To some evolutionary researchers, parental love is a mechanism to foster the transfer of genes through successive generations. Feeling emotionally attached to our children, most humans feed and protect our offspring and prepare them to thrive as adults, including as breeders. But when did this start for fathers – did our common ancestor with chimps and bonobos do the same? Recent research among chimpanzees suggests that the answer is yes.
The origin of paternal love has been an open question because chimps and bonobos – our two closest cousins among living species – mate without forming lasting “pair bonds,” or couples. So, unlike in many human couples, a chimp mother can’t count on protection and other aid from a male companion who would likely be the father of her offspring. This can be a matter of life and death among chimps, as a sympathetic adult male can save a child from infanticide – most often by other females. But hanging out with lactating mothers and infants comes at a cost to the male chimps’ other activities, such as foraging and competing for status. Given these trade-offs, have chimps evolved a way for males to favor their own progeny?
To answer this question, a team of U.S. researchers worked with data from the much-studied chimps of Gombe National Park, Tanzania, to discern whether males spent more time with their offspring than with other infants. Presumably, this extra attention would lead to greater protection.
After crunching numbers from observations made over 25 years, that’s what they found. Males spent significantly more time with their recently born progeny than they did with other newborns. And the time they spent decreased over the first year, as the danger of infanticide waned.
But might the males have cozied up to the mothers to improve their chances of mating again, rather than to protect the infant? Apparently not. The researchers determined that the fathers’ attention didn’t result in more breeding with the lactating mothers. Thus, they infer that the primary result – and main evolutionary cause – was the protection of offspring.
If so, then this suggests that the ancestors we share with chimps and bonobos – more than six million years ago – acted similarly. That is, before couples evolved, both parents nurtured newborns.
Doubts
But there are good reasons to wonder about these conclusions. First, it’s hard to observe chimps in the wild – much less to identify their genetic relationships. So the sample size (17 males, 49 mother-infant pairs) is quite small for such a sweeping conclusion – especially the claim that fathers spent time preferentially with their children rather than with the mothers. It would be helpful to know whether the fathers spent time preferentially with the mothers during their pregnancy, or whether they mated, backed off for months, and then started hanging around only after the infant was born.
Another reason to doubt the robustness of their dataset is that the numbers reveal no relationship between a male’s rank and his likelihood of reproducing a second time with the mother of his offspring. Yet rank has been established as a predictor of mating success among chimpanzees.
In any case, why wouldn’t male chimps hang around for both reasons? Wouldn’t the evolutionary advantage be greatest if the father’s presence protected the infant and resulted in another pregnancy?
Why?
Whether males preferred the company of particular females or their offspring or both, intriguing questions arise regarding the process. For example, if males favor particular females, why do their interactions fall off over the first year after the birth of their joint offspring?
Conversely, by what process would a male chimp feel the urge to hang around his progeny? After all, it’s unlikely that chimps make the cognitive connection between sex and birth (some human groups don’t) or have any concept akin to fatherhood. It’s unlikely that a chimp would recognize physical similarities between himself and his offspring, given the different appearance of infants and the absence of mirrors or photos to reveal his own appearance. In fact, the father decreases his time around the infant as the infant matures and therefore resembles him more closely.
Perhaps the father gets distracted by his next offspring after a few months. But that leaves the question of how he is attracted to any of his offspring. Does the infant emit some pheromone? Does it wear off after several months?
The researchers raise another possibility: that chimp fathers aren’t seeking out their offspring or their mothers; instead, the mothers bring the offspring to the father. Why they do so less over time would still require explanation, as would the process by which they feel the urge to spend time with the father.
In short, the researchers present evidence that, by an unknown mechanism, chimp fathers spend more time with their offspring than with other young chimps. Perhaps our common ancestors did, too. The researchers make a plausible argument that this pattern provides some protection against infanticide. How this happens and whether it has other evolutionary effects remain mysterious.
Reference
Murray CM,Stanton MA, Lonsdorf EV,Wroblewski EE,Pusey AE. 2016. Chimpanzee fathers bias their behaviour towards their offspring. R. Soc. opensci. 3:160441. http://dx.doi.org/10.1098/rsos.160441
Tracy Duvall 