Scientists have been chronicling the lives of spotted hyenas, seeing links between their brain size and social intelligence.

By CARL ZIMMER

For the past two decades, Kay E. Holekamp has been chronicling the lives of spotted hyenas on the savannas of southern Kenya.
She has watched cubs emerge from their dens and take their place in the hyena hierarchy; she has seen alliances form and collapse.
She has observed clan wars, in which dozens of hyenas have joined together to defend their hunting grounds against invaders.
“It’s like following a soap opera,’’ said Dr. Holekamp, a professor at Michigan State University.
The lives of spotted hyenas, she has concluded, share some profound similarities with our own.
In both species, a complex social world has driven the evolution of a big, complex brain.
Scientists have puzzled over the enormous size of the human brain.
It is seven times larger than one would predict for an average mammal of our size.
Many of our extra neurons are in a region called the frontal cortex, where much of the most sophisticated thought takes place.
To understand how we ended up with such a strange organ, many scientists have turned to our fellow primates.
They also have large brains, although not as large as our own.
It turns out that primates with a big frontal cortex tend to live in large groups.
Primates may be pushed into larger groups thanks to predators or to patchy sources of food like fruit trees.
As their numbers grow, natural selection may favor social intelligence.
The primates form long-term alliances with each other and compete with rivals.
They begin to keep track of a larger and larger social network.
A boost in social intelligence can lead to an evolutionary edge for primates.
“ Spotted hyenas live in a society just as large and just as complex as a baboon,’’ Dr. Holekamp said, noting that spotted hyenas live in the largest social groups of any carnivore.
“We’re talking about 60 to 80 individuals who all know each other individually.
’’ Spotted hyena societies have one dominant female at the top, and a series of hyenas below her.
The hierarchy reveals itself most vividly when it is time to eat.
When one or two hyenas make a kill, other members of the clan will join them to fight over the prey.
But the dominant female always wins.
There are times, however, when the entire group of hyenas comes together.
Spotted hyena clans patrol the borders of their territory together, marking it with their urine.
“When the whole group territory is on the line,’’ Dr. Holekamp said, “all these unrelated individuals join forces and engage in a clan war.
’’ Spotted hyenas belong to a family of four species, and the other three live in different kinds societies, she said.
Brown hyenas, for instance, live in much smaller clans that range up to about 14 animals.
Striped hyenas live in even smaller groups of a single female and no more than three adult males.
The most solitary of all the spotted hyena’s relatives is the aardwolf.
Instead of hunting or scavenging meat, they have shifted to a diet of termites.
A male and female aardwolf will live as a monogamous pair .
Dr. Holekamp wonders if this range of social arrangements is reflected in the structure of hyena brains.
From a CT-scan of a hyena skull, it is possible to reconstruct the three-dimension structure of the brain it held.
In recent months, Dr. Holekamp has been working with Sharleen Sakai and Barbara Lundrigan, both at Michigan State, to survey dozens of skulls from all four species in the hyena family.
Their preliminary results indicate hyenas follow the same rule as primates.
“It’s just what the social complexity hypothesis would predict,’’ Dr. Holekamp said.
“The hyenas with the simplest social systems have the tiniest frontal cortices.
The spotted hyena, which lives in the most complex societies, has far and away the largest frontal cortex.
’’ The brown and striped hyenas, with intermediate social systems, have intermediate brains.
By comparing hyenas and primates, as well as other mammals, Dr. Holekamp believes it will be possible to get a full picture of how intelligence evolves.
“There’s a tremendous support for the social brain hypothesis,’’ she said, “but I think that in order to understand the origin of intelligence we have to think more broadly than that.’’