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The costs of signalling are often expressed in terms of increased predation risk to the signaller; however, whether signalling predators also incur costs due to eavesdropping by prey and may attempt to reduce these costs is less well studied. In this study, we investigated whether there is a trade-off between intraspecific communication through scent marking and the risk of alerting prey in a wild population of a stalking predator, the Eurasian lynx. We followed lynx tracks in the snow and recorded scent marks and evidence of hunting behaviour along these tracks. Lynx preferred conspicuous objects for marking and increased scent marking rate when walking along linear structures, such as forest roads. This association was strongest when lynx were hunting, while there was only a weak correlation when no evidence of hunting could be detected. On tracks with evidence of hunting behaviour, lynx engaged less in scent marking. The relationship was most evident during the mating season, when lynx increased scent marking rates while they were not hunting. We further expected lynx to mark most in areas where they had recently hunted successfully, but time and distance to the last kill were not associated with scent marking rate. Our study supports the hypothesis that lynx face a trade-off between enhancing the detection probability of scent marks by conspecifics and avoiding eavesdropping by prey, and indicates that scent marking rate is influenced by several factors.

Benefits and costs of group living may be differentially influenced by kinship or familiarity and depend on context. For example, aggregation with kin may be costly in a mating context due to the risk of inbreeding, but beneficial in social interactions because it may reduce within-group competition. Research investigating aggregation behaviour in different contexts is scarce, especially in gregarious invertebrates. In the present study, we investigated the aggregation among adult European earwigs (Forficula auricularia) by comparing the aggregation behaviour of groups composed of individuals from two different families with the aggregation dynamics of groups composed of individuals from the same family (siblings). Aggregation behaviour was measured both during the night (active phase) and during the day (inactive phase). Our results showed enhanced aggregation among individuals from the same family at night, but lack of such preferences during the day. These findings demonstrate that earwigs are able to recognize familiar relatives and that this information is used for aggregation decisions at night, while they are most active, but not during the day when they rest.

Many insects raise their offspring on organic substrates or in the soil where microorganisms are abundant. Microbes may pose a serious threat to offspring development and survival by either decomposing food resources or directly infecting the offspring. Selection to cope with these effects may favor social defenses, for example, through forms of parental care that can limit or eliminate these threats to offspring fitness. In this study, we experimentally tested if maternal egg attendance in the European earwig Forficula auricularia has a function as a social defense against mold infection of eggs by manipulating exposure of eggs to mold spores and the presence of the mother in a fully factorial design. Furthermore, we investigated the potential roles of egg grooming behavior and maternal transfer of chemicals as underlying mechanisms. As predicted, the beneficial effect of egg attendance on hatching success was significantly enhanced when eggs were exposed to the mold. Females significantly increased their egg grooming duration in response to mold exposure of her eggs, and the quantity of chemicals (identified as hydrocarbons) was maintained among attended eggs but decreased substantially among unattended eggs. Maternal transfer of chemicals was confirmed in extractions of glass beads that were mingled into attended or unattended clutches. This study shows that maternal egg attendance in the European earwig has a social defense function protecting offspring against mold infection. The maternal egg grooming behavior seems to be key for this effect, probably through both the mechanical removal of spores and the continued application of chemical substances on the egg surface.

Parental care is beneficial for offspring, but costly for parents. Thus, it seems maladaptive to care for unrelated offspring unless the additional offspring provide direct benefits to the carer's own offspring or discrimination costs are high. Females of the European earwig, Forficula auricularia, provide maternal care for their own offspring (nymphs), but also for foreign nymphs that join their brood. Nymphs preferentially kill and cannibalize nonsiblings and individuals that are smaller or in poor condition. Here, we tested the effects of nymph condition and food availability on brood mixing in F. auricularia. We experimentally manipulated nymph condition by feeding or food depriving groups of unrelated nymphs. The resident groups remained with their mothers whereas the mothers were removed from the foreign groups. Since nymph condition determines the direction of cannibalism, we hypothesized that brood mixing is dependent on nymph condition and food availability in the environment. Our results show that brood mixing was initially driven by condition-dependent effects on nymph activity, and later by the fed foreigners invading resident broods, suggesting that foreigners control brood mixing, independent of food availability. These results are consistent with our hypothesis that the dynamics of brood mixing are condition dependent, possibly mediated by corresponding cannibalism threats.

