Mexican spadefoot toad embryos that sense lots of shrimp in the water before they are born grow bigger and stronger jaws, ready for their first meals after hatching

Some tadpoles can respond to their environment even before birth, hatching with stronger jaws if they sense prey in the water while still embryos.

The Mexican spadefoot toad (Spea multiplicata) is a species of small, round frog usually found in arid desert environments. As tadpoles, the frogs cope with unpredictable climate conditions by adjusting their diets. Some grow up eating a little bit of everything. Others develop into specialised carnivores, even snacking on other tadpoles – this lets them grow faster so they can escape their drying ponds.

Researchers have long thought tadpoles have to get a taste of meat to spark their rapid transformation into carnivores, but Emily Harmon at the University of North Carolina at Chapel Hill thought they might be getting some kind of signal earlier in their development.

Her team ran two experiments, exposing 30 groups of 20 sibling spadefoot toad embryos to an environment replete with fairy shrimps, and another 30 groups to a neutral environment with no shrimp. As this is before hatching, the toads were still just eggs and couldn’t actually feed on any of the shrimp.

Yet, they must have sensed the presence of shrimp in the water – a phenomenon called anticipatory plasticity. The team found that two days later, the shrimp-exposed tadpoles hatched with moderately larger and stronger jaws – their jaws were about 5 per cent wider and could exert 12 per cent greater force than those of the other tadpoles. And they were 0.07 millimetres bigger on average.

“They’re not able to eat yet, but they hatch out with these traits that will allow them to become better carnivorous competitors when they are able to start eating the next day,” says Harmon. “These not-even-fully-cooked embryos are processing this information”, she says, but how exactly the toad embryos glean information from their environment is still unclear.

Research also needs to hone in on whether the tadpoles are sensing prey while embryos, or while newly hatched larvae, or both, says Karen Warkentin at Boston University.

This seems to be among the first examples of an amphibian using anticipatory plasticity to improve traits used for “offence”, says Warkentin, but that is probably just because researchers haven’t been looking. There are various known cases of animals deploying it for “defence” purposes, such as common frogs hatching with shorter bodies and deeper tail fins if they sense predator cues as embryos.

There is reason to believe many other animals are capable of this trick for offence or defence. “I think we don’t really know,” says Warkentin. “But it’s probably more widespread than we’re currently aware of.”

Journal reference: Biology LettersDOI: 10.1098/rsbl.2022.0613