Age-related patterns of reproductive success among female mountain gorillas

Abstract

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.

Aged Black-and-Gold Howler Monkey Female (Alouatta caraya): A Sign of Reproductive Senescence?

“…Similar results were reported in mountain gorillas (Gorilla gorilla gorilla), which support the occurrence of menopause in captivity [Atsalis and Margulis, 2006], but its absence in the wild [Robbins et al, 2006]. In orang- utan species (Pongo pygmaeus), Knott [1996] found evidence of significantly lower urinary estrone conjugate levels and the absence of mating, suggesting that energy imbalance had significant effects on fecundity.…”

Age‐related changes in molar topography and shearing crest length in a wild population of mountain Gorillas from Volcanoes National Park, Rwanda

“…This association is thought to exist due to the mother's inability to effectively process a mechanically tough diet using senesced teeth, reflected by an increase in fecal particle size, which results in less available energy for lactation (King et al, 2005). Unlike the significant decrease in infant survival observed for old P. edwardsi mothers, gorilla mothers experience a very slow decrease in infant survival with age (Robbins et al, 2006). While many ecological and social factors influence infant survivorship in primates (Pusey et al, 1997;Bales et al, 2000;Domb and Pagel, 2001;Altmann and Alberts, 2005;Silk et al, 2009), the ability of gorilla mothers to maintain high infant survival even into the final years of life suggests that these individuals do not experience energetic deficits as King et al (2005) inferred for P. edwardsi mothers.…”

Lifetime reproductive success of female mountain gorillas

“…The population of those groups has grown by 3-5% per year throughout this study, so we expect that k ind will typically be above one, and it will place diminishing value on reproduction later in life (Miller et al, 1998;. However adult females are almost always nursing or pregnant, so we see relatively little variation in how the timing of their reproduction is distributed throughout adulthood (Robbins et al, 2006;Robbins et al, 2011), and therefore we expect a close correlation between LRS and k ind .…”