequency in the population. Simulations carried out with variable agent memory constraints, different numbers of observers who initiated information propagation, and a variety of network types suggested that the single most important factor in making sense of information was the number of independent sources that agents could consult. Broadly, our model suggests that despite the distortion information is subject to in the real world, it is nevertheless possible to make sense of it based on simple Darwinian computations that integrate multiple sources.

Ecosystems are fundamentally connected to one another, although empirical documentation of the links between them, especially the links between marine and terrestrial ecosystems, has been limited. This study examined the foraging behavior and ecology of an understudied invertebrate, the hermit crab Coenobita compressus, which is remarkably abundant at the nexus of marine, terrestrial, and freshwater ecosystems. Data collected between 2008 and 2011 in the Osa Peninsula of Costa Rica revealed that C. compressus plays a generalist scavenger role across ecosystems. Beach almond fruits (Terminalia catappa) were a staple of the crabs’ diet, but the crabs also commonly consumed a variety of other fruit species and various parts of plants (nuts, seeds, flowers, husk, stems, buds) as well as algae, fungus, animal feces, and the carcasses of insects, jellyfish, conspecifics, other crustacean species, freshwater and saltwater fish, sea turtles (eggs and adults), and mammals. The crabs fed on these resources in prominent social aggregations, also forming such aggregations as they exchanged the aquatically derived gastropod shells that they use as homes while roaming the land. The size of the crabs’ aggregations around plant versus animal resources and around terrestrial versus aquatic resources did not differ, underscoring the species’ generalist foraging behavior. Broadly, these results suggest that C. compressus has one of the most diverse diets of any crustacean and plays a critical role in recycling organic matter and nutrients. This species might also mediate the passage of resources across different ecosystems, although further research is needed to explore this possibility.

Understanding the process by which limiting resources are incorporated into populations is a major goal of ecology. While many studies have examined this dynamic process using essential resources like homes, few of these studies have involved homes that can be transported by their occupants. This study introduced over a thousand transportable homes into a population of terrestrial hermit crabs Coenobita compressus, animals that carry their homes with them wherever they travel. These new homes were tracked between years to test key predictions about the temporal dynamics the homes would generate, and the spatial and structural changes the homes would undergo as they were used by the population. When moving into new homes, crabs dropped off their old homes directly at the exchange site, and the number of such traded-in homes peaked rapidly in time. Traded-in homes were under half the diameter of new homes, a difference apparently magnified by social formations involving vacancy chains. After crabs moved into new homes, they carried the homes away from the exchange site. The following year, these homes were displaced a distance four orders of magnitude times their diameter, thus penetrating extensively through the population. Between years, crabs also remodeled the internal architecture of the homes, creating homes that were more spacious and less of a burden to carry. These results suggest that transportable homes generate novel ecological dynamics along temporal, spatial and structural dimensions, which are a direct consequence of their transportability.

Hermit crabs are critically dependent upon gastropod shells for their survival and reproductive fitness. While anecdotal reports have suggested that hermit crabs may be capable of removing live gastropods from their shells to access the essential shell resource, no systematic experiments have been conducted to investigate this possibility. This paper reports experiments on both marine (Pagurus bernhardus) and terrestrial (Coenobita compressus) hermit crabs in which crabs were paired in the laboratory with the gastropods whose shells they inhabit in the field. Pairings included both shelled and naked crabs and spanned the full range of the gastropod life cycle. Neither marine nor terrestrial hermit crabs were successful at removing live gastropods from their shells. Furthermore, only a small fraction of the crabs (5.7%) were capable of accessing shells in which the gastropod had been killed in advance, with its body left intact inside the shell. Finally, although hermit crabs readily entered empty shells positioned on the surface, few crabs (14.3%) were able to access empty shells that were buried just centimeters beneath them. These results suggest that hermit crabs are constrained consumers, with the shells they seek only being accessible during a narrow time window, which begins following natural gastropod death and bodily decomposition and which typically ends when the gastropod's remnant shell has been buried by tidal forces. Further experiments are needed on more species of hermit crabs as well as fine-grained measurements of (i) the mechanical force required to pull a gastropod body from its shell and (ii) the maximum corresponding force that can be generated by different hermit crab species' chelipeds.

