In nature, tadpoles encounter food on substrates oriented at different angles (e.g. vertically along stems, horizontally on the bottom). We manipulated the orientation of food-covered surfaces to test how different orientations of surfaces affect tadpoles’ feeding efficiency. We studied taxa that differed in the oral morphology of their larvae and position in the water column. We hypothesized that species would differ in their ability to graze upon surfaces at different orientations and that differences in the tadpoles’ feeding ability would result in different growth rates for the species. The orientation of food-covered surfaces did not affect the growth rate of species with bottom-dwelling tadpoles (whose growth rate varied only between species). Among species with midwater tadpoles, some appear to have a generalist strategy and experienced a high relative growth rate on numerous substrate orientations whereas others achieved high growth rates only on flat substrates (i.e. at 0° and 180°). We conclude that oral morphology constrains tadpoles’ ability to feed at different substrate orientations, and this could lead to niche partitioning in structurally complex aquatic environments. Because physical parameters of the environment can affect tadpoles’ growth rate, characterizing these features might help us better understanding how competition structures tadpole assemblages.
1 When studying speciation, researchers commonly examine reproductive isolation in recently diverged populations. Polymorphic species provide an opportunity to examine the role of reproductive isolation in populations that may be in the process of divergence. 2 We examined a polymorphic population of Plethodon cinereus (red-backed salamanders) for evidence of sympatric ecological separation by colour morphology. Recent studies have correlated temperature and climate with colour morphology in this species, but no studies have looked at differences in diet or mate choice between colour morphs. We used artificial cover objects to assess salamander diet, mating preference and surface activity over a 2-year period at a field site in north-eastern Ohio. 3 We detected differences in diet between two colour morphs, striped and unstriped. The diets of striped individuals were significantly more diverse and were made up of more profitable prey than the diets of unstriped salamanders. 4 Opposite sex pairs were made up of individuals of the same colour morph and striped males were found more often with larger females than were unstriped males. 5 We corroborate findings of earlier studies suggesting that the unstriped form is adapted to warmer conditions. Unstriped individuals were the first to withdraw from the forest floor as temperatures fell in the late fall. We found no evidence that the colour morphs responded differently to abiotic factors such as soil moisture and relative humidity, and responses to surface temperatures were also equivocal. 6 We conclude that the two colour morphs exhibit some degree of ecological separation and tend to mate assortatively, but are unlikely to be undergoing divergence given the observed frequency of intermorph pairings.
Abstract L eptodactylus labyrinthicus tadpoles are known predators of anuran eggs and hatchlings, but they are also able to filter‐feed in the water column and scrape food off of firm substrates. We evaluated and compared these alternative feeding behaviors in relation to feeding kinematics and the shape of the mouth with high‐speed digital imaging. We tested the hypotheses that (1) L . labyrinthicus tadpoles use functionally different feeding kinematics when feeding on alternative food sources and (2) that the jaw sheaths of L . labyrinthicus tadpoles deform less during filter‐feeding and substrate grazing compared with more common tadpoles not so specialized for macrophagous carnivory. Our results show that filtering and scraping feeding behaviors differ significantly in both kinematics and shape of the mouth. During filter‐feeding, tadpoles display longer gape cycles and attain a narrower maximum gape earlier in the cycle compared with substrate grazing. Jaw deformation during opening and closing phases of the gape cycle is more pronounced during grazing on firm substrates. This deformation contributes to the achievement of a wider maximum gape during feeding. These differences appear to reflect behavioral adjustments by the tadpoles to maximize food intake. Feeding in tadpoles of L . labyrinthicus is not restrained by their typical carnivorous morphology. On the contrary, L . labyrinthicus tadpoles seem to be opportunistic feeders able to obtain nutrients from a variety of food sources by using different feeding strategies.
