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The association of immobility and camouflage is widespread as a defensive mechanism in prey from varied taxa. However, many experiments assessing the reaction of prey to predator cues are conducted under artificial laboratory conditions. In a previous experiment we observed the tadpoles of Ololygon machadoi (Hylidae) to respond to predator visual and/or chemical cues by choosing backgrounds that improve their disruptive properties, but detected no associated reduction of movement. Here we experimentally demonstrate this response in the species' natural habitat, on backgrounds where the tadpoles are likely to achieve their best camouflage. We also tested whether previous experiences could influence both background choice and immobility in O. machadoi tadpoles. These novel experimental results suggest that a defensive behavior-i.e., reduction of movement-in these tadpoles is more strongly expressed under the natural conditions where they evolved, compared to laboratory conditions where prey and predator were brought into closer contact. Besides, previous experiences are likely to play an important role in expressed defensive responses.

The ability to respond to background colour is an important feature of species that might benefit from background matching camouflage. Tadpole colour patterns vary and could be associated with several functions, including defense. Because tadpoles are exposed to a wide array of visually oriented predators, they represent good models to study defensive colouration and associated behaviours. We tested whether a potentially disruptively camouflaged tadpole with a dark body crossed by yellow bars (Ololygon machadoi) is able to respond differently to matching light and dark natural background colours and an artificial blue contrasting background. We used a syntopic contrasting black tadpole (Bokermannohyla martinsi) as a control, expecting it not to respond to background colour in search for camouflage. Ololygon machadoi tadpoles chose light over blue backgrounds under threat, as expected, however they did not show preferential use of dark vs. blue backgrounds. Bokermannohyla martinsi did not respond to any combination of background colours. Our results suggest that O. machadoi tadpoles are able to respond to background colour, and may favor matching backgrounds under some circumstances. The potentially disruptive colouration of O. machadoi tadpoles may increase their repertoire of escape strategies, background matching being one of the options to escape predation.

We examined the gut contents of a fossil giant pelobatid tadpole from the late Oligocene of Enspel, Germany, and discovered that it contains mainly pollen from spruce (Picea) but also pollen from pine (Pinus), beech (Fagus), and elm (Ulmus). Pollen in the gut of the fossil tadpole and other plant fossil records from this locality suggest that the regional vegetation around Enspel was characterised by mixed conifer and broadleaved forests with a prominent deciduous angiosperm component. Palaeoclimatic estimations indicate that the area endured a fully humid warm temperate climate with a hot to warm summer and a distinct temperature difference between seasons. The pollination period of potential modern analogues of the plant taxa discovered in the gut of the fossil tadpole hints that it was feeding on pollen floating on the water surface of the ancient Enspel Lake during late spring or earliest summer. Comparable analyses from the guts of extant pelobatid tadpoles have shown a broad spectrum of food remains, including whole/or parts of algae, protists, protozoans, macrophytes, rotifers, crustaceans, and pollen. It seems that living pelobatid tadpoles are indiscriminate and opportunistic feeders able to adapt and change their feeding behaviour according to the available food source. The time of origin and evolution of pollen feeding in tadpoles is obscure. Our discovery indicates that pelobatid tadpoles were already feeding on pollen in the late Oligocene, and the amount and purity of pollen filling the gut suggests intentional surface feeding. [ABSTRACT FROM AUTHOR]

The structure of many aquatic food webs is influenced by tadpole survivorship and duration of seasonal occupancy, which depend on tadpole diet. Pollen has been occasionally reported to occur in tadpole diets, and the available studies show that tadpoles have a strong preference for this supposedly nutritive resource. Thus, we aimed to test whether a diet composed of pollen from varied plant species would boost tadpole growth, development, and agility/mobility, compared to a regular artificial balanced and nutritive diet. Tadpoles in initial developmental stages 25 and 30 grew more when fed with pollen, compared to artificial food and a mixture of both diets. Those initially in stage 30 advanced more developmental stages during one month when fed exclusively with pollen. Tadpoles fed with pollen moved longer distances in response to an aversive stimulus simulating a predatory attack than tadpoles fed with artificial food or a mixed diet. Our results corroborate the hypotheses of pollen being a very important food source to aquatic consumers such as tadpoles, being likely to influence the structure of aquatic food webs through tadpole nutrition and modulation of growth/development speed, as well as tadpole ability to escape predators. [ABSTRACT FROM AUTHOR]

