No abstract

Tadpole diet is likely to vary in response to environmental conditions and nutritional needs throughout growth and development. We investigated seasonal variation in diet composition of Bokermannohyla saxicola tadpoles and compared diets between two developmental stages with a significant difference in size. We found that the diet of B. saxicola tadpoles was dominated by periphytic algae, in accordance with their benthic habits. Considering number of cells ingested, tadpole trophic niches were broader in more advanced developmental stages. Tadpole trophic niches were narrower during the summer (wet season) than during the winter (dry season), which may reflect increased consumption of more energetic food items during the warm period when primary productivity is expected to be higher. Tadpole metabolism is likely to be higher in the summer and increased energetic needs might be supplied in this manner. However, results differed when biovolume was considered instead of number of cells ingested, with larger items assuming a greater importance and niches being usually larger in the summer. In these cases, the increased ingestion of diatoms (likely to be more nutritive) in the summer may decrease the relative importance of large algae (e.g., Mougeotia sp.) that form the bulk of the diet. Both food availability/accessibility and tadpole feeding behaviour driven by nutritional needs may influence patterns of food acquisition. Given the importance of biofilms to tadpole diet, studies on the mechanisms by which tadpole nutritional needs and environmental conditions interact are likely to provide important insights into the dynamics of aquatic food webs.

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.

Tadpoles are important components of many aquatic food webs as they influence populations at other trophic levels and, ultimately, energy transfer between aquatic and terrestrial ecosystems. What tadpoles consume is a key factor to their ecological functions. Neotropical anuran assemblages include many species with a larval stage, however, the diets of neotropical tadpoles are relatively poorly known. We performed a systematic review on the available literature on in situ studies of neotropical tadpole diets and found out that most studies are descriptive and temporally restricted, failing to consider seasonal variations. Studies frequently also fail to consider possible ontogenetic dietary variations or tadpoles' capacity to select food based on what is available to them in their habitats. We contributed to this topic by investigating seasonal variations and comparing the diets of two developmental stages of Scinax curicica tadpoles. Tadpole diet varied between dry and rainy seasons, but not between the studied developmental stages. These results, together with published studies, suggest that multiple ecological/physiological factors are likely to influence tadpole food intake and should not be overlooked. Studies on neotropical tadpole feeding ecology, digestion capacity, physiological effects of different diets and microbiome versus diet interactions are highly desirable but largely unexplored. Such studies are likely to add valuable information to understand tadpole roles in aquatic food webs, their growth and development performance resulting from food acquisition and assimilation, and consequently their effects on other species in aquatic and terrestrial ecosystems.

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. Prey species are under constant selection to escape predation. They can develop predator specific responses, but some responses seem to be widespread, such as spatial avoidance of predators and reduction of activity 1. In this context prey species that count on camouflage to reduce the probability of being detected by predators tend to remain motionless, aiding to the protective properties of their colouration 2,3. Reduction of movement is likely to reduce detection probability and can emerge as a spontaneous response when the detected threat is at a distance (background threat), but an approaching (immediate) threat may otherwise elicit an escape response 4. Previous experience can also determine the reaction of prey to specific predators 5 , however there is evidence that evolutionary pressures may have led prey species to develop different defensive mechanisms and express each one in appropriate contexts determined by the assessed threat 4. Both innate and learned defensive behaviours can be expressed even in very early life stages 6,7. Tadpoles, the larvae of biphasic anuran amphibians, are frequently employed in experiments conducted to study the reaction of prey to predator cues 8 , due to their broad range of predators and defensive mechanisms 1. It has been shown that many species of tadpoles reduce movement in the presence of predators 1,9. However, these studies are usually conducted under laboratory or mesocosm conditions, sometimes within reduced spaces in artificial standardized containers where predator cues are manipulated, other factors kept equal, to record tadpole reaction to particular cues 1,10. Under such circumstances, both visual and chemical stimuli have been