Temporal stability in color patterns and body size variation in a six-year monitored population of the aposematic toad (Melanophryniscus rubriventris)

Abstract

Color polymorphisms in poison frogs have intrigued evolutionary ecologists, as understanding the mechanisms that sustain them is of great interest. Temporal variation in selection is considered crucial for maintaining color polymorphism, and monitoring aposematic signals provides valuable insights into such temporal dynamics. We conducted a six-year study on red-belly toads, Melanophryniscus rubriventris, analyzing color patterns and body size variation across five reproductive seasons. Our findings show stable color morph ratios over time, with no discernible differences between sexes and no interdependence between dorsal and ventral patterns. Notably, darker morphs, mostly melanic, comprised only about 25% of the population, contrasting with the higher frequency of conspicuous aposematic combinations. Intermediate morphs were more prevalent, while the most extreme conspicuous dorsal pattern was rare. Regarding body size, females were significantly larger and in better body condition than males. We observed a significant decrease in snout-vent length for males between 2021 and 2022, while females showed a significant increase between 2020 and 2022. Interestingly, there was a phenotypic association between color patterns and body size, with males exhibiting black bellies having smaller snout-vent lengths compared to those with other ventral patterns, particularly red bellies. Our study suggests that dorsal and ventral color patterns may respond differently to selection. The stability of color patterns within the population could be upheld through complex interactions involving stabilizing and sexual selection. We emphasize the importance of long-term studies and recommend that future research considers reproductive ecology and individual behavioral decisions in this polymorphic aposematic species. By incorporating these aspects, we can gain a more comprehensive understanding of the mechanisms driving phenotypic variation.