By studying the social traits of non-human apes, we can better interpret our own.
For instance, it is not uncommon for us to lose touch with friends as we age. We may attribute the reasoning to increased life responsibilities but new data conducted by a team of researchers from Harvard’s Department of Human Evolutionary Biology offers a more compelling explanation.
In it, the authors employed chimpanzee models to thoroughly explore social selectivity in older populations.
The findings concerned a lifespan theory known as socioemotional selectivity. As we get older, we begin to prioritize our relationships based on what we get from them.
With a decreasing time horizon, relationships that bring us any degree of tension get swapped out for ones that do not, or they’re eliminated completely. Turns out chimps experience a similar process.
“As humans age, we prioritize established positive friendships over the new, but risky, socializing we do when we are young. It has been hypothesized that this shift may come as our own sense of mortality kicks in,” the authors wrote in the new paper. “Though there is evidence of some sense of time among nonhuman animals, it seems unlikely that they have the same impending sense of mortality that we experience; thus, these results suggest that a different, and deeper, the mechanism may be at play.”
The results were derived from 78,000 hours worth of data on male chimp behavior collected between 1995 and 2016. The chimps involved in the report were between the ages of 15 and 58 years old.
Given that male chimpanzees typically display stronger social bonds than female chimps, it was telling that the former becomes progressively more reclusive over time.
The fifteen-year-old subjects displayed a median of 2.1 one-sided friendships and 0.9 mutual friends compared to the average of .6 one-sided friends and 3 mutual friends observed among the 40-year-old chimps. Friendships were primarily assessed via grooming behavior.
“The really cool thing is that we found that chimpanzees are showing these patterns that mirror those of humans,” Alexandra Rosati, an assistant professor of psychology and anthropology at the University of Michigan and one of the paper’s lead authors, explained in a release. “There’s really a pressing need to understand the biology of aging. More humans are living longer than in the past, which can change the dynamics of aging.”
For the chimps, a positivity bias became pretty apparent. Their tolerance for tense social dynamics seemed to decrease over time so that the few one-sided relatiiohips they maintained into old age revolved around grooming sessions and many quiet moments.
“Even though chimps are very smart, they do not understand they’re going to die,” the authors explained. “Much more likely something else is going on in chimps to explain why their relationships become more positive as they get older, and then the question is, is what applies to chimps the same as what applies to humans?
We see individuals having these more lopsided friendships and then as they age they start really spending time with individuals that reciprocate. When you have this kind of mutual friendship, you actually groom that individual more, so these older chimps have these mutual friendships and they’re actually grooming those individuals quite a bit. They’re really invested in these relationships.”
Even though the behaviors indexed above do not appear to be a direct reaction to mortality, there may be evidence to support an evolutionary survival modus.
Perhaps establishing relationships with those who reciprocate our affection reduces biomarkers relevant to longevity.
“Aging male chimpanzees have more mutual friendships characterized by high, equitable investment, whereas younger males have more one-sided relationships. Older males are more likely to be alone, but they also socialize more with important social partners. Further, males show a relative shift from more agonistic interactions to more positive, affiliative interactions over their life span,” the authors concluded.
“It raises the possibility that we are seeing behavioral systems that have been shared evolutionarily back to our common ancestor, around seven or eight million years ago.”