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As wildlife populations are declining, conservationists are under increasing pressure to measure the effectiveness of different management strategies. Conventional conservation measures such as law enforcement and community development projects are typically designed to minimize negative human influences upon a species and its ecosystem. In contrast, we define “extreme” conservation as efforts targeted to deliberately increase positive human influences, including veterinary care and close monitoring of individual animals. Here we compare the impact of both conservation approaches upon the population growth rate of the critically endangered Virunga mountain gorillas (Gorilla beringei beringei), which increased by 50% since their nadir in 1981, from approximately 250 to nearly 400 gorillas. Using demographic data from 1967–2008, we show an annual decline of 0.7%±0.059% for unhabituated gorillas that received intensive levels of conventional conservation approaches, versus an increase 4.1%±0.088% for habituated gorillas that also received extreme conservation measures. Each group of habituated gorillas is now continuously guarded by a separate team of field staff during daylight hours and receives veterinary treatment for snares, respiratory disease, and other life-threatening conditions. These results suggest that conventional conservation efforts prevented a severe decline of the overall population, but additional extreme measures were needed to achieve positive growth. Demographic stochasticity and socioecological factors had minimal impact on variability in the growth rates. Veterinary interventions could account for up to 40% of the difference in growth rates between habituated versus unhabituated gorillas, with the remaining difference likely arising from greater protection against poachers. Thus, by increasing protection and facilitating veterinary treatment, the daily monitoring of each habituated group contributed to most of the difference in growth rates. Our results argue for wider consideration of extreme measures and offer a startling view of the enormous resources that may be needed to conserve some endangered species.

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.

Infanticide can be a major influence upon the social structure of species in which females maintain long-term associations with males. Previous studies have suggested that female mountain gorillas benefit from residing in multimale groups because infanticide occurs when one-male groups disintegrate after the dominant male dies. Here we measure the impact of infanticide on the reproductive success of female mountain gorillas, and we examine whether their dispersal patterns reflect a strategy to avoid infanticide. Using more than 40 years of data from up to 70% of the entire population, we found that only 1.7% of the infants that were born in the study had died from infanticide during group disintegrations. The rarity of such infanticide mainly reflects a low mortality rate of dominant males in one-male groups, and it does not dispel previous observations that infanticide occurs during group disintegrations. After including infanticide from causes other than group disintegrations, infanticide victims represented up to 5.5% of the offspring born during the study, and they accounted for up to 21% of infant mortality. The overall rates of infanticide were 2–3 times higher in one-male groups than multimale groups, but those differences were not statistically significant. Infant mortality, the length of interbirth intervals, and the age of first reproduction were not significantly different between one-male versus multimale groups, so we found no significant fitness benefits for females to prefer multimale groups. In addition, we found limited evidence that female dispersal patterns reflect a preference for multimale groups. If the strength of selection is modest for females to avoid group disintegrations, than any preference for multimale groups may be slow to evolve. Alternatively, variability in male strength might give some one-male groups a lower infanticide risk than some multimale groups, which could explain why both types of groups remain common.

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Variance in male reproductive success is expected to be high in sexually dimorphic mammals, even when it is modulated by the costs and benefits of group living. Here, we investigate the variance in reproductive success of male western gorillas (Gorilla gorilla), a highly dimorphic primate with long-term male-female associations, using 12.5 years of data collected at Mbeli Bai in northern Congo. Access to mates and offspring survival were both major sources of variance in male reproductive success. Males with larger harems had lower offspring mortality with no apparent reduction in female fertility or observed tenure length, so the size of harems did not seem to be limited by female feeding competition or by the risk of takeovers and infanticide by outsider males. The lower mortality in larger harems may reflect improved vigilance against predators, and females may cluster around males that enhance offspring survival. Thus, this study illustrates how a detailed analysis of the components of male reproductive success can shed light on the interrelated social and ecological aspects that affect it.