Parental care typically enhances offspring fitness at costs for tending parents. Asymmetries in genetic relatedness entail potential conflicts between parents and offspring over the duration and the amount of care. To understand how these conflicts are resolved evolutionarily, it is important to understand how individual condition affects offspring and parental behaviour and whether parents or offspring make active choices in their interactions. Condition effects on offspring have been broadly studied, but the effect of parental condition on parentoffspring interactions is less well understood, in particular in species where care is facultative and offspring have the option to beg for food from the parents or to self-forage. In this study, we carried out two experiments in the European earwig Forficula auricularia, a system where females provide facultative care, in which we manipulated female condition (through a high-food and low-food treatment) and the degree by which mothers and offspring could make active choices. In a first experiment, where female mobility was limited, female condition had no significant effect on the rate of offspring self-foraging, which increased with nymph age. In a second experiment, nymph access to food was limited and females in poor nutritional condition provided food to significantly fewer nymphs than high condition females. In both experiments, offspring attendance remained at a constantly high level and was independent of female condition even after experimental separation of females and offspring. Our results show that earwig nymphs do not use cues of female condition to adjust rates of self-foraging, that females control food provisioning depending on their own condition, and that females and nymphs share control over offspring attendance, a form of care not influenced by female condition.

In group-living or social species intraspecific predation and cannibalism can lead to substantial decreases in inclusive fitness because related individuals may encounter one another. Therefore, selection on the ability and accuracy of kin recognition to avoid losses in inclusive fitness is expected. Competition and relatedness are two key factors for the evolution of siblicide and cannibalism. However, our knowledge of the temporal dynamics and the accuracy of kin-mediated siblicide and cannibalism is scarce and experiments have usually focused on either relatedness (kin recognition) or weight asymmetry (dominance and competition) as independent components. In an experiment we set up mixed-kinship groups of three nymphs of the European earwig, Forficula auricularia Linnaeus, a gregarious and (sub-)social species, to investigate the combined effects of relatedness and weight asymmetry on the temporal dynamics of siblicide and cannibalism. We found that the temporal dynamics were jointly influenced by relatedness and weight asymmetry among nymphs. Weight asymmetry effects on siblicide and cannibalism were stronger between unrelated individuals than between related individuals as shown by a significant relatedness∗weight asymmetry interaction on survival patterns. The opportunity to choose between related and unrelated individuals did not enhance the accuracy of kin recognition but induced the above interaction between relatedness and weight asymmetry on cannibalistic outcomes. These results show that relatedness and weight affect siblicidal and cannibalistic behaviour, and that their combined effect depends on the social context.

Parent–offspring conflict theory predicts the evolution of offspring solicitation signals that can influence the amount and/or the duration of parental investment. Short-term effects of offspring solicitation signals on parental food provisioning have been widely demonstrated, but persistent effects of offspring signals on the maintenance of parental care have been rarely studied. Also, the relation between the amount of care provided to the brood and how it is distributed among individual offspring within a brood is not well enough understood. Here, we investigated in the European earwig (Forficula auricularia) the effects of offspring condition-dependent chemical signals on the maintenance of maternal care among broods and the distribution of maternal food within broods. Mothers were isolated from their brood for 3 days and continuously exposed to chemical signals extracted from broods of experimentally manipulated nutritional state. After re-introducing mothers to their brood, a range of maternal behaviours were quantified. We found that earwig mothers groomed their offspring significantly more after exposure to chemical extract from high-food brood in comparison with mothers exposed to extract from low-food brood, which in turn displayed significantly more aggressive behaviour. Furthermore, we manipulated offspring individual nutritional condition within the brood to evaluate the effect of offspring state on the within-brood food distribution. Within broods, poorly fed individuals received significantly more food than well-fed individuals, probably due to scramble competition. These results show that earwig nymphs express multi-component condition-dependent signals and behaviours differentially affecting maternal care provisioned to the brood and the distribution of care within broods.

