Phenotypic Plasticity in Juvenile Frogs That Experienced Predation Pressure as Tadpoles Does Not Alter Their Locomotory Performance

Anuran species can respond to environmental changes via phenotypic plasticity, which can also result in ecological impacts across the life history of such species. We investigated the effects of predation pressure (i.e., the non-consumption effect) from the dragonfly larva (Anax parthenope) on the phenotypical change of tadpoles into juvenile frogs (specifically the black-spotted pond frog, Pelophylax nigromaculatus), and also analyzed the impact of morphological changes on locomotory performance after metamorphosis. The experiments on predator impact were conducted in the laboratory. Body length, weight, development timing, and metamorphosis timing in the presence of dragonfly nymphs were measured in both tadpoles and juvenile frogs. The body and tail shapes of the tadpoles, as well as the skeletal shape of the juvenile frogs, were analyzed using landmark-based geometric morphometrics. Furthermore, the locomotory performance of the juvenile frogs was tested by measuring their jumping and swimming speeds. Tadpoles that had grown with predators possessed smaller bodies, deeper tail fins, and slower development rates, and they waited longer periods of time before commencing metamorphosis. Having said this, however, the effect of predator cues on the body length and weight of juvenile frogs was not found to be significant. These juvenile frogs possessed longer limbs and narrower skulls, with subtle morphological changes in the pelvis and ilium, but there was no subsequent difference in their swimming and jumping speeds. Our results showed that the changes in anatomical traits that can affect locomotor performance are so subtle that they do not affect the jumping or swimming speeds. Therefore, we support the view that these morphological changes are thus by-products of an altered tadpole period, rather than an adaptive response to predator-escape ability or to post-metamorphosis life history. On the other hand, delayed metamorphosis, without an increase in body size, may still be disadvantageous to the reproduction, growth, and survival of frogs in their life history following metamorphosis.

Keystone nonconsumptive effects within a diverse predator community

The number of prey killed by diverse predator communities is determined by complementarity and interference among predators, and by traits of particular predator species. However, it is less clear how predators' nonconsumptive effects (NCEs) scale with increasing predator biodiversity. We examined NCEs exerted on Culex mosquitoes by a diverse community of aquatic predators. In the field, mosquito larvae co-occurred with differing densities and species compositions of mesopredator insects; top predator dragonfly naiads were present in roughly half of surveyed water bodies. We reproduced these predator community features in artificial ponds, exposing mosquito larvae to predator cues and measuring resulting effects on mosquito traits throughout development. Nonconsumptive effects of various combinations of mesopredator species reduced the survival of mosquito larvae to pupation, and reduced the size and longevity of adult mosquitoes that later emerged from the water. Intriguingly, adding single dragonfly naiads to ponds restored survivorship of larval mosquitoes to levels seen in the absence of predators, and further decreased adult mosquito longevity compared with mosquitoes emerging from mesopredator treatments. Behavioral observations revealed that mosquito larvae regularly deployed "diving" escape behavior in the presence of the mesopredators, but not when a dragonfly naiad was also present. This suggests that dragonflies may have relaxed NCEs of the mesopredators by causing mosquitoes to abandon energetically costly diving. Our study demonstrates that adding one individual of a functionally unique species can substantially alter community-wide NCEs of predators on prey. For pathogen vectors like mosquitoes, this could in turn influence disease dynamics.

Plastic hatching timing by red-eyed treefrog embryos interacts with larval predator identity and sublethal predation to affect prey morphology but not performance

Many animals respond to predation risk by altering their morphology, behavior, or life-history. We know a great deal about the cues prey respond to and the changes to prey that can be induced by predation risk, but less is known about how plastic responses to predators may be affected by separate plastic responses occurring earlier in life, particularly during the embryonic period. Embryos of a broad array of taxa can respond to egg- or larval-stage risks by altering hatching timing, which may alter the way organisms respond to future predators. Using the red-eyed treefrog (Agalychnis callidryas), a model for understanding the effects of plasticity across life-stages, we assessed how the combined effects of induced variation in the timing of embryo hatching and variation in the larval predator community impacted tadpole morphology, pigmentation and swimming performance. We found that A. callidryas tadpoles developed deeper tail muscles and fins and darker pigmentation in response to fish predators, either when alone or in diverse community with other predators. Tadpoles altered morphology much less so to dragonfly naiads or water bugs. Interestingly, morphological responses to predators were also affected by induced differences in hatching age, with early and late-hatched tadpoles exhibiting different allometric relationships between tail height and body length in different predator environments. Beyond induced morphological changes, fish predators often damaged tadpoles' tails without killing them (i.e., sublethal predation), but these tadpoles swam equally quickly to those with fully intact tails. This was due to the fact that tadpoles with more damaged tails increased tail beats to achieve equal swimming speed. This study demonstrates that plastic phenotypic responses to predation risk can be influenced by a complex combination of responses to both the embryo and larval environments, but also that prey performance can be highly resilient to sublethal predation.

Ecological immunogenetics of life-history traits in a model amphibian

Major histocompatibility complex (MHC) genes determine immune repertoires and social preferences of vertebrates. Immunological regulation of microbial assemblages associated with individuals influences their sociality, and should also affect their life-history traits. We exposed Xenopus laevis tadpoles to water conditioned by adult conspecifics. Then, we analysed tadpole growth, development and survivorship as a function of MHC class I and class II peptide-binding region amino acid sequence similarities between tadpoles and frogs that conditioned the water to which they were exposed. Tadpoles approached metamorphosis earlier and suffered greater mortality when exposed to immunogenetically dissimilar frogs. The results suggest that developmental regulatory cues, microbial assemblages or both are specific to MHC genotypes. Tadpoles may associate with conspecifics with which they share microbiota to which their genotypes are well adapted.

Intraspecific variation of temperature-induced effects on metamorphosis in the pool frog (Rana lessonae)

Understanding the factors that affect the process of metamorphosis in species with complex life cycles, and in particular their variation within and among populations, has been rarely explored until recently. We examined the effects of temperature environment on several metamorphic characteristics in three populations of the pool frog ( Rana lessonae Camerano, 1882) by rearing individuals at two temperature environments (20 and 25 °C). Higher temperature shortened the metamorphic period and reduced the absolute mass loss, although there was no difference between the temperatures in the percentage of mass lost. No differences among the populations were detected, but there was significant intrapopulation variation both in the mean and in the plasticity for the duration of metamorphosis. These results indicate that several aspects of metamorphosis are plastic in amphibians, these traits may have considerable intrapopulation variation, and that temperature is a strong factor affecting the process of metamorphosis.