Distorted sex ratio at birth in the captive pygmy hippopotamus, Hexaprotodon liberiensis

Abstract

The total captive population of the pygmy hippopotamus Hexaprotodon (=Choeropsis) liberiensis has a highly female-biased sex ratio at birth (41% males), which exceeds most of the other known distorted sex ratios in captive mammals. Deviation from an even sex ratio was not compensated by a higher juvenile mortality in females. I examined the possible causes of that deviation and could not find any association between offspring sex and inbreeding, parity, or mother's age. I found, however, a significant difference in sex ratios among different zoological gardens, suggesting that husbandry influenced sex at birth in the pygmy hippopotamus. Further analysis suggested that high feeding intensity and "hands-on" husbandry favored production of daughters. However, not all the observed patterns and relationships could be explained by classic sex ratio theories.