Aggression levels among individuals can severely increase under high density or shortage of crucial resources, sometimes resulting in individuals killing conspecifics. This is not uncommon in family groups of diverse taxa, where the dependent offspring compete for the limited resources provided by their parents. Killing a nest mate can relax the level of competition, and cannibalism provides a direct nutritional benefit. However, nest-mate killing bears the risk of reducing indirect fitness if the victim is related (i.e., siblicide), imposing selection on kin-recognition abilities. Based on this hypothesis, we predicted that first instar juveniles (nymphs) of the European earwig (Forficula auricularia) kill and cannibalize unrelated nest mates earlier and more often than related nest mates and that cannibalism has a direct nutritional benefit in terms of survival. We tested these predictions experimentally by establishing related and unrelated pairs of nymphs and recorded survival, aggregation behavior, and cannibalistic outcomes in the absence of alternative food sources. In order to obtain expected survival probabilities of victims and survivors in the absence of any interaction, we simulated virtual nymph pairs based on survival data of singly held control individuals. As predicted, victims lived for less time and survivor for longer than expected from the simulated survival distributions, demonstrating nest-mate killing and cannibalism. Furthermore, unrelated individuals were killed significantly earlier and were more often cannibalized than related individuals. The survival patterns of victims and survivors were quantitatively consistent with the expectations of Hamilton's rule. Our study shows that earwig nymphs recognize kin and adjust their nest-mate killing and cannibalistic behavior as predicted under the hypothesis of kin-selected siblicide and cannibalism. Copyright 2010, Oxford University Press.

Posthatching maternal care such as food provisioning and protection has evolved several times in insects, allowing offspring (larvae, nymphs) to interact with their mothers and potentially influence their investment. The evolutionary conflict over the duration and amount of parental care is thought to promote the evolution of offspring begging behaviours either as honest signals of need or as competitive signals with the potential to manipulate parents into investing more. In most social insects, parental care is not obligatory and may represent a less derived state than in vertebrate systems making them more appropriate to test ancestral conditions for the evolution of begging signals. Here, we review forms of maternal care in insects ranging from protection to food provisioning and evidence of offspring begging behaviours influencing maternal care, including condition-dependent chemical cues produced by offspring that may turn out to be solicitation pheromones. Since behavioural parent–offspring interactions are embedded in the reproductive and developmental physiology of mother and offspring, we stress the need for behavioural studies to be complemented by physiological measurements which will allow us to understand better the nature of conflict resolution. We propose a hypothetical mechanism of maternal care regulation by direct internal chemical signals (i.e. hormones) and indirect external chemical signals (i.e. solicitation pheromones) influencing maternal reproductive physiology and future reproduction. Social insect species and the integrated study of behavioural interactions and physiological/reproductive consequences may represent promising new experimental systems for direct tests of the evolution of begging signals, complementing current research on parent–offspring conflict.

Sibling and parent–offspring conflicts arise mainly over the amount and distribution of parental care, especially food. In altricial bird species where the young depend on parents for obtaining food, parents may control sibling competition by the choice of their respective provisioning locations. In great tits, the parents use fixed provisioning positions on the nest rim that are determined early in the breeding cycle and maintained until fledging. The two parents may choose positions that are close to each other, or far apart, and thereby increase or relax the pressure for optimal feeding positioning among nestlings. As an inspiration to this study we previously found that the two parents provide food from closer positions if the nest is infested by ectoparasites. Here, we tested the hypothesis that the parental choice of relative provisioning locations could be strategically used to control nestling competition. We forced parents to feed from either one or two provisioning locations and assessed the induced change in nestling movement, weight gain, and food distribution among siblings. We show that the angular distance between male and female locations influences the level of behavioural competition and affects nestling weight gain and food distribution. It is the first evidence for hole-nesting birds, where it was assumed that the nestling closest to the entrance hole was fed first, that the apparent choice of feeding positions by parents could be a way of controlling sibling competition and thereby also taking partial control over the outcome of parent–offspring conflict.