Fever is an evolutionarily conserved and effective organismal response used to reduce infection burden. Although ectotherms are unable to induce a fever endogenously because they only produce negligible heat from their metabolism, they can increase their body temperature in response to infection by selecting warm microclimates ("behavioral fever"). For amphibians, behavioral fever is hypothesized to be critical in their defense against pathogens because many amphibian immune parameters are more effective at warm temperatures. We explored this topic using the pathogenic amphibian chytrid fungus (Batrachochytrium dendrobatidis; "Bd"), a fungal pathogen implicated in the worldwide amphibian declines. Behavioral fever in amphibians should be an effective defense against Bd because fungal growth is temperature dependent and does not survive above 30°C. We conducted a laboratory experiment in which we allowed Bd-infected and sham inoculated metamorphic American toads (Anaxyrus americanus) to behaviorally thermoregulate. During the experiment, we recorded the body temperature of each toad and also determined their Bd infection intensity. Bd-infected toads behaviorally increased their body temperatures through the course of an infection compared to sham inoculated toads, which did not change their body temperatures. In addition, toads with higher initial Bd infection intensities (measured prior to the start of the thermoregulation experiment) had higher average body temperatures compared to the body temperature of toads with lower infection intensities. Collectively, these findings build upon our understanding of the relationship between temperature and disease in the amphibian-Bd system and offer hope that amphibians might be able to induce behavioral fevers to clear their Bd infections in nature.
Climate change might drive species declines by altering species interactions, such as host-parasite interactions. However, few studies have combined experiments, field data, and historical climate records to provide evidence that an interaction between climate change and disease caused any host declines. A recently proposed hypothesis, the thermal mismatch hypothesis, could identify host species that are vulnerable to disease under climate change because it predicts that cool- and warm-adapted hosts should be vulnerable to disease at unusually warm and cool temperatures, respectively. Here, we conduct experiments on Atelopus zeteki, a critically endangered, captively bred frog that prefers relatively cool temperatures, and show that frogs have high pathogen loads and high mortality rates only when exposed to a combination of the pathogenic chytrid fungus (Batrachochytrium dendrobatidis) and high temperatures, as predicted by the thermal mismatch hypothesis. Further, we tested various hypotheses to explain recent declines experienced by species in the amphibian genus Atelopus that are thought to be associated with B. dendrobatidis and reveal that these declines are best explained by the thermal mismatch hypothesis. As in our experiments, only the combination of rapid increases in temperature and infectious disease could account for the patterns of declines, especially in species adapted to relatively cool environments. After combining experiments on declining hosts with spatiotemporal patterns in the field, our findings are consistent with the hypothesis that widespread species declines, including possible extinctions, have been driven by an interaction between increasing temperatures and infectious disease. Moreover, our findings suggest that hosts adapted to relatively cool conditions will be most vulnerable to the combination of increases in mean temperature and emerging infectious diseases.
Differences in host behavior and resistance to disease can influence the outcome of host-pathogen interactions. We capitalized on the variation in aggregation behavior of Fowler's toads (Anaxyrus [ = Bufo] fowleri) and grey treefrogs (Hyla versicolor) tadpoles and tested for differences in transmission of Batrachochytrium dendrobatidis (Bd) and host-specific fitness consequences (i.e., life history traits that imply fitness) of infection in single-species amphibian mesocosms. On average, A. fowleri mesocosms supported higher Bd prevalences and infection intensities relative to H. versicolor mesocosms. Higher Bd prevalence in A. fowleri mesocosms may result, in part, from higher intraspecific transmission due to the aggregation of tadpoles raised in Bd treatments. We also found that, independent of species, tadpoles raised in the presence of Bd were smaller and less developed than tadpoles raised in disease-free conditions. Our results indicate that aggregation behavior might increase Bd prevalence and that A. fowleri tadpoles carry heavier infections relative to H. versicolor tadpoles. However, our results demonstrate that Bd appears to negatively impact larval growth and developmental rates of A. fowleri and H. versicolor similarly, even in the absence of high Bd prevalence.
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