Background illumination and its intensity are crucial factors in visual interaction among organisms. In the present study, we used tadpoles of eight sympatric anuran species (Duttaphrynus melanostictus, Microhyla ornata, Uperodon globulosus, Kaloula taprobanica, Euphlyctis cyanophlyctis, Fejervarya orissaensis, Polypedates maculatus, and Polypedates teraiensis) to examine species-specific responses towards background illumination (shade and light), combined with four treatments [without food and predator (F−P−), with food and without predator (F+P−), without food and with predator (F−P+), and with food and with predator (F+P+)], with four coloured backgrounds (green, red, brown, and black) during different hours of a day (early morning, late morning, noon, and afternoon). Anuran tadpoles choose shaded sides over lighter sides, as such microhabitats are used as an effective means of camouflage, but most of the M. ornata tadpoles preferred the opposite. The observations were similar for red, green, and brown backgrounds simulating different natural conditions. The developmental stage and co-occurrence of tadpoles of multiple species do not affect their behavioural responses. These results draw attention to the importance of low illuminated/shaded backgrounds for behavioural responses of anuran tadpoles to environmental changes. [ABSTRACT FROM AUTHOR]

Geographical patterns of phenotypic variation are the outcome of a complex array of evolutionary and environmental factors. Studies on the correlates of the geographic variation in morphological characters can be useful to understand the drivers of phenotypic differences and because intraspecific variation in morphology can impact either local adaptation of any given species or higher-level ecosystem processes. The morphology of larval amphibians responds to multiple factors, including wetland level and surrounding land use. However, understanding the relative influence of such environmental factors on the morphology of tadpoles remains challenging. We used geometric morphometrics to describe the intraspecific morphological variation in tadpoles of Scinax squalirostris (Anura: Hylidae) in freshwater wetlands ranging along the transition between the Pampa and Atlantic Forest biomes in southern Brazil. Specifically, we assessed the relative influence of spatial and environmental (aquatic predators, water chemistry and hydroperiod, crop area) factors to tadpole body size and shape. The size of S. squalirostris tadpoles was affected by spatial factors and pH. Smaller-sized tadpoles were observed in the Pampa–Atlantic Forest transition. Allometry-free changes in the shape of tadpoles were associated both to wetland and upland factors. Tadpoles in larger ponds showed globular bodies and higher ventral fins, while tadpoles in ponds with higher electrical conductivity showed smaller tails. Tadpoles in ponds surrounded by larger crop areas showed longer fins. Our results are useful to understand the importance of wetland and upland effects on the morphology of widely distributed aquatic species, especially given the ongoing scenarios of land cover modification and climate change. [ABSTRACT FROM AUTHOR]

The evolution of defensive coloration is dependent upon a complex combination of variables related to the prey itself, the environmental context, and predator vision/behavior. Some animals reflect ultraviolet (UV) light, which is visible to predators such as birds. Here, we report the first case of UV reflectance by a tadpole (Ololygon machadoi, Hylidae) and we investigate whether it influences tadpole detection/predation by a bird (Saltator similis, Passeriformes) on backgrounds of three different colors (two blending – yellow and dark – and one not blending – blue – to tadpole colors). We also tested whether a previous experience of the birds with the prey species would influence the search/attack effort. Although many predation attempts were recorded, birds did not consume many tadpoles. Birds attempted to/preyed upon more tadpoles on dark backgrounds in the presence rather than the absence of the UV portion of the spectrum; attempt/predation on the other backgrounds (yellow, blue) did not change between the UV conditions. The differential contrast of dark and yellow tadpole body parts against specific backgrounds may aid to disruptive properties. The interruption of tadpole body contour was probably enough to prevent detection on yellow and blue backgrounds based on low achromatic contrasts of yellow and dark body parts against such backgrounds, respectively. On dark backgrounds, however, an increase in the chromatic contrast of yellow bars caused by UV reflectance may have reached a threshold and increased tadpole detectability. Birds spent more time searching for tadpoles and visited more frequently the trays in the first than in the second trial, and several tadpoles were regurgitated after ingestion, suggesting that the existence of a potential aposematic role of the UV reflectance cannot be disregarded. Birds also spent more time inspecting blue backgrounds, which may relate to their unfamiliarity with such backgrounds. UV reflectance in O. machadoi tadpoles unveals a potential adaptative value of this feature for the larval stage, and our results indicate influence on tadpole detection by birds. The effects of such adaptation in natural habitats and its occurrence in other species deserve further studies. Significance statement: Defensive colorations that hamper prey detection can play an important role on prey survival, which depends on background matching ability, prey and predator behavior, and predator visual system. Tadpoles are preyed upon by a variety of visually oriented predators and background matching is likely to aid survivorship through decreased detectability. We show for the first time that tadpoles of Ololygon machadoi, although apparently benefited by camouflage, reflect UV wavelenghts, becoming more visible and maybe providing some kind of aversiveness signaling to birds. In natural conditions, however, the water column on deep stream microhabitats usually occupied by the tadpoles is expected to filter UV wavelengths and improve background matching, at least for some specific background colors. [ABSTRACT FROM AUTHOR]

