Convergent Evolution: Hyenas Offer Clues To The Human Past
When anthropologists work to reconstruct the lives of our own ancestors we bring together multiple sources of information. We look at fossils and material culture, such as ancient tool technologies. We even look at animals alive today whose behavioral patterns might provide clues to our past.
When it comes to these animal models, we think first of apes. Chimpanzees, bonobos and gorillas are our closest living primate relatives. Our Homo lineage was able to survive and thrive via expansion of our geographic range — throughout periods of ecological and climate instability — in ways that these other apes were not.
So why not look for clues from more distant animal kin? How about animals who live in complex societies but also thrive reproductively, such that they live at higher densities over wider regions than do apes? In a recent paper in the journal Current Anthropology, Jennifer E. Smith, Eli M. Swanson, Daphna Reed and Kay E. Holekamp suggest that the spotted hyena fits the bill and is an under-appreciated source of information about human evolution.
I like Smith et al.'s perspective because it shows how an understanding of convergent evolution may help us learn more about our past. Convergent evolution is the process by which distantly related species — like human ancestors and spotted hyenas — evolve similar traits as they adapt over time to similar ecological and social environments.
So let's unpack all this. I'll begin with the spotted hyenas (Crocuta crocuta) themselves.
As the Current Anthropology article explains, about 85 percent of terrestrial mammalian carnivores are solitary outside of mating and parental-care contexts. Spotted hyenas are different.
They're more socially complex than carnivores like wolves, lions and wild dogs. These animals live in small groups where many adults of the same sex tend to be related to each other. By contrast, spotted hyenas live in groups that may exceed 100 animals, many of whom have no kinship ties. Within these social units, the hyenas cooperate in hunting and in defending their food and territory. They may even communally rear their young.
Here's a fact I didn't know: spotted hyenas are the most abundant social carnivores in sub-Saharan Africa. In other words, they're not just getting by demographically, they're doing really well. As lead author Jennifer Smith noted in an email message to me earlier this month:
In contrast to most other carnivores, spotted hyenas possess an extraordinary ability to cope with the demands of life in a socially and ecologically dynamic landscape and to persist at high densities despite the energetic demands of being a top predator.
So we begin to see why spotted hyenas have something in common with our group-living Homo ancestors who, around two million years ago, began to eat more meat (probably through both scavenging and cooperative hunting, with high-quality tubers important as well). These ancestors soon expanded their range both inside and outside of Africa, coping successfully with climate fluctuation over time.
But exactly what is it that hyenas are doing that apes aren't doing? Smith sent me answers to this question. First, some significant hyena behaviors do overlap with ape behavior, especially chimpanzees. Hyenas and chimpanzees live, for example, in what are called fission-fusion societies where animals join small parties with constantly shifting membership.
It's a fascinating combination of hyena behaviors that sets this species apart and brings the convergent-evolution framework most sharply into focus.
Cooperative hunters and meat-eaters, spotted hyenas show impressive flexibility both in their diet (e.g., cracking open bones to extract marrow or eating termites) and in their reproductive behavior. Their pace of reproduction, Smith et al. believe, can speed up or slow down according to "the availability and acquisition of energy-rich foods."
The key idea is that hyenas are better at this than other social carnivores, and human ancestors were better at it than other primates. Smith put it to me this way:
Just as flexibility in female reproduction among spotted hyenas exceeds that of most extant carnivores, the plastic reproductive responses typical of early Homo appear to surpass those of chimpanzees or gorillas.
In other words — my words now — our lineage began to turn out babies more efficiently than other apes, probably because of our high-quality diet and some form of enhanced cooperative parental care.
Sure, Homo sapiens evolved cognitive and linguistic behavior that no other mammal did. In key ways, though, our adaptive strategies seem to be the spotted hyena's strategies, too. I think that's a very cool thing to know.
You can keep up with more of what Barbara is thinking on Twitter: @bjkingape