During conflicts, animals may perform displays that convey information about their future antagonistic behavior. Although theory once predicted that such signals of "intent" would be utterly susceptible to dishonesty, empirical studies have established that animals sometimes do signal their intentions. It remains unclear, however, what level of honesty exists within such signals. Here I report interactive-model experiments designed to expose instances of falsified intent and to reveal how frequently signalers within an invertebrate population lie about their impending hostile actions. Hermit crabs (Pagurus bernhardus) were given the opportunity to aggressively threaten an approaching model, and their subsequent behavior was then examined after the model fled from them or probed them to the point of imminent collision. Discrepancies between an individual's advertised intention and what it actually did next were infrequent. If the model fled, nearly all crabs remained in place, regardless of whether they had initially threatened. If the model probed, crabs that threatened reliably stood their ground, thus backing up their threat, whereas those that did not threaten tended to back down and retreat. Much of what has been regarded as "lying about intent" in prior nonexperimental studies may actually represent uncontrolled noise, especially recipient-response dynamics.

Sociality provides a unique opportunity for animals to acquire information and learn from others. Especially during foraging, where trial-and-error food selection might be fatal, conspecifics could act as valuable sources of information. During a six-year study across captive, semifree ranging, and wild Old World monkeys, I investigated whether individuals garnered olfactory-based information from their group mates that could guide their feeding decisions. Each of three study species [mandrills (Mandrillus sphinx), drills (M. leucophaeus), and olive baboons (Papio anubis)] performed a prominent muzzle-muzzle behavior, potentially enabling individuals to smell others' mouths and determine via olfaction what foods their conspecifics had chosen. This muzzle-muzzle behavior (1) was preferentially directed by naïve, younger individuals toward more experienced, older individuals, (2) occurred specifically while recipients were chewing and hence emitting the most potent chemical cues, (3) was typically followed by the actor consuming the very same food type the recipient had been eating, (4) was elicited most often in response to experiments involving novel foods, and (5) occurred less frequently as initially novel foods became more familiar. In contrast to this evidence for information acquisition, there was little support for previous proposals suggesting that muzzle-muzzle functions as a social display. Instead, the omnivorous diets and intensely social lifestyles of mandrills, drills, and baboons, may have each favored a convergent form of information acquisition: seeking out the breath of knowledgeable conspecifics to help decide what foods are safe to eat.

Whether animal signals convey honest information is a central evolutionary question, since selection pressures could, in some circumstances, favour dishonesty. A prior study of signalling in hermit crabs proposed that the cheliped extension display of Pagurus bernhardus might represent such an instance of dishonesty. A limitation of this conclusion, however, was that honesty was defined in the context of size assessment, neglecting the potential information that displays might transmit about signallers' variable internal states. Recent analyses of signalling in this same species have shown that its displays provide reliable information about the amount of risk crabs are prepared to tolerate, which therefore might enable signallers to use these displays to honestly convey their motivation to take such risks. Here we test this ‘honest advertisement of motivation’ hypothesis by varying crabs' need for food and analysing their signalling during simulated feeding conflicts against a model. When crabs were starved for 1–5 days, they dropped significantly in weight. Despite this decrement in resource-holding potential and energy reserves, crabs were more likely to perform cheliped extension displays the longer they were food deprived. Longer-starved crabs, whose subjective resource value was greater, also displayed at a higher rate and were more likely to risk seizing the food from the model. We conclude that cheliped extension is a reliable indicator of crabs' internal state and suggest how this honest signal might operate in conflicts over a variety of other resources in addition to food. We propose that future studies detecting apparent dishonesty should analyse many possible signal–state correlations before concluding a signal is actually dishonest.

Studies of intraspecific behavioral variability have documented cases where behaviors are present in some populations or groups but are absent in others. In some cases these differences cannot be explained by recourse to environmental or genetic variation, and may instead represent "traditions". Despite many examples of animal traditions in acoustic communication, relatively few examples exist of gestural traditions. Here I report on a study of communication across eight captive groups of mandrills (Mandrillus sphinx) in which a prominent gesture (Hand extension) was unique to two groups. Habitat variability, genetic differences, and sampling bias were not sufficient to account for the gesture's limited distribution across the study groups. Within the two groups where the gesture did occur only the juveniles in the group performed it, consistently directing it toward adults. Quantitative analysis of the contexts and responses associated with the gesture suggested that juveniles utilized it to provoke adults. Moreover, the gesture appeared to minimize the risk juveniles incurred while inciting adults, suggesting that repeated social interactions shaped the gesture's form. Interestingly, both the groups where the gesture emerged contained few juveniles. With limited play partners, these juveniles may have resorted to harassing adults as an alternative social play outlet. The creation of this novel gesture may thus be due to the combined influence of a shortage of play partners and of the increased free time for playful social exploration afforded by captivity. Although juveniles frequently "eavesdropped" on dyadic interactions involving the gesture and would subsequently initiate an interaction with the recipient of the gesture, there was no definitive evidence for social transmission; the gesture could instead have been independently invented by each juvenile.

