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

The total captive population of the pygmy hippopotamus Hexaprotodon (= Choeropsis) liberiensis had a highly female biased sex ratio at birth (41% males), which exceeded most other known distorted sex ratios in captive mammals. Deviation from an even sex ratio was not compensated by a higher juvenile mortality of females. I examined 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 of the observed patterns and relationships could be explained by classic sex ratio theories.