Female burrower bugs (Sehirus cinctus, Hemiptera: Cydnidae) show extended care for their offspring. They guard their clutch and feed the hatched nymphs up to the third larval instar. Previous research indicated that nymphs partly regulate maternal food provisioning, but how nymphs accomplish this is unknown. We tested the hypothesis that nymphs solicit maternal provisioning by condition-dependent chemical signalling and we postulated the existence of a solicitation pheromone. Clutches of 30 nymphs were handreared in either low- or high-food conditions. After moult to second instar, cuticular compounds were extracted in hexane. An independent set of test mothers caring for offspring were subsequently exposed to extracts of nymphs from either the low- or the high-food treatment. Two control groups were also involved, one exposed to the solvent hexane and one with no treatment. As predicted for a solicitation pheromone, test mothers exposed to extracts from nymphs reared under low food provisioned more than those exposed to extracts from nymphs reared under high-food treatment. Contrary to our expectation, however, nymph extracts had an overall inhibiting effect on maternal provisioning. The effects of extract exposure on maternal provisioning were short-lasting, suggesting that the critical cues may be volatile. Our results suggest complex chemical communication in burrower bug families for the short-term regulation of maternal provisioning, potentially involving both provisioning-releasing solicitation pheromones and inhibiting chemical cues.

Scent-marking is widespread among mammals and has been observed in many felid species. Although the behaviour is well-described, little is known about its function in wild felid populations. We investigated patterns of scent-marking and its role in intra- and intersexual communication among resident and non-resident Eurasian lynx Lynx lynx by observing interactions among wild lynx at natural marking sites by means of infrared camera traps. Marking activity of resident animals showed a peak during the mating season and was lowest during the time when females gave birth and lactated. Both sexes scent-marked, but male lynx visited marking sites much more often than females and marked relatively more often when visiting a site. Most visits to marking sites were by residents but we also observed scent-marking by non-residents. Juveniles were never observed marking. We found no evidence of lynx regularly renewing scent-marks after a certain 'expiry date' but the presence of a strange scent-mark triggered over-marking. Males responded similarly to the presence of another individual's scent-mark, irrespective of whether it was the top- or the underlying scent-mark in a mixture of scent-marks they encountered. Our results suggest that marking sites could serve as 'chemical bulletin boards', where male lynx advertise their presence and gain information on ownership relationships in a given area. Females placed their urine marks on top of the ones left by resident males, but further studies are needed to explain the functions of over-marking in females.

Solicitation displays by offspring and responses by parents are commonly regarded as phenotypic expressions of the conflict over parental investment between genes expressed in offspring and their parents. Models for the resolution of the parent–offspring conflict have assumed genetic variation for levels of both offspring solicitation and parental response in order to allow offspring and parental strategies to coevolve and the conflict to be evolutionarily resolved. Empirical research has focused on the phenotypic predictions of these models, and little is known about the genetics of parent–offspring interactions. We review the existing evidence for genetic variation in offspring solicitation behaviours and parental responses from a variety of animal species. This evidence confirms the fundamental assumption of genetic variation in offspring solicitation and parental response, indicating that offspring and parental strategies can (co-) evolve. We discuss potential reasons for the maintenance of genetic variation in offspring solicitation and parental response, and the implications genetic variation and covariation and/or parental effects may have for the resolution of the parent–offspring conflict. Further research on the genetic bases of parent–offspring interactions may be crucial for understanding the resolution of parent–offspring conflict and the coevolution of the behaviours involved.

