Jos Kramer, Maximilian Körner, Janina MC Diehl, Christine Scheiner, Aytül Yüksel-Dadak, Teresa Christl, Philip Kohlmeier and Joël Meunier
Decades of research on mammals and birds have taught us that parental care almost always renders the presence of parents beneficial to immature offspring regardless of the prevailing environmental conditions. This is because offspring in these animals are often fully dependent on resources provided by their caring parents, and thus benefit from their presence even under the most adverse conditions. This strong dependency on parental care, however, is itself the result of a long evolutionary history. During earlier stages of the evolution of family life, offspring were actually largely independent of parental care, since they could forage on their own early on. Surprisingly, the role of this early onset of foraging has been overlooked so far; and yet, it could have been of fundamental importance for the evolution of family life, as it might have put juveniles into direct competition with their own parents. The costs of this parent-offspring competition might have counteracted the benefits of early family life, especially where environmental conditions were harsh and food was scarce.
Here, we investigated the presence and effects of parent-offspring competition in the European earwig Forficula auricularia, an insect with non-obligatory family life reminiscent of an ancestral state. In this species, mothers routinely clean, protect and feed their newly-emerged offspring (called nymphs), while the offspring retain their ability to forage independently and thus can survive on their own. To shed light on the occurrence of parent-offspring competition, we raised nymphs under food limitation either together with or without their mother, and determined whether the food consumption of mothers and offspring (as reflected by their weight gains) affected their fitness. We showed that maternal presence under food limitation was almost always detrimental, and that offspring suffered especially in situations where their mothers consumed an excessive portion of the scarce food. Interestingly, mothers in a bad condition showed the least self-restraint in competing with their offspring. These highly competitive mothers likely monopolized the food to prevent starvation, or to increase their later investment into another brood. Overall, our findings reveal that parent-offspring competition under harsh conditions can counteract the benefits of early (facultative) forms of parental care. Where food was too scarce, parent-offspring competition even could have prevented the evolution of family life altogether.
Image caption: Earwig family. Joël Meunier