Behavioral changes constitute a common response of aquatic animals to predator chemical and/or visual cues. Tadpoles frequently use such signals to evaluate predator presence/activity and, consequently, alter their own behavior to reduce detectability and susceptibility to predation. The present study aimed to test how tadpoles of Bokermannohyla martinsi and Ololygon machadoi respond to the presence of a predator (Belostoma testaceopallidum), regarding background choice and activity levels. Results showed that O. machadoi tadpoles increased the use of yellow backgrounds in the presence of the predator perceived through either chemical or visual cues. Bokermannohyla martinsi, on the other hand, did not show preference for any specific background color or changed activity levels in the presence of any predator cue. Our results corroborate the hypothesis of disruptive coloration as a defense strategy in O. machadoi and indicate that the visible and active tadpoles of B. martinsi may employ a different defensive strategy, possibly relying on a more efficient post detection escape mechanism. This hypothesis is corroborated by the higher activity levels of B. martinsi and lack of response to predator presence regarding either modulation of activity levels or background choice. [ABSTRACT FROM AUTHOR]

Hydroperiod length has been identified as a major driver in community assembly in freshwater systems. Yet we generally lack an understanding of how morphological traits respond to water level decrease under natural conditions. Here, we studied variation in body size and shape in lateral view in tadpoles of Scinax squalirostris and Odontophrynus americanus inhabiting ponds in superhumid highland plateau in southern Brazil. Hydroperiod did not affect tadpole size in either species. In relation to body shape, S. squalirostris tadpoles from long hydroperiod ponds had shorter tails and deeper tail fins than tadpoles from other hydroperiods. Predator presence was positively associated with hydroperiod, restricting our ability to separate their effects on tadpoles morphology. For O. americanus, tadpole shape was not affected by pond hydroperiod. We demonstrated that, in natural environment, the influence of hydroperiod on tadpole morphology might depend on species-specific characteristics, such as behavior and life-history traits. Our results indicate that local context may lead to different effects of hydroperiod, and hydroperiod alone can only partly explain the variation on tadpole shape. [ABSTRACT FROM AUTHOR]

Little consensus exists on the role of food partitioning in the organization of tadpole assemblages. We studied trophic ecology of syntopic tadpoles through the analysis of gut contents, selectivity, and stable isotopes to assess the occurrence of food partitioning in tadpole assemblages. Tadpoles (n = 194) were collected in three wetlands and corresponded to four species: Elachistocleis bicolor (Eb), Scinax nasicus (Sn), Physalaemus albonotatus (Pa), and Dendropsophus sp. (D); and belonged to four ecomorphological groups (EMGs): suspension feeders (Eb), nektonic (Sn), benthic (Pa) and macrophagous (D). Sn, and Pa showed low selective diet and a wider trophic spectrum than Eb and D, which mainly consumed one or two food categories. Diet overlap was higher between Sn and Pa. Still, Sn and Pa presented some differences in the food resources consumed. Stable isotopes analysis showed that Eb, Sn, and Pa had a lower trophic position than D, explained by the high contribution of animal food oligochaete in D diet, in contrast to the importance of algae in the diet of Eb, Sn, and Pa. Diet specialization of some species, combined with the low dietary and isotopic overlap among the ecomorphological groups, suggests that trophic partitioning facilitates coexistence of syntopic tadpoles. [ABSTRACT FROM AUTHOR]