Mock et al. (2011) in the following referred to as ‘‘MDS’’, point out some weaknesses and inconsistencies in begging research, and they convincingly place signaling of need (Godfray, 1991) and signaling of quality (Grafen, 1990) at eye level as 2 competing hypotheses for the evolution of offspring begging signals. Hopefully, the 2 hypotheses will be placed to balanced tests in the future. They further point out that the widespread hunger experiments are not tests of signals of ‘‘need’’ as envisaged by Godfray (1991) (see Royle et al. 2002 for previous, albeit different, criticism) because hunger is a poor predictor of offspring fitness returns on investment. They propose instead to treat hunger-dependent begging as conceptually separate proximate ‘‘signals of hunger.’’ The relation between proximate control of begging by hunger and ultimate causes of condition dependence of begging by need/quality is critical and possibly also at the heart of some of the confusion surrounding the concept of ‘‘need’’, I agree. However, I don’t think hunger effects on begging are as detached from evolutionary theory as the authors suggest. Godfray (1991) and others assumed in their models a single begging–provisioning interaction between an offspring and a parent. Under such simplistic conditions, more hungry offspring would actually often stand to gain more in terms of fitness from the single expected provisioning, and hunger would be a useful proxy for need. The core limitation of hunger as operational definition for need in experimental research is the characteristic repetitiveness of parent–offspring interactions in nature. The highly dynamic nature of hunger effects on offspring begging, begging effects on parental provisioning, and provisioning effects on offspring hunger makes hunger a highly transient state, concealing in very short time any potential relation between hunger and offspring fitness gains. But even then, we don’t have to exile hunger effects on begging to a purely proximate realm. Hunger experiments measure how offspring behaviorally react to variation in food provisioning at a given moment in time (Kolliker 2003). This causal relation was termed ‘‘effect of supply on demand—or ESD—mechanism’’ (Mock and Parker 1997; Parker et al. 2002; Kolliker 2003) or ‘‘demand reaction norm’’ (Smiseth et al. 2008). The slope of the demand reaction norm describes how begging encodes information about variation in recent food intake and comprises the proximate mechanisms that translate food intake into variation in begging (e.g., digestive efficiency, genetic variation, hormonal or ontogenetic changes; see Wright and Leonard 2002). Together with the parental supply reaction norm, the demand reaction norm determines the levels of provisioning and begging in the repeated interactions (Kolliker 2003) and plays a role in mediating the behavioral dynamics between parents and offspring (Dobler and Kolliker 2009). Finally, like reaction norms in general, the demand reaction norm is a potentially heritable and evolving trait (Smiseth et al. 2008), maybe even one of key interest in the evolution of family interactions. Thus, I would not go so far as to request a ‘‘moratorium on the hunger experiment’’ (MDS). Hunger experiments are often (and reasonably) the first step into the study of parent– offspring interactions in less well-studied organisms in which begging is not as obvious to the human observer as in birds. In such systems, they remain an essential tool to identify candidate begging traits. Furthermore, the ‘‘at least as an end to itself’’ part in MDS’s proposal seems essential to me, although the authors do not elaborate on it. Little is known about causal agents of interindividual variability in demand reaction norms or how selection operates on them. For example, is there heritable variation, ontogenetic shifts, and growth/learning/environmental effects on the slope of the demand reaction norm? Is the demand reaction norm expected to evolve differently if begging evolves as a signal of need as opposed to a signal of quality or as a means of scramble competition? Does sibling rivalry select for more or less hunger-sensitive begging? Such questions have not been broadly enough studied—neither theoretically nor experimentally. In my opinion, hunger experiments should continue to play an important role in research on begging but properly applied and interpreted: Hunger may usually not reflect need. The hunger effect on begging is a measurement of an offspring trait—the slope of the demand reaction norm. Most scientists in the field probably would agree with MDS that testing for behavioral control (bc) is of prime importance in future research on begging (Royle et al. 2002). Theoretically, bc is a model assumption needed to define which party makes active choices, that is, to which degree parents impose selection on offspring or vice versa. However, empirically, it is not so obvious what kind of trait bc should be and how we should test or observe this potentially cryptic trait. As a cautionary note, the term shares some of the properties that rendered need ambiguous and misleading. Like need, ‘‘bc’’ is theoretically straightforward, intuitively appealing, and empirically difficult to grasp, test, and quantify. While I agree with MDS that careful observation is essential, I doubt that observation alone will be sufficient in many cases. Manipulative experiments will be required to causally disentangle bc and to generalize beyond the limits of the behavioral repertoire of a given species [see Hinde et al. (2010) for an approach to disentangle bc based on coadaptation logic]. I think this will be important to avoid erroneously assigning bc based on easily observable behaviors that are interpreted as control behaviors on intuitive grounds.

