The importance of evolutionary timelines when explaining the evolution of parental care strategies

Comparative research on the evolution of parental care has followed a general trend in recent years, with researchers gathering data on clutch size or egg size and correlating these traits with ecological variables across a phylogeny. The goal of these studies is to shed light on how and why certain life history strategies evolve. However, results vary across studies, and we rarely have results explaining why the observed pattern occurred, leaving us with further hypotheses to test. By using a combination of comparative methods, we provide an explanation of how such patterns emerge based on the evolutionary timeline of the investment in eggs and life-history traits; this combination also allowed us to pinpoint why the pattern occurred. We do so with data on freshwater crayfish, which are ideal for such investigations because they exhibit a diversity in body size, post-ovipositional care strategies (associated with burrowing), and pre-ovipositional investment in eggs. Specifically, we tested whether a strong dependence on burrows is related to pre-ovipositional investment in eggs (i.e., larger eggs or more eggs).

We found no correlation between burrowing and the size or number of eggs crayfish lay; instead, body size was the best predictor of the number of eggs (but not the size of eggs) that each species lays. Interestingly, our analysis suggests that crayfish ancestors had a small clutch size, relatively large eggs, and a weak connection to burrows. Thus, the shift to heavily relying on burrows appeared after this lineage had already evolved relatively large eggs, which gives insights into the colonization of freshwater by an ancestral astacidean ancestor. While other studies show that the connection between life history and egg investment is not straightforward, our study provides a clear evolutionary timeline of the interplay between the evolution of pre-ovipositional parental care and life history strategies. Furthermore, our work showcases how merging multiple phylogenetically informed approaches can disentangle the origin and evolution of life history traits.