Bigger mother toads are not better for tadpole survival

As the world's climate continues to change, many frog, toad and salamander species worldwide are facing extinction. To help preserve these animals, researchers and conservationists have heroically turned to breeding them in captivity and releasing them as adults back into the wild. Unfortunately, many of these programs are having a hard time getting their amphibians to mate and, of those animals that do, generally few lay eggs and a relatively low number of the young survive. These breeding programs go above and beyond to provide the animals with ideal breeding and rearing conditions, yet the number of amphibians being released back into the wild is often well below the goal. In an attempt to increase amphibian survival odds, many of these programs keep their animals well fed, working under the assumption that bigger, heavier animals lay more eggs, which eventually hatch and grow into hardier adults that are better able to survive in challenging environments. However, Emily Harmon, Tianxiu Li, Patrick Kelly, Catherine Chen, David Pfennig and Karin Pfennig at the University of North Carolina at Chapel Hill, USA, have begun to suspect that this feeding practice is doing more harm than good, suggesting that obesity may be contributing to the low egg-laying rate and high mortality of these breeding programs.Using data from three separate experiments collected across 3 years (2019–2021) on the Mexican spadefoot toad, Spea multiplicata, Harmon and colleagues analyzed how the relationship between body mass and length of overfed parent toads affects tadpole survival. Before 2019, the common practice in the Pfenning lab was to feed each toad one to two crickets a week. But beginning in 2019, toads were fed two extra crickets on alternating weeks. Although there were slight differences between the three experiments, each involved the researchers pairing a female and a male toad from the same population and recording the hatching success and tadpole survival of the resulting eggs. Using a calculation to compare the relationship between body mass and size, the researchers scored the body condition of the toads in each pairing. Those that had a ‘high’ score had greater mass for their size, and those that had a ‘low’ score were much leaner. The scores from these three experiments were then compared with the scores from wild toads caught in Arizona, USA, over 21 years (1998–2019), which typically had good tadpole survival rates. This allowed Harmon and the team to compare how overfeeding the adults affected the ability of tadpoles to hatch and survive.The researchers found that although tadpoles born to males with more mass had a much higher chance of survival, the more mass the females carried decreased offspring survival across all three experimental years. The researchers also found that the length of time a female was kept in the lab prior to breeding experiments also decreased the chance of tadpole survival. Although Harmon and colleagues were unable to definitively say whether the decrease in tadpole survival is due to overfeeding or obesity, their data suggest that the extra mass these females are carrying and how long they have been in the lab strongly contribute to the struggle that captive breeding programs are having to produce enough adult amphibians to stabilize populations in the wild.As climate change continues to cause worldwide amphibian declines, captive breeding programs will play an integral role in preserving surviving species. Many amphibians are built to take advantage of short periods of food availability and thus have evolved to quickly turn food into body mass. The findings of Harmon and colleagues emphasize the need to ensure that the breeding animals in these programs are in the best condition, and show that bigger animals are not always better breeders. Understanding the ecological and nutritional needs of amphibians will be crucial to ensuring the survival of these threatened animals in a changing world.