Age-related patterns of reproductive success among female mountain gorillas.

A key goal of life history theory is to explain the effects of age and parity on the reproductive success of iteroparous organisms. Age-related patterns may be influenced by changes in maternal experience or physical condition, and they may reflect maternal investment trade-offs between current versus future reproduction. This article examines the influences of age and parity upon the interbirth intervals (IBI), offspring survival, and birth rates of 66 female mountain gorillas in the Virunga Volcano region from 1967-2004. Fertility was relatively low for females below age 12; improved as they matured; and then declined as they aged further. Primiparous mothers had 50% higher offspring mortality and 20% longer IBI than second-time mothers, though only the difference with IBI was statistically significant. The length of subsequent IBI was positively correlated with birth order but not with the mother's age. Mountain gorillas showed no evidence of an extended postreproductive lifespan. Age-related patterns seem most likely to reflect changes in the physical condition of the mother, but more detailed studies are needed to quantify those physical differences, and to obtain behavioral evidence that would provide more direct measures of maternal investment and experience.