For grazing animals that share spatial and alimentary resources, the relationship between differences in oral morphology and the physical properties of substrates can help explain how and where species forage. The substrates may require different efforts from animals to access and remove food from their surfaces. Variation in oral morphology may produce differences in animals' feeding efficiency. We tested whether one substrate characteristic, the surface texture (i.e., smooth, striated and rough), influences the growth and food consumption rates of anuran larvae from nine species with different oral morphologies. Tadpoles with few keratinized oral structures and those with more gaps in the marginal papillae row were more efficient grazing on smooth and rough surfaces, respectively. This may indicate possible feeding specializations. Conversely, tadpoles with a high number of labial tooth rows, regardless of the number of gaps in these structures, and those with only a dorsal gap in the marginal papillae row were equally efficient feeding upon all substrate textures. Tadpoles with the generalized labial tooth row formula 2(2)/3(1), had higher growth rates than the other species, suggesting an adaptive significance for this common oral morphology. We demonstrated that species differ in feeding efficiency when grazing on substrates with different textures. This can help elucidate the adaptive significance of variation in tadpole oral morphology. We also provide insights on resource selection and niche partitioning among species, especially for those whose diets do not differ in quantity or quality, as it is common in anuran larval assemblages. [ABSTRACT FROM AUTHOR]

Background matching and disruptive coloration are common strategies used by animals to increase concealment, whereas motion-dazzle may prevent capture after recognition. Studies have related background matching to habitat dependency and survival success, whereas for animals with highly contrasting patterns it has been shown that they are able to explore a broader range of habitats due to disruptive coloration, and possibly via motion-dazzle. However, the effects of these strategies are likely to be influenced by body size and to work better for smaller species. We applied phylogenetic comparative methods to test the hypothesis that smaller snapping shrimps (genus Alpheus) with high-contrast stripes would be able to utilize more microhabitats than non-striped and larger species. We used a published phylogeny of the American species of Alpheus , studies that have described alpheid microhabitats and size, and high-resolution photographs of each species in the phylogeny. Our categorical analysis suggested that generalist snapping shrimps are more likely to have stripes than specialist shrimps, and this effect was stronger in smaller species. Similarly, we found an interacting effect of body size and habitat use on the degree of luminance contrast: smaller generalist species had higher contrast values than average-sized and habitat-specialist species. Therefore, predators, body size and frequency of microhabitats are likely to have influenced the evolution of colour patterns in Alpheus. [ABSTRACT FROM AUTHOR]

Amphibians are sensitive to anthropogenic habitat alterations but also respond to natural drivers of assemblage composition at many levels. Additionally, they are usually hard to detect in field inventories. We used a multiscale approach, from microhabitat to the landscape levels, to try to understand the effects of natural changes, and try to distinguish them from the effects of landscape level anthropogenic changes, on dynamic and diverse anuran assemblages, taking imperfect detection into account. We conducted thorough field inventories in 16 streams at Serra do Cipó, in the southern portion of Espinhaço Mountain Range, southeastern Brazil, during two time periods separated by 16 years. We compared species richness and diversity between periods, sampling both tadpoles and adult frogs. We quantified tadpole microhabitat availability, alterations in immediate riparian vegetation, and changes in classes of land cover within buffers around streams (adult habitats) to test for their effects on species composition. We also tested for effects of human occupancy around streams on nestedness and turnover components of species diversity. Microhabitats and riparian vegetation explained some of the changes in species composition (or detection) between time periods. Nestedness seemed to be influenced by the stability of the landscape. Detectabilities were too low to support robust occupancy estimates for most species. Natural changes in local habitats occupied by anurans in montane meadows are likely to influence species distribution. Some species with robust estimates experienced change in their occupancy over the studied 16-year interval, although no anthropogenic causes could be directly associated with such changes. The low detectability of most species, even with thorough sampling effort, makes it very hard to detect amphibian declines and possibly tell them apart from natural population fluctuations. New techniques are needed that improve species detectability in such diverse tropical habitats. [ABSTRACT FROM AUTHOR]

Tadpoles are important components of food webs at aquatic ecosystems; however, their trophic roles remain understudied compared to fish. Morphology indicates general aspects of tadpole diets (e.g., food size), but a fine association between preferred microhabitats and diet composition remained to be tested. We tested whether syntopic tadpoles differed in microhabitat use and in diet composition according to food availability in different microhabitats. We also tested whether types of microhabitats were consistent in food availability or if food availability would vary among streams. Considering volumes of different food groups ingested, the diets of tadpoles could be partly explained by the microhabitats where they were sampled, but not by streams or even species. Considering number of items of each food group, the diet of tadpoles varied according to stream, microhabitat, and species. Volumes of items of each food group differed among microhabitats, but not among streams. All the studied species could be classified as primary consumers based on the dominance of algae (especially Zygnematophyceae and Bacillariophyceae) in their diets. Patterns of food consumption of tadpoles were complex and responded to variables at a very fine spatial scale, suggesting that they may be sensitive even to small alterations in stream dynamics. [ABSTRACT FROM AUTHOR]