In the wild, mistakenly caring for a rival’s offspring can be a costly exercise, so being able to identify your own young is important. A new study finds that male brilliant-thighed frogs aren’t too picky about whose tadpoles they offer ‘piggybacks’ to, but females will only carry tadpoles that she believes are hers.

The brilliant-thighed poison frog (Allobates femoralis) is a ground-dwelling species of frog native to the tropical forests of South America. The female frog lays eggs on fallen leaves within a male’s territory, which are then fertilised by the male. After three weeks, the male frog returns to where the eggs were laid and offers a ‘piggyback’ to the awaiting tadpoles. The tadpoles hop aboard and are transported to the nearest body of water to continue development into adulthood.

However, it is not always the male that does the tadpole transport. “Females only do so when the male is not in his territory at this time,” explains lead researcher Eva Ringler from the University of Vienna.

Transporting tadpoles is a risky operation, but essential for the survival of the frog’s offspring. Not only is the journey fraught with predators, the males that transport tadpoles also leave their territories open to seizure by rivals. “Transport therefore only makes sense when the risk that is taken serves the survival of one’s offspring,” says Ringler.

To find out more about these piggybacking behaviours, Ringler and her team set up a series of experiments to find out under what conditions the frogs would transport unrelated tadpoles, and whether these behaviours differed between the sexes.

When male and female frogs within a terrarium were presented with only unrelated tadpoles, the majority of males transported the foreign offspring while none of the females did. When the frogs were presented with both their own offspring as well as unrelated tadpoles, the majority of males weren’t picky and transported both related and unrelated tadpoles. However, the majority of females only transported their own tadpoles and ignored those she determined were unrelated.

In a final experiment to test how females determined which offspring were theirs, the researchers replaced the frog’s own offspring with unrelated tadpoles in the same location. They found that that all of the females transported these unrelated tadpoles, suggesting that they identify their offspring solely by their location.

Ringler explains that these contrasting behaviours reflect the different reproductive behaviours of the sexes. Male frogs tend to guard a large territory and will assume that all tadpoles within this area are theirs. This tactic reduces the risk of neglecting their own offspring but may benefit other males by transporting their unrelated tadpoles too.

Female frogs, however, have a much higher risk of transporting unrelated tadpoles and neglecting their own, and so are more selective about who they give piggybacks to and rely much more on the ability to identify their own offspring.

In order to remember the exact positioning of their tadpoles after three weeks, the team suggest that the female frogs possess an impressive inner GPS. “Further research is needed to clarify just how the females remember the exact location of oviposition in the dense rain forest,” suggests Ringler.

Reference

Ringler E, Pašukonis A, Ringler M, Huber L (2016) Sex-specific offspring discrimination reflects respective risks and costs of misdirected care in a poison frog. Animal Behaviour 114: 173-179. DOI http://dx.doi.org/10.1016/j.anbehav.2016.02.008

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