The Evolutionary Loss of Paternal Care Is Associated With Shifts in Female Life-History Traits

Parental care can increase the fitness of parents through increased offspring survival but can also reduce reproductive output by limiting time and energy allocated to additional mating opportunities. The evolutionary origin of parental care is often associated with shifts in life-history traits (e.g., high investment in few, large offspring, slow offspring growth), but little is known about whether the evolutionary loss of care is associated with reciprocal shifts in the same life-history traits. Here, we capitalize on the divergence of parental care between ecotypes of three-spined stickleback (Gasterosteus aculeatus) to test for associations between parental care and life-history traits. While males from most stickleback populations provide care, an unusual "white" ecotype has recently lost paternal care. We found support for the hypothesis that the evolutionary loss of paternal care is associated with shifts in female life-history traits; relative to females of the ecotype with paternal care, females of the white ecotype that lack paternal care produced clutches with a similar overall mass and a greater number of smaller eggs, despite their smaller body size, suggesting lower per-offspring investment. We did not detect an ecotypic difference in embryonic development rate, metabolic rate, or offspring age at hatching, contrary to the 'safe harbor hypothesis'. These results support the theory that behavioral traits such as parental care co-evolve with other life-history traits and highlight opportunities for future study of the underlying causal mechanisms.