Although the technical problem-solving expertise of nonhuman primates has been investigated extensively in captivity, few species have been tested in their natural habitats. Here I examine the physical cognition of wild savanna baboons (Papio anubis), a species that occupies an omnivorous foraging niche in which a variety of embedded food items are extracted and processed. Baboons were tested on three puzzles, each involving high-quality food that required removal from a novel obstruction: (1) a string-pulling puzzle in which food was hung from tree branches, (2) a twig-dipping puzzle in which food was embedded in a vertical tube, and (3) a stick-pushing puzzle in which food was contained in a horizontal conduit. The baboons failed to solve the second and third puzzles even when tools had been appropriately positioned in advance. And although they solved the first puzzle, their actions (running while holding food that was still attached to the string), suggested they did not fully comprehend the string's connective property. The baboons' performance might reflect the time constraints of life in the wild, which relative to captivity may provide fewer opportunities for the development of understanding about the physical properties of objects and their potential uses as tools. Further experiments on the physical cognition of baboons and many other primate species in their natural habitats would help test this ontogenetic hypothesis. Such field experiments would be especially fruitful if they continued to target extractive foragers like baboons: these experiments could simultaneously provide a test of phylogenetic hypotheses that invoke extractive foraging as the key stimulus for brain expansion in savanna-dwelling hominids.

When interacting organisms have opposing genetic interests, the integrity of communication systems may be undermined. For signaling in such conflict circumstances to remain evolutionarily stable, cheaters must be handicapped. Agonistic threat signals, however, are not always constrained or costly to produce, and yet these signals occur in the severest of conflicts where strong incentives exist for dishonesty. A leading hypothesis for how reliability is stabilized under these conditions is that signaling entails a risk, making signalers vulnerable to injury. Here I experimentally alter vulnerability to show how risk can modify organisms' willingness to escalate disputes, affecting the use of threat signals. The vulnerability to injury of hermit crabs (Pagurus bernhardus) was manipulated by varying the exposure of their soft uncalcified abdomens. When faced with potentially damaging conspecific attacks, more vulnerable crabs were conflict averse, showing reluctance to claim ownership over contestable food, frequently retreating from threats, and refraining from threatening others. The risk an organism can bear in escalated conflict can thus mediate its agonistic behavior and usage of threats. Postural nuances can consequently provide reliable information about aggressive intentions despite minimal production costs and opposing interests between communication parties. Copyright 2007, Oxford University Press.

At an ultimate, evolutionary level, manipulation entails behaviour that enhances the fitness of the manipulator even if at times inflicting a cost on the fitness of the manipulated. At a proximate, mechanistic level, mind-reading entails an ability on the part of a manipulator to attribute beliefs to those it manipulates. Both manipulation and mind-reading were investigated during a study of food-associated calling by semi free-ranging mandrills in Gabon. Mandrills were initially provisioned at a feeding cage (whose opening generated loud noises) and under these conditions calling occurred at high rates, particularly among middle-ranking individuals. When mandrills were then fed away from the cage (where potential alerting cues were absent) the calling rate dropped drastically and discoverers of the resource suppressed loud calls, producing only soft grunts. The latter conditions provided an increased opportunity for solitary consumption, and analyses showed that discoverers benefited substantially in terms of ingested food from a reduced presence of group mates. Loud calls, however, were produced by later arrivals and a correlation existed between the production of these calls and more group mates arriving at the resource. Loud calls may have enabled later arrivals to access monopolized resources, since by divulging information they could attract other, particularly lower-ranking group mates whose arrival engendered a diversion. Withholding calls (information suppression) and producing calls (information leakage) thus each appeared functionally manipulative, their usage depending only on whether they benefited the signaler. Acoustic playbacks were consistent with this hypothesis: compared to controls, food-associated calls resulted in more arrivals and also a trend toward quicker resource depletion and increased consumption by mandrills other than the discoverer. Yet upon hearing playbacks of a group mate's calls, non-vocal discoverers consuming the resource did not disguise their actions in anticipation of their group mates' arrival. In fact, some of their actions (like making repeated forays with unconcealed food) appeared wholly inconsistent with mind-reading.

