Altruism in Forest Chimpanzees: The Case of Adoption

In recent years, extended altruism towards unrelated group members has been proposed to be a unique characteristic of human societies. Support for this proposal seemingly came from experimental studies on captive chimpanzees that showed that individuals were limited in the ways they shared or cooperated with others. This dichotomy between humans and chimpanzees was proposed to indicate an important difference between the two species, and one study concluded that “chimpanzees are indifferent to the welfare of unrelated group members”. In strong contrast with these captive studies, consistent observations of potentially altruistic behaviors in different populations of wild chimpanzees have been reported in such different domains as food sharing, regular use of coalitions, cooperative hunting and border patrolling. This begs the question of what socio-ecological factors favor the evolution of altruism. Here we report 18 cases of adoption, a highly costly behavior, of orphaned youngsters by group members in Taï forest chimpanzees. Half of the adoptions were done by males and remarkably only one of these proved to be the father. Such adoptions by adults can last for years and thus imply extensive care towards the orphans. These observations reveal that, under the appropriate socio-ecologic conditions, chimpanzees do care for the welfare of other unrelated group members and that altruism is more extensive in wild populations than was suggested by captive studies

Male-Mediated Gene Flow in Patrilocal Primates

BACKGROUND: Many group-living species display strong sex biases in dispersal tendencies. However, gene flow mediated by apparently philopatric sex may still occur and potentially alters population structure. In our closest living evolutionary relatives, dispersal of adult males seems to be precluded by high levels of territoriality between males of different groups in chimpanzees, and has only been observed once in bonobos. Still, male-mediated gene flow might occur through rare events such as extra-group matings leading to extra-group paternity (EGP) and female secondary dispersal with offspring, but the extent of this gene flow has not yet been assessed. METHODOLOGY/PRINCIPAL FINDINGS: Using autosomal microsatellite genotyping of samples from multiple groups of wild western chimpanzees (Pan troglodytes verus) and bonobos (Pan paniscus), we found low genetic differentiation among groups for both males and females. Characterization of Y-chromosome microsatellites revealed levels of genetic differentiation between groups in bonobos almost as high as those reported previously in eastern chimpanzees, but lower levels of differentiation in western chimpanzees. By using simulations to evaluate the patterns of Y-chromosomal variation expected under realistic assumptions of group size, mutation rate and reproductive skew, we demonstrate that the observed presence of multiple and highly divergent Y-haplotypes within western chimpanzee and bonobo groups is best explained by successful male-mediated gene flow. CONCLUSIONS/SIGNIFICANCE: The similarity of inferred rates of male-mediated gene flow and published rates of EGP in western chimpanzees suggests this is the most likely mechanism of male-mediated gene flow in this subspecies. In bonobos more data are needed to refine the estimated rate of gene flow. Our findings suggest that dispersal patterns in these closely related species, and particularly for the chimpanzee subspecies, are more variable than previously appreciated. This is consistent with growing recognition of extensive behavioral variation in chimpanzees and bonobos

The presence of a close kin does not increase the likelihood of adoption in Taï chimpanzees (Fisher exact test: p = 0.463).

<p>The presence of a close kin does not increase the likelihood of adoption in Taï chimpanzees (Fisher exact test: p = 0.463).</p

Successful adoptions of orphaned infants in three communities of Taï chimpanzees.

<p>quick  =  adoption occurred within days after the death of the mother.</p><p>* =  Bonnie was first identified when she had already been adopted by her suspected brother, Clyde.</p><p>** =  Yayo has been observed to be carried by 4 other adult males of the East Group during the 4 months of his adoption, but Fredy was the main adopter.</p>d1<p> =  adoption interrupted by the death of the orphan.</p>d2<p> =  adoption interrupted by the death of adopter.</p>i<p> =  adoption actively interrupted by adopter.</p><p><u>Relation</u>: Sister/Brother =  older sibling of the orphan, Friend =  A♀ was a friend of the deceased mother, NR =  confirmed as not related following genetic testing, uk = unknown.</p

The adult male Porthos carrying on his back the adopted female infant Gia.

<p>The adult male Porthos carrying on his back the adopted female infant Gia.</p

Number of orphans and adoptions seen in the 27 years of observation of three study groups of the Taï chimpanzee project.

<p>Number of orphans and adoptions seen in the 27 years of observation of three study groups of the Taï chimpanzee project.</p

Orphans adopted do not present higher likelihood of surviving for 2 years the death of their mother than non-adopted orphans in Taï chimpanzees (X2 = 0.37, df = 1, p = 0.54).

<p>Orphans adopted do not present higher likelihood of surviving for 2 years the death of their mother than non-adopted orphans in Taï chimpanzees (X2 = 0.37, df = 1, p = 0.54).</p

The adult male Porthos with his adopted female infant Gia.

<p>A) Porthos cracking and sharing nuts with Gia, B) Porthos carrying Gia on his back.</p

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

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