DOMINANCE RANK AND PARENTAL INVESTMENT IN SWINE (SUS SCROFA DOMESTICUS)

Using domestic swine, we tested the general prediction from life histo ry theory that females increase their investment in offspring with inc reasing age and parity. Because increased investment may have a greate r beneficial impact on the lifetime reproduction of sons than daughter s, we also tested the prediction that older females would invest more in sons than in daughters compared to younger females. Finally, we exa mined whether age- or parity-related patterns of change in reproductiv e effort were associated with differences in the social dominance rank s of females. Female swine from a large number of domestic breeds were assigned to social groups, and their dominance ranks were determined based on the outcome of agonistic encounters. The prediction that olde r females produce larger litters was supported, but the increase was r elated only to age, not to parity Across all ages, high-ranking female s produced a greater proportion of sons than low-ranking females. Cont rary to our prediction, there was no rank-related change in the propor tion of sons born with increasing age or parity. However, the mean bod y masses offspring born to high-ranking females increased with increas ing maternal age and parity, but this was not the case for offspring o f low-ranking females. Studies of free-ranging groups of swine are nee ded to determine whether an increase in body mass at birth would have different effects on the reproduction of sons or daughters.