Costly sons do not lead to adaptive sex ratio adjustment in pilot whales, Globicephala melas

Maternal investment in reproduction and parental care is an important determinant of both offspring and maternal fitness. However, optimal investment strategies may differ depending on offspring sex, potentially resulting in a sex-biased distribution of maternal resources or adaptive variation in offspring sex ratio. We used morphometric and genetic data collected from over 3400 long-finned pilot whales in 40 pods to investigate whether females experience differential costs of producing sons and daughters and whether they differentially invest in male and female offspring. We found that male calves grew faster than female calves during the first 5 years of life, suggesting that sons may require greater investment from lactating mothers. This appeared to result in mothers experiencing a higher cost to future reproductive opportunities when producing male offspring as the presence of dependent sons (but not daughters) reduced the probability that a female would be pregnant. Despite these costs, we found no evidence that mothers adaptively adjusted their investment in sons and daughters according to their body condition or their social and physical environment. These results suggest that mothers may be constrained from biasing investment in the sexes, or that additional benefits may be masking such costs.