The chimpanzee males who adopt orphans

The value of long-term field studies: Christophe Boesch and colleagues report on adoption in the Taï Forest chimpanzee study population – where more than 30 years of observations have produced 18 well-defined cases of adoption of orphaned individuals. They considered “adoption” to be the provision of maternal care (e.g., carrying, feeding, food sharing, defense) for more than two months. It’s possibly unfortunate terminology, as it leads to headlines like mine. Yet it is really interesting behavior.

It would be nice to say that these cases represent 18 happy endings, but these adoptions did not increase the probability of survival compared to orphaned individuals who did not receive ongoing care. There were a couple of cases where females breastfed orphaned infants “for many years,” but there seem to be several sad stories too.

Sometimes, the care for the orphaned juveniles was given by males:

Remarkably, all adult males of the East Group that adopted young orphans went a step further by investing in unweaned small infants and carrying them dorsally during travel for many months (see Figures 3 and 4 of Porthos with Gia) (Table 3). Since, Ta chimpanzees walk about 8 km per day on average, this represents a notable investment. Porthos' adoption of Gia lasted for 17 months, until his death due to Anthrax, and he was seen to carry her even in extremely risky situations, such as during encounters with neighboring communities [26]. Furthermore, some males were seen to share their night nest with their adopted infant (Table 3). Fredy, the 3rd ranking male of the East Group, adopted Victor, the son of Vanessa, who died from Anthrax in late December 2008, and shared his nest with him every night, carried him on his back for all long travels, and shared the Coula nuts he opened from December 2008 to July 2009. For example, on February 17th, Fredy cracked 196 Coula nuts for 2h05mn and shared pieces of 79% of them. This gives a measure of the altruistic investment made in an unrelated infant.

That sounds pretty amazing. I think it’s very relevant to human evolution, as orphaning must have been very common with the high mortality rates of the past.

The authors propose that adoption is basically a side effect of prosocial behavior in these chimps brought on by leopard predation:

[T]he resulting high predation pressure exerted by these cats seems to have promoted strong within-group solidarity in the form of care for all injured individuals as well as joint coalition defense against the leopards [16], [26]. Once established, this care for the welfare of others seems to have been generalized to new social contexts, including adoption [26]. Any discussions about the evolution of altruism must include the caveat that dissimilar socio-ecological conditions will lead to important population differences in both chimpanzees and humans and we need to remain very careful before making any claims about species differences.

Well, if there was such a simple psychological correlation between various kinds of prosocial behavior, it would make life a lot easier for those of us trying to figure out what happened to humans.


Boesch C, Bolé C, Eckhardt N, Boesch H. 2010. Altruism in forest chimpanzees: the case of adoption. PLoS ONE 5:e8901. doi:10.1371/journal.pone.0008901