The Indus Valley bullfrog (Hoplobatrachus tigerinus) and the Asian common toad Duttaphrynus melanostictus) are widely distributed on the Indian subcontinent. Their mating behavior is spectacular: during the reproductive phase the males change their color from an inconspicuous brown to bright yellow. Despite this flashy behavior, little is known about the biology of these animals.
This research is designed to examine the mating behavior of the animals in more detail.
As so-called explosive breeders, all the individuals in the population arrive at the spawning waters almost synchronously to reproduce. The first results show that the reproductive event and the concurrent color change of the males start in early June after the first intense monsoon storms. Within one day, hundreds of sexually mature animals aggregate in waterbodies created by the strong rainfall. The males typically outnumber the females, which they actively seek out and immediately embrace as soon as they are discovered. The whole show is over in one or two days, leaving the spawn deposited in the water. Thereafter, no yellow frog or toad can be spotted far and wide.
Another aim is to determine the potential functions of the conspicuous mating coloration.
The temporary and reversible color change is presumably a visual signal that serves in inter- or intra-sexual communication. Several different explanatory models need to be investigated.
The yellow color could signal the quality of the male and help the female select suitable reproductive partners. In the dense aggregations, it could also serve as a distinguishing feature between the two sexes and therefore promote efficient partner selection.
In the case of the Indus Valley bullfrog, the yellow body color could also strengthen the contrast to the blue vocal sac, which when inflated or pulsating might help the animals to orient themselves or be a deciding factor in partner selection.
In cooperation with Susanne Stückler, a PhD candidate at the University of Vienna, spectral measurements and physiological experiments are being conducted both at the zoo and in India to answer questions regarding such a potential signaling role and provide insights into their behavioral biology in general.
Behavior experiments with variously colored frog models are expected to show how the body coloration influences partner choice and confrontations between the males.
Asian common toads have long been bred in the zoo’s Rainforest House. These animals are being used to help determine what controls the rapid color change in the males.
This project is supported by the Austrian Academy of Sciences (DOC25701) and the Society of the Friends of Schönbrunn Zoo.
Susanne Stückler, Samantha Cloer, Walter Hödl, Doris Preininger (2022): Carotenoid intake during early life mediates ontogenetic colour shifts and dynamic colour change during adulthood, Animal Behaviour, Volume 187, 121-135, ISSN 0003-3472, https://doi.org/10.1016/j.anbehav.2022.03.007