Consistent inter-individual variation in behaviour over time and across contexts has been reported for a wide variety of animals, a phenomenon commonly referred to as personality. As behavioural patterns develop inside families, rearing conditions could have lasting effects on the expression of adult personality. In species with parental care, conflicts among family members impose selection on parental and offspring behaviour through coadaptation. Here, we argue that the interplay between the evolution of personality traits (i.e. boldness, exploration, activity, aggressiveness and sociability) expressed outside the family context and the specialized behaviours expressed inside families (i.e. offspring begging behaviour and parental response to offspring solicitations) can have important evolutionary consequences. Personality differences between parents may relate to the typically observed variation in the way they respond to offspring demand, and dependent offspring may already express personality differences, which may relate to the way they communicate with their parents and siblings. However, there has been little research on how personality relates to parental and offspring behaviours. Future research should thus focus on how and why personality may be related to the specialized parent and offspring behaviour that evolved as adaptations to family life.

Theory for the evolution of social interactions based on continuous strategies often assumes for simplicity that expressed behaviours are independent of previous encounters. In reality, however, such dependencies are likely to be widespread and often strong, generating complex behavioural dynamics. To model this process and illustrate potential consequences for the evolution of behavioural interactions, we consider the behavioural dynamics of the interaction between caring parents and their demanding offspring, a prime example of long series of interdependent and highly dynamic interactions. These dynamics can be modelled using functions describing mechanisms for how parents and their offspring respond to each other in the interaction. We establish the general conditions under which the behavioural dynamics converge towards a proximate equilibrium and refer to such converging interactions as behaviourally stable strategies (BSSs). We further demonstrate that there is scope for behavioural instability under realistic conditions; that is, whenever parents and/or offspring ‘overreact’ beyond some threshold. By applying the derived condition for behavioural stability to evolutionary models of parent–offspring conflict resolution, we show by numerical simulations that evolutionarily stable strategies (ESSs) of current models are not necessarily behaviourally stable. Because behavioural instability implies that expressed levels of behaviours deviate from the ESS, behavioural stability is required for strict evolutionary stability in repeated behavioural interactions.

The occurrence of parental food regurgitation as a form of parental care in (sub-) social insects has been little studied and is largely based on anectodal and indirect evidence. However, understanding the behavioural mechanisms mediating the benefit of parental care is critical to advance research on the evolution of family interactions. Here, we report results from a study where we experimentally tested the hypothesis that European earwig (Forficula auricularia) females regurgitate food to their nymphs. We used a simple experimental method based on food dyes as colour markers to separate maternal food regurgitation and nymphal self-feeding as the two components of food intake by nymphs. Two different food dyes were used to label the food offered to the earwig mother and the nymphs respectively. By analysing the colour of the gut content of the nymphs, we demonstrate significant food transfer from the mother to the nymphs. This study demonstrates unambiguously that maternal food regurgitation exists in earwigs and presents a simple and easily applicable technique to trace maternal food regurgitation in the study of insect parental care and family interactions.

The distribution of food among altricial bird nestlings is the result of an interaction between parental feeding decisions and scramble competition between nestmates. Both young and parents can potentially be in control of the outcome of this interaction. In great tits, each parent feeds from a fixed location on the nest rim, thereby forcing nestlings to choose between the father's and the mother's location. It was previously found that hungry nestlings approached the female preferentially and were more likely to be fed, appearing as if females showed a stronger preference to feed hungry young than males. However, nestlings were free to move in that study, and the effects of nestling positioning could not be disentangled from those of parental food allocation decisions. Here, we experimentally divided broods into two halves and randomly assigned each half of the brood to one side of the nest cup where only one parent could feed them. One nestling in each half-brood was food deprived to manipulate short-term hunger state. Both parents showed a similar preference to feed the more hungry nestlings, suggesting that the previously observed difference was due to offspring positioning rather than active parental choice. Our study shows that food allocation is partially under nestling control and suggests that nestlings adjust positioning and begging behaviour to the profitability of a given position in the nest cup.