The functions of two visual signals of the mandrill [silent bared-teeth face (SBTF) and crest-raise (CR)] were investigated by quantifying the probability of each signal occurring across a set of distinct contexts. The motivation for the investigation was twofold: (1) SBTF had been interpreted in diverse and sometimes contradictory ways, and (2) CR had been interpreted as a distinct signal from SBTF, despite indications that the two signals grade into one another. In our investigation we considered four functions for both SBTF and CR (threat, submissive, conciliatory, and ambivalent), and we made specific predictions about the relative probability that a signal with each of these functions should occur in different contexts. To determine if SBTF and CR represent a single graded signal, we analyzed them separately and together. We predicted that if they represent a single graded signal, then they should exhibit similar patterns of occurrence across contexts when they were analyzed separately, and that these patterns should be strengthened when they were analyzed together. The results showed that both SBTF and CR met the predictions for conciliatory signals, occurring most often in non-aggressive, non-hostile contexts and least often in aggressive, hostile contexts. The results were also consistent with the hypothesis that SBTF and CR represent a single graded signal.

Honest signaling of intentions was initially rejected by evolutionary game theory because it was believed to be susceptible to cheating mutants. In theory, however, conditions may exist that enable this type of signaling to remain evolutionarily stable. Two such conditions have been proposed: (1) signaling may entail performance risks by making the sender vulnerable to attack ('vulnerability handicap' model), or (2) individuals may recognize one another, remember the outcome of past interactions, and interact repeatedly ('individual recognition' model). In this paper, I tested predictions deriving from both models by analyzing behavioural sequences involving threats in a species of Old World monkey (Mandrillus sphinx). The results were consistent with the individual recognition model. Subordinates (judged competitively inferior based on approach-avoid interactions) rarely elicited favorable responses with their threats, which were generally ignored. In addition, the honesty of dominants was frequently tested since subordinates responded favorably to dominants' threats less than half the time. When dominants experienced unfavorable responses to their threats they were significantly more likely to direct further aggression at the subordinate, thus demonstrating an honest commitment to the conflict. The results did not support the vulnerability handicap model since receiver retaliation against threats was essentially nonexistent. Though in partial support of the vulnerability handicap model (a) riskier threats were more effective, and (b) senders were less likely to perform the riskiest threats against more dominant individuals. In primates and other animals that live in long-term, stable groups, individual recognition may be especially important in maintaining honest signaling of intentions. Vulnerability handicaps, however, could also play a role in maintaining honesty in such species, but primarily during interactions between unfamiliar individuals that are transferring groups, or during dominance reversals.

Summary Organisms can receive not only a genetic inheritance from their ancestors but also an ecological inheritance, involving modifications their ancestors made to the environment through niche construction [1]. Ecological inheritances may persist as a legacy, potentially generating selection pressures that favor sociality. Yet, most proposed cases of sociality being impacted by an ecological inheritance come from organisms that live among close kin and were highly social before their niche construction began [2]. Here, I show that in terrestrial hermit crabs ( Coenobita compressus ) — organisms that do not live with kin and reside alone, each in its own shell — niche-construction drives social dependence, such that individuals can only survive in remodeled shells handed down from conspecifics. These results suggest that niche construction can be an important initiator of evolutionary pressures to socialize, even among unrelated and otherwise asocial organisms.

