The Levels of Gelada Society

Gelada society throughout the Ethiopian mountains is multilayered and dynamic. At its most reductionary level, it consists of two types of fundamental “blocks” or “units”. The most common is the “reproductive unit” or “harem”. These units consist of a dominant female elder, her close female kin (usually one to twelve females), their offspring, a dominant male leader, and occasionally a few “subordinate” or “follower” males. The second type of unit is the “all-male” or “bachelor” unit. These consist of anywhere between two and fifteen males. As we will see, these all-male groups can play a crucial role in the genetic diversity of a reproductive unit and have a significant effect on the power dynamics of life in the Ethiopian Highlands. 

Expanding our scope outwards, the next largest strata of gelada society is called the “band”. But don’t expect to see any geladas holding trumpets or glockenspiels. This band is composed of somewhere between two to about twenty-seven reproductive units and several all-male units. The band members generally sleep and forage within close range of each other. You can also think of the “band” as an “ecological unit”. Since the gathering of units in a band is concentrated within a geographical home range, they influence the distribution of resources and other species within that ecosystem, such as predators, availability, and functionality of grass, roots, rhizomes, and tubers. Part of the benefit of a band over a single unit is greater strength in numbers. Higher numbers mean there are more eyes to look out for predators or available food patches.    

Bands typically share overlapping home ranges with between one and four other bands. These overlapping groups compose a “community”. Communities are less tightly knit than bands with members of the same community associating less than 50 percent of the time whereas units in the same band interact more than 50 percent of the time. The interactions between bands within a community allow for a unique type of fission-fusion to occur. Essentially, “fission-fusion” refers to a type of social organization where the composition of a group changes as individuals move in or out of the “band” or “unit”. Fission, or the splitting of a group, can occur for a number of reasons including a change in food availability or conflicts within a group. These community gatherings can provide a vital opportunity for young males to take over or join reproductive units.  

Finally, you have the “herd” which is made up of about sixty reproductive units that are sometimes from different bands. When herds come together they can be as large as a thousand individuals or more. Woah, