A Mexican spadefoot toad at El Morro National Monument, USA. Photo credit: US National Park Service via Wikimedia Commons. This image is in the public domain.
Every year, you'll find millions of tadpoles in the short-lived ponds caused by heavy rains. These tadpoles need to grow fast to make sure they escape from their evaporating habitat before the water dries up in the summer. However, growing up fast can come with its own set of problems. Tadpoles need lots of energy to grow quickly, meaning they need to eat more calories. Generally, the tadpoles of Mexican spadefoot toads (Spea multiplicata) eat decaying plant and animal matter called detritus. However, some lucky tadpoles hatch in ponds that are also home to tiny shrimp. While these protein-packed treats could help the tadpoles grow faster, they could also cause changes to the tadpoles after they've left the water behind. Alex Shephard, Sydney Jacobsen and Cristina C. Ledón-Rettig of Indiana University, USA, wanted to see whether a change in diet of the tadpoles might change the behavior and hormone levels of the toads when they emerge onto land as juveniles.
Shephard and the team collected 13 pairs of mating toads from two small ponds in the San Simon Valley, Arizona. After the toads had laid eggs, the researchers returned the parents to the ponds, and raised the eggs until the tadpoles hatched. Once the tadpoles were out of their eggs, the team fed some only their normal detritus diet, but other tadpoles had live shrimp as well. The toad tadpoles that had shrimp mixed in with their diet turned into young toads ∼10 days faster than those that ate detritus alone. But did this different diet also lead to changes in the young toads?
Normally, toads that develop faster are bolder as juveniles. To test whether this trend continued when the baby toads had a different diet, the scientists placed each toad in the center of a small chamber that contained 15 crickets – one of their favorite snacks – and recorded how long it took the toad to move, how many times it tried to eat a cricket and how much of the chamber it explored over the course of 10 min. The toads that grew fastest while just eating decaying matter moved sooner, tried to eat more crickets and explored more of the chamber than the ones that grew slower. Interestingly, toads that grew the quickest on the shrimp diet started moving sooner but didn't attack more crickets or explore more than the slower growing ones. So, their different diet as tadpoles changed the usual link between growing fast and being bold – would it do the same to other aspects of the toads?
Shephard next measured how much of the stress hormone corticosterone the toads made normally compared with when they were slightly more stressed. Toads that grew fastest on the usual diet had a bigger jump in their levels of stress hormone than those that grew more slowly, but the toads that grew the fastest on the shrimp diet weren't any different from the ones that grew the slowest. Maybe the toad's telomeres would tell a different story.
The length of a telomere, which are the regions of DNA at the end of chromosomes, generally indicates how well an animal can maintain its cells. When an animal grows quickly, telomeres can get shorter, but that didn't happen in these toads. In fact, the faster growing detritus-eating toads had longer telomeres than the slower growing ones. Shephard suggests that the faster growers might be generally healthier, allowing them to devote their energy to both growing faster and keeping their telomeres longer. Whatever the reason, a simple change in diet as a tadpole can make a slow growing spadefoot toad behave and react just like the rest.
