Susanne Stückler

Many species reproduce by participating in explosive breeding events, where hundreds of individuals converge at specific locations for a short time period to mate. During these spectacular aggregations, many males must actively search and compete for a limited number of females. One idea is that sexual selection in explosive breeding species favors the evolution of communication strategies facilitating easy detection of males and females. As such, the emergence of dynamic sexual dichromatism, a reversible change of body coloration of one sex, might act as a visual signal promoting rapid mate recognition in dense breeding aggregations. We tested this idea in Asian Common Toads (Duttaphrynus melanostictus), an explosive breeder in which males quickly turn from brown to bright yellow for up to two days during mating. Thus, we first explored whether color differences of brown and yellow are sufficient to be perceived and distinguished by toads using a color vision model. In a set of behavioral experiments, we then tested the interactions of male toads with a yellow (resembling a breeding male) and a brown (resembling a female) 3D model toad to determine the functional significance of yellow male coloration during mating. Finally, we compared male body color, morphological parameters, and body condition of mated and non-mated males to investigate if these traits are influenced by securing a mate or female mate choice. Our results show that dynamic yellow coloration displayed by males can be easily discriminated against brown female conspecifics from a toad's point of view. During model experiments, males had twice as much physical contact with brown models and clasped them 40 times more often than yellow models. Coloration and several morphological traits did not differ between mated and non-mated males; hence, securing a mating partner was not influenced by variations in color or morphology. Our study supports the idea that dynamic sexual dichromatism repeatedly evolved as a visual signal mediating fast mate recognition in tropical and temperate anuran species solving a common problem in time-limited, dense breeding aggregations, by minimizing same-sex harassment and mismatch.

In Lower Austria’s Waldviertel region, artificial drainage ditches were constructed in the last century in order to use peatlands for forestry, agriculture, and peat extraction. By now, many of these peatlands are part of the Natura 2000 network and therefore gradually undergo restoration measures, which aim to rewet the peatlands. For this, the ditches are blocked with wooden dams, leading to a water runback, which in turn fills the ditches and peat pools. Such artificial water bodies generally depict secondary habitats for many species. Here, we investigated the amphibian fauna in four restored peatlands in the Waldviertel region and measured abiotic factors of the aquatic habitats to answer the question whether blocked ditches and peat pools are valuable secondary habitats for amphibians. We characterized the microhabitats of amphibians based on various structures and vegetation. Additionally, this study provides a basic assessment of reptile species in the investigated peatlands. During our assessment, we observed 1520 individuals of eight amphibian species, 64 individuals of four reptile species, and characterized 12 different microhabitats. Despite the low pH values of 3.2–4.2, four amphibian species and amphibian spawn were detected in Schwarzes Moos. Our results indicate that peat pools, drainage ditches, and open moorlands are potential habitats for amphibians and reptiles, making their conservation and management an important factor in the protection of amphibian and reptile species.

A diversity of defence colourations that shift over time provides protection against natural enemies. Adaptations for camouflage depend on an organism’s interactions with the natural environment (predators, habitat), which can change ontogenetically. Wallace’s flying frogs (Rhacophorus nigropalmatus) are cryptic emerald green in their adult life stage, but juveniles are bright red and develop white spots on their back 1 month after metamorphosis. This latter conspicuous visual appearance might function as antipredator strategy, where frogs masquerade as bird or bat droppings so that predators misidentified them as inedible objects. To test this idea, we created different paraffin wax frog models—red with white spots, red without white spots, green, and unpainted—and placed them in equal numbers within a > 800 m² rainforest house at the Vienna Zoo. This environment closely resembles the Bornean rainforest and includes several free-living avian predators of frogs. We observed an overall hit rate of 15.5%. A visual model showed that the contrast of red, green and control models against the background colouration could be discriminated by avian predators, whereas green models had less chromatic difference than red morphs. The attack rate was significantly greater for red but was reduced by half when red models had white spots. The data therefore supports the hypothesis that the juvenile colouration likely acts as a masquerade strategy, disguising frogs as animal droppings which provides similar protection as the cryptic green adult colour. We discuss the ontogenetic colour change as a possible antipredator strategy in relation to the different habitats used at different life stages. Significance statement: Predation pressure and the evolution of antipredator strategies site at the cornerstone of animal-behaviour research. Effective antipredator strategies can change in response to different habitats that animals use during different life stages. We study ontogenetic shifts in colour change as dynamic antipredator strategy in juvenile and adult Wallace’s flying frogs. We show that the unusual colour pattern of juveniles (bright red with small white spots) likely functions as a masquerade of animal droppings. Specifically, we show that white dotting, which can be associated with animal faeces, acts as the main visual feature that turns an otherwise highly conspicuous individual into a surprisingly camouflaged one. To our knowledge, this is the first experimental exploration of a vertebrate masquerading as animal droppings.

Many animals communicate by rapidly (within minutes or seconds) changing their body coloration; however, we know little about the physiology of this behaviour. Here we study how catecholaminergic hormones regulate rapid colour change in explosive breeding toads (Duttaphrynus melanostictus), where large groups of males gather and quickly change their colour from brown to bright yellow during reproduction. We find that both epinephrine (EP) and/or norepinephrine (NE) cause the toads' skin to become yellow in minutes, even in the absence of social and environmental cues associated with explosive breeding. We hypothesize that natural selection drives the evolution of rapid colour change by co-opting the functional effects of catecholaminergic action. If so, then hormones involved in ‘fight or flight’ responses may mechanistically facilitate the emergence of dynamic visual signals that mediate communication in a sexual context.