The evolution of parental care and family group formation critically depends on offspring survival benefits and parental fecundity costs of care under given ecological conditions. Investigations of the functional significance of care in insect species that exhibit facultative parental care have been relatively rare but may be of particular interest for better understanding of benefit and cost schedules at an early evolutionary stage. In this study, aspects of benefits and costs of care were addressed in the sub-social European earwig (Forficula auricularia; Dermaptera: Forficulidae) by manipulating the presence of tending mothers and brood size in a fully crossed experimental design. Larvae growing in broods tended by their mother or of reduced size showed a higher survival probability than larvae growing in untended or large broods, as predicted if maternal care is beneficial and shaped by a trade-off between number and quality of offspring. Analysis of patterns of food consumption and developmental time further suggested that the benefit of maternal attendance is mediated by the maternal provisioning of food, while the quality–quantity trade-off seemed to be driven by sibling rivalry. Further, tending mothers delayed the production of a second clutch, indicating a potential cost of care in terms of lifetime fecundity. This study experimentally shows benefits and potential costs of maternal care and family group formation in the European earwig. More detailed behavioural experiments will be required to fully understand how behavioural interactions among family members mediate these reproductive outcomes.

Evolutionary theory of parent-offspring conflict explains begging displays of nestling birds as selfish attempts to influence parental food allocation. Models predict that this conflict may be resolved by honest signaling of offspring need to parents, or by competition among nestmates, leading to escalated begging scrambles. Although the former type of models has been qualitatively supported by experimental studies, the potential for a begging component driven by scramble competition cannot be excluded by the evidence. In a brood-size manipulation experiment with great tits, Parus major, we explored the scramble component in the begging activity of great tit nestlings by investigating the mechanisms of sibling competition in relation to brood size. While under full parental compensation, the feeding rate per nestling will remain constant over all brood sizes for both types of models; the scramble begging models alone predict an increase in begging intensity with brood size, if begging costs do not arise exclusively through predation. Great tit parents adjusted feeding rates to brood size and fed nestlings at similar rates and with similar prey sizes in all three brood-size categories. Despite full parental compensation, the begging and food solicitation activities increased with experimental brood size, whereas nestling body condition deteriorated. These findings support a scramble component in begging and suggest that the competition-induced costs of food solicitation behavior play an important role in the evolution of parent-offspring communication. Copyright 2003.

Sex allocation theory predicts that the allocation of resources to male and female function should depend on potential fitness gain realized through investment in either sex. In the great tit (Parus major), a monogamous passerine bird, male resource-hording potential (RHP) and fertilization success both depend on male body size (e.g., tarsus length) and plumage traits (e.g., breast stripe size). It is predicted that the proportion of sons in a brood should increase both with male body size and plumage traits, assuming that these traits show a father-offspring correlation. This was confirmed in our study: the proportion of sons in the brood increased significantly with male tarsus length and also, though not significantly, with the size of the breast stripe. A sex ratio bias in relation to male tarsus length was already present in the eggs because (1) the bias was similar among broods with and without mortality before the nestlings' sex was determined, and (2) the bias remained significant when the proportion of sons in the clutch was conservatively estimated, assuming that differential mortality before sex determination caused the bias. The bias was still present among recruits. The assumption of a father-offspring correlation was confirmed for tarsus length. Given that both RHP and fertilization success of male great tits depend on body size, and size of father and offspring is correlated, the sex ratio bias may be adaptive. [KEYWORDS: body size, great tits, Parus major, resource holding potential, sex allocation, sexual selection]