Public information offers a valuable means for social foragers to determine the relative quality of foraging patches. Despite much evidence that foragers use public information based on others’ feeding behavior, no experiments have examined whether foragers might use public information based on others’ competitive behavior, particularly the collective commotion that can be generated by aggregations. Such commotion could potentially provide a rich source of public information: as foragers compete in a patch with an especially high value resource, their heightened competition intensity could enable eavesdropping foragers to target this superior patch, based simply on its higher level of collective commotion. To test the hypothesis that the level of collective commotion is used as public information by eavesdropping foragers I conducted field experiments on terrestrial hermit crabs Coenobita compressus. These animals engage in collective competitive interactions in foraging patches for food and shells, generating variable levels of commotion across different quality patches. By experimentally manipulating the level of collective commotion in sham aggregations in the wild I show that a higher level of commotion is exploited by eavesdropping foragers to differentially target more valuable patches. Broadly, these results highlight an underappreciated significance of competitive by-products and higher- order collective pheno mena as forms of public information for foragers.

Cuantificar la variacion de la biodiversidad es un objetivo importante de la biologia. Sin embargo, debido a que tales inventarios, en especial para la biodiversidad marina, son costosos, dificiles y consumen mucho tiempo, hay un gran valor en indicadores simples que permitan determinar la biodiversidad de una region determinada de manera eficiente. Hicimos un inventario de las conchas usadas como casas moviles terrestres por los cangrejos ermitanos (Coenobita compressus) en la Peninsula de Osa, Costa Rica, un “punto algido” de biodiversidad. Se registraron 41 especies, el mayor numero registrado en cualquier poblacion de cangrejos ermitanos. Proponemos que la “diversidad del mercado de vivienda” en los cangrejos ermitanos puede proporcionar un indice de biodiversidad ecosistemica conveniente, lo que podria facilitar las comparaciones entre sitios. ABSTRACT Quantifying variation in biodiversity is an important goal of biology. However, because inventorying biodiversity, especially marine biodiversity, is costly, difficult, and time consuming, there is great value in simple metrics that can reliably indicate a given region’s biodiversity and that can be efficiently gathered. We inventoried shells used as portable houses by terrestrial hermit crabs (Coenobita compressus) in Osa Peninsula, Costa Rica, a biodiversity hotspot. Forty-one species were recorded, the largest number registered for any hermit crab population. We propose that housing market diversity in hermit crabs might provide a convenient biodiversity index of ecosystems, potentially facilitating comparisons across different sites.

Architectural creations occur throughout the animal kingdom, with invertebrates and vertebrates building structures such as homes to maximize their Darwinian fitness. Animal architects face many trade-offs in building optimally designed structures. But what about animals that do not build, and those that only remodel the original creations of others: do such secondary architects face similar trade-offs? Recent evidence has revealed that hermit crabs—animals well known for opportunistically moving into remnant gastropod shells—can also act as secondary architects, remodelling the shells they inherit from gastropods. Remodelling has only been found among terrestrial hermits (Coenobitaspp.), not marine hermits. Here we investigate the potential trade-offsCoenobita compressusfaces from remodelling by subjecting its remodelled and unremodelled homes to controlled engineering crush tests, which parallel the homes being crushed by predators. While remodelled homes are significantly more spacious and lightweight than unremodelled homes, we find that the homes attain these beneficial qualities at a cost: a reduced resistance to being crushed. Hermit crabs may therefore only remodel their homes to thresholds set by the bite force of their predators. Our results suggest that, like primary animal architects, which face trade-offs when optimizing architectural designs, secondary animal architects face trade-offs when remodelling such designs.

BackgroundHuman societies exhibit a rich array of gestures with cultural origins. Often these gestures are found exclusively in local populations, where their meaning has been crafted by a community into a shared convention. In nonhuman primates like African monkeys, little evidence exists for such culturally-conventionalized gestures.Methodology/principal findingsHere I report a striking gesture unique to a single community of mandrills (Mandrillus sphinx) among nineteen studied across North America, Africa, and Europe. The gesture was found within a community of 23 mandrills where individuals old and young, female and male covered their eyes with their hands for periods which could exceed 30 min, often while simultaneously raising their elbow prominently into the air. This 'Eye covering' gesture has been performed within the community for a decade, enduring deaths, removals, and births, and it persists into the present. Differential responses to Eye covering versus controls suggested that the gesture might have a locally-respected meaning, potentially functioning over a distance to inhibit interruptions as a 'do not disturb' sign operates.Conclusions/significanceThe creation of this gesture by monkeys suggests that the ability to cultivate shared meanings using novel manual acts may be distributed more broadly beyond the human species. Although logistically difficult with primates, the translocation of gesturers between communities remains critical to experimentally establishing the possible cultural origin and transmission of nonhuman gestures.