Living along the cliffs of highland Ethiopia is a fascinating species of baboon called the Gelada baboon. They are magnificent creatures and stand apart from the rest of the primates due to their chests. They are bare and bright red.
Both males and females of the so-called bleeding heart baboon have hourglass shaped areas of bare, red skin on their chests. Female geladas, upon reaching sexual maturity, develop blister-like bumps around the edges of these skin patches.
They swell and change color from whitish to red in conjunction with the female's estrous cycle. They are an obvious indicator of fertility, somewhat like the fleshy rump patches of other baboons, chimps and bonobos.
Because gelada baboons spend most of their day sitting on their behinds munching grasses, they don't signal ovulation on their rears like many other primates do. What good is a hidden signal? So it's displayed where it can be seen — on their chests. Some evolutionary psychologist think something similar drove the evolution of human female sexual signals. Instead of sporting swollen labia like chimps and bonobos do when they ovulate, human females have a much more subtle signal in the form of a different pair of red lips. The ones on our faces. It's a much more subtle signal. No wonder men find women mysterious!
Gelada baboons live alongside steep cliffs. At night, they climb down the rocks to sleep, safely away from predators. They huddle together for warmth. Nights in the mountains of Ethiopia are chilly. Males and female have evolved thick fur to cope with the conditions. Males have especially thick hair concentrated around their heads, neck and chest. They look very much like lions. It is said that Ethiopian warriors use their manes in traditional costumes.
Their unusual appearance and striking presence is one thing, but perhaps the most interesting aspect of these primates is their social structure. It is one of the most complex among primates and is often cited as a model for understanding what the social structure of ancestral humans might have been like.
Gelada baboon social structure is also especially interesting because it contrasts sharply with a closely related species of baboon — the hamadryas baboon. Both species of baboon live in Ethiopia in the highlands in groups of hundreds of individuals. Bands upwards of 400 have been spotted in one place, but they recognize only the few who live in their troop and especially their harem. Harems are formed of 2 to 8 females, their young and one dominant male. Sometimes harems are referred to as OMUs or one male units to differentiate their social system from that of human harems, but the concept is basically the same. Often, gelada and hamadryas baboon harems have a hanger-on or follower male who is too young, too inexperienced, or too low ranking to acquire his own harem. And that's where some of the stark differences between the two species can be seen.
Hamadryas males actively herd their females using visual and vocal threats. Sometimes they bite. A nip to the back of the neck corrects wayward females who may be intent on defecting to another harem or joining up with a follower male. It can get vicious and stressful from a female's point of view. Hamadryas follower males also try to steal the females — even females that are juveniles not yet mature enough to reproduce. They kidnap the youngsters and guard them until they are old enough to mate with. They do take good care of their young charges by grooming them, helping them up and down cliffs, and watching out for other signs of danger.
Gelada females, in contrast, have much more relaxed lives. Males don't herd them or try to abscond with them or their babies. They live lives characterized more by sisterly bonds than domestic abuse. Gelada females enjoy very tight bonds with other females in their group, spending most of their grooming time on each other. In contrast, their hamadryas counterparts focus most of their grooming on 'the boss.'
Why do gelada and hamadryas females live such different social lives?
The answer is actually remarkably simple: Gelada females are family. Hamadryas females are totally unrelated to each other. Kinship is a powerful determinant of behavior among primates.
Why are gelada females all related to one another?
At sexual maturity, when juveniles complete puberty and become capable of reproduction, they must leave their natal troop, their birth troop, to avoid inbreeding. Among geladas, it's the males who leave. For hamadryas, it's the females who leave. When they join new troops, they find themselves living with unrelated females who are more interested in the dominant male than each other. For gelada females it's just the opposite.
Are the bonds of sisterhood powerful deterrents for male violence? I think the story of the gelada and hamadryas baboons suggests so.