Mountain gorilla tug-of-war: silverbacks have limited control over reproduction in multimale groups

Abstract

To determine who fathers the offspring in wild mountain gorilla groups containing more than one adult male silverback, we genotyped nearly one-fourth (n = 92) of the mountain gorillas (Gorilla beringei beringei) living in the Virunga Volcanoes region of Africa. Paternity analysis of 48 offspring born into four groups between 1985 and 1999 revealed that, although all infants were sired by within-group males, the socially dominant silverback did not always monopolize reproduction within his group. Instead, the second-ranking male sired an average of 15% of group offspring. This result, in combination with previous findings that second-ranking males fare best by not leaving the group but by staying and waiting to assume dominance even if no reproduction is possible while waiting, is not consistent with expectations from a reproductive skew model in which the silverback concedes controllable reproduction to the second-ranking male. Instead, the data suggest a "tug-of-war" scenario in which neither the dominant nor the second-ranking male has full control over his relative reproductive share. The two top-ranked males were typically unrelated and this, in combination with the mixed paternity of group offspring, means that multimale gorilla groups do not approximate family groups. Instead, as long-term assemblages of related and unrelated individuals, gorilla groups are similar to chimpanzee groups and so offer interesting possibilities for kin-biased interactions among individuals.

Fig. 1.

Reproductive careers of mountain gorilla males. Thick dark bars indicate the dominant silverback, whereas thick light bars indicate the second-ranking male. Bars are hatched during periods when the dominance relationships were unknown. Thin bars span the ages of 7–12 years. Males below the age of 7 are indicated by dotted lines. The circles represent offspring and are each placed on the line of the assigned father at the estimated time of conception. Striped circles are untyped offspring. The one typed offspring for which paternity could not be assigned, although the dominant male was excluded, is indicated by a circled x. em, emigration out of the group.