Social insect colonies can achieve feats of astonishing complexity, like building termite hills. The lives of individual termites and the relationships between them are far from complicated, however: Each member of the group has its own strictly defined role, and unrest is virtually nonexistent as long as the queen is alive. A similar silent agreement governs the lives of mammal species such as mole rats. One pair makes babies and everybody else takes care of them, creating an incredibly stable, efficient and successful living arrangement.
On the other end of the spectrum, the social lives of some other mammals are many times more complex; individuals compete—sometimes violently—for higher positions in their respective hierarchies, employing misleading behavioral tactics or even forming political alliances against rivals. Why are some mammals so well behaved as members of a society, whereas others cause so much trouble? A recent analysis of dozens of social mammal species in Ecology Letters may help explain what's going on.
Zoologists Dieter Lukas at the Max Planck Institute for Evolutionary Anthropology and Tim Clutton-Brock at the University of Cambridge scoured the scientific literature for details of the average levels of genetic relatedness among members of various social mammal groups. They used a measure based on the amount of genetic variation these animals share in changeable stretches of DNA that do not code for genes and are therefore largely left alone by natural selection, making them a better indicator of relatedness than sections that may be evolving.
The researchers found that group members from cooperatively breeding species in which just one pair reproduces—such as mole rats, meerkats and beavers—are consistently more closely related than in mammal species whose individuals form more complex social relationships, such as chimpanzees, elephants, hyenas and dolphins.
But isn’t this to be expected, if all their offspring are from the same parents?
Yes, Lukas says, cooperative breeding reinforces itself. But he contends that it could only have emerged when group members were closely related already; the genes that make a mammal care for the offspring of others, he reasons, were far more likely to make it into the next generation if those others were family. Thus he believes relatedness was likely a cause—not a consequence—of cooperative breeding.
So how do mammals recognize relatives? “They usually rely on very basic cues,” Lukas says, “such as a history of growing up together. So others are simply perceived as close relatives or not.” Cooperative breeders seem to pay especially close attention to such cues. “In meerkats, the dominant female knows that while she’s still pregnant, any offspring around are not her own,” he explains, adding that she will often personally eliminate these juveniles. Some cooperatively breeding mongoose females have developed a cunning way to avoid this kind of situation, however: “Multiple females may give birth within the same hour, in the same burrow,” Lukas says, “so that nobody knows their own offspring, and no young are killed.”
The timing of birth may in fact be what explains the levels of relatedness in the first place, Lukas adds, and this is something he hopes to explore in future work. “Group members are likely more closely related [in species in which] females deliver large litters at a time, creating clusters of siblings,” he explains. This kind of family planning may be more common in seasonal environments where food is plentiful only during certain periods. In places without such variation, females may opt to raise one young at a time, often from different fathers.
When everyone is related and just one pair reproduces, life is quite straightforward, Lukas says. A meerkat or mole rat behaves as either a dominant or subordinate individual; as long as the dominant pair is around and nobody else gets pregnant, violence is relatively rare. “Only when a dominant dies there will likely be intense aggression, and animals often get killed,” he notes. But if you’re a chimpanzee or a hyena, coalitions change and hierarchical positions are under threat most of the time—so one must constantly be on guard.
This may be why, in Lukas and Clutton-Brock’s analysis, lower levels of relatedness among group members do not appear to be linked just to higher levels of violence but to more brainpower as well. This idea may come as a surprise to some, as other scientists have hypothesized that species are more likely to evolve higher intelligence if, like humans, individuals help others care for their young. That idea never sat very well with researchers studying species such as meerkats, Lukas contends.
“Meerkats are clearly not smart,” as Lukas puts it. “Their solution to almost anything is digging.” So the need for more sophisticated brains may be more likely explained by the complex relationships we have in common with chimpanzees and other mammals that live in groups with members that are not so closely related.
Evolutionary biologist Susanne Shultz at the University of Manchester, who has studied links between social life and brain evolution, agrees that the challenges to animals living in groups consisting of highly or barely related individuals are quite different. “I think this distinction is an important one, and this analysis makes a massive contribution to the discussion,” Shultz says. “Not just about the nature of social complexity, but the evolution of cooperation as well.”
Yet when applied to our own species, the distinction becomes more fuzzy. Humans are unusual, Lukas adds, since our social lives appear to combine the best of both worlds. Like chimpanzees, we form complex relationships with others outside our families. We even care for their kids—something chimpanzees never do, and mole rats only do for relatives. But like social insects, we cooperate on such a scale that we can achieve things small groups could never pull off.
“The only force that might explain this is culture,” says evolutionary biologist Jacobus Boomsma at the University of Copenhagen, who studies social insects and agrees there are “essentially two ways for animals to be social.” That may be why for humans, it takes a village to raise a child.