Evidence for male allocation in pipefish?

Sexual differences in the extent and type of parental care lie at the heart of sexual selection theory, and evolution resulting from parental conflict has produced some striking behavioural and morphological adaptations. In a study of male pregnancy in Gulf pipefish, Paczolt and Jones showed that more eggs were transferred to the male's brood pouch and more offspring survived following mating with large females (preferred by males) than with small (less preferred) females. Although the authors conclude that the lower survival of embryos from small females is most consistent with males actively removing resources from these offspring, no data are presented to directly support this hypothesis (ref. 2, and Supplementary Information therein) and the data do not refute the alternative explanation that differential egg survival is caused by female effects mediated by variation in fecundity and egg size or quality. We argue that only by experimentally manipulating female attractiveness separately from the quality of eggs deposited in the brood pouch can the extent of sexual conflict in this role-reversed system be assessed.