The evolutionary conflict over the amount of resources transferred between a parent and its offspring may be resolved by honest signalling of 'need' by offspring and parental investment in relation to signalling level. In birds, biparental care is the norm and evidence that male and female parents differ in their investment pattern in individual offspring is growing. In an experiment on great tits, Parus major, we investigated how and why parents differ in food allocation when responding to similar chick signals, which supposedly uniquely reflect the chick's nutritional condition. Nestling hunger level was manipulated by food deprivation and hand-feeding. Subsequent filming revealed that parents fed from significantly different locations on the nest and thereby forced chicks to choose between them when competing for favourable positions. Deprived nestlings approached, and fed ones retreated (or were displaced by siblings) from, positions near the female. No such behaviour was observed towards the male. Females allocated more feeds than males to the food-deprived nestlings. The results are discussed in terms of nestling competition for access to 'begging patches'. By varying their 'begging patch' value, parents may exploit competitive inter-sibling dynamics to influence the outcome of competition among chick phenotypes (e.g. 'need', size, sex). Parent birds may thereby exert considerable control over the information content of chick begging behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.

1. The transmission mode and reproductive rates of a parasite are usually associated with its virulence. In this study, two experiments were carried out to quantify horizontal transmission rates and reproductive dynamics of hen fleas infesting a population of great tits. Our results provide experimental data on certain factors affecting the population dynamics of an ectoparasite in a host population. 2. Immigration of adult hen fleas occurred in 72% (29 out of 40) of previously deparasitized great tit nests. The mean infestation intensity was 5.8 adult fleas per nest and did not vary seasonally. Adult hen flea distribution within the host population was aggregated and did not differ from a negative binomial distribution. 3. In nests experimentally infested with 40 adult fleas, two discrete flea cohorts were found at the end of the hosts' breeding attempts. The first cohort consisted of adult fleas, the second cohort was much larger and consisted of second and third instar larvae. No first instar larvae were observed in the second cohort. A small proportion of larvae was found in cocoons. 4. The day in the season and the daily mean temperatures during the birds' 'brooding period' (i.e. from the start of incubation until the last young fledged or died) were not significantly correlated with the number of fleas in the nests. These results suggest that climatic and seasonal factors do not have significant effects on flea reproduction. 5. The number of adult fleas and larvae was not significantly different between nests where all chicks died and nests where at least one young P, Cged. Host reproductive success affected the number and proportion of larvae in cocoons which was lower in nests of birds that failed compared with nests of birds that fledged young. The proportion of larvae in cocoons increased with the number of days in the 'brooding period' of the birds. 6. Among infested nests, the number of adult fleas and the total number of larvae were positively correlated with the fresh mass of the nests, suggesting that density-dependent mechanisms within nests affect flea numbers. Great tits appeared to increase the mass of their nests following experimental infestations with hen fleas. 7. In nests of birds that fledged at least one young, the number of adult fleas and larvae was not significantly correlated with measures of the hosts' breeding performance (chick mass and number of fledglings). These results suggest that flea reproductive success does not depend on the hosts' reproductive performance. 8. These results emphasize the need to study the effects of host responses on survival and reproductive rates of parasites, A knowledge of these effects is essential for the understanding of population dynamics, dispersal and life-history traits of the parasites.

Parent birds often feed from predictable sites at the nest and nestlings compete for access to positions close to them. Such scrambling for stable parentally favoured positions may relate to optimal foraging theory. For efficient foraging, nestlings should be able to associate begging payoffs with the available positions in the nestcup and use this information to position themselves before parental arrival at the nest. We tested this prediction experimentally in the great tit by keeping nestlings temporarily in the laboratory where they were housed individually in nestboxes identical to their natural boxes. After a period of food deprivation, we quantified their positioning, taking the nest entrance as a reference. Parental feeding locations were measured from videos made at the natural nests, again relative to the nest entrance. As predicted, nestlings in the laboratory boxes chose positions significantly closer to parental feeding sites than expected by chance. Thus, they had knowledge about their parents' habit of feeding from predictable sites, enabling them to navigate and forage efficiently in the nestcup.