Lisa M. Schulte and Michael Mayer. (2017) Journal of Zoology, DOI: 10.1111/jzo.12472
Parental care can be found in many different animal taxa and it is highly beneficial for the survival of the offspring. This is also the case for Neotropical poison frogs: most species guard and moisten their terrestrial clutches and then transport their tadpoles to appropriate water bodies. The poison frog Ranitomeya variabilis, however, is a little bit different: it already deposits its eggs inside of phytotelmata (small water bodies within plants). Once the tadpoles are ready to hatch, the male typically returns to transport each of them separately into other phytotelmata. The separation is important because the tadpoles of this species not only compete for the same resources within the small pools, but are also cannibalistic among each other. However, on various occasions we found that males did not return for their offspring and let their tadpoles hatch all together into the same phytotelm.
This made us wonder if such abandoned tadpoles, facing competition and cannibalism, actively seek parental care in form of transportation whenever they get the chance – i.e. as soon as a frog enters their pool. In order to test this hypothesis, we conducted experiments where tadpoles of the same clutch that shared a pool with their siblings were presented with con- and heterospecific frogs, and both moving and non-moving frog models.
Our results revealed that abandoned tadpoles actively approached the frogs and sometimes even climbed onto their backs – although the frogs had not entered the pools with the purpose of tadpole transportation. Furthermore, this behaviour was not restricted to the biological parents, and even unfamiliar species that normally occur in rivers and never enter phytotelmata were approached by the tadpoles. This leads us to the assumption that the benefits for the tadpoles to escape sibling competition and cannibalism are so high that they even accept the risk of undirected transportation by heterospecific frogs. The plastic models, however, did not trigger any behaviour, possibly because tadpoles do not recognize frogs by visual or tactile cues only, but might be dependent on chemical or multiple stimuli.
The latter is also the case in related egg-feeding species where tadpoles beg for food. We therefore suggest that our observations might be comparable to begging behaviours – with the difference that in our study system the tadpoles seem to be begging for transportation instead of food. Another approach to interpret our results is a potential parent-offspring conflict, where the tadpoles require more care than the adult frogs are willing to give. Both interpretation approaches open up a wide range of new questions, highlighting that our study system should be of great relevance for anybody interested in animal parental care.
Lisa M. Schulte