Birth Sex Ratio and Infant Mortality Rates in Captive Western Lowland Gorillas

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Abstract
A study of over 300 births to captive female gorillas revealed a significant trend for more male offspring to be born to older mothers and an especially marked tendency for very young mothers (less than about 8 years old) to give birth to a very low proportion of male offspring (31%). This finding is investigated with respect to the differing costs and benefits to the mother of rearing male and female offspring at different ages and stages of physical development. It is concluded that the observed pattern is likely to result from selective pressures on females to rear males at time when they can provision them most effectively, as male reproductive success probably varies more than that of females and may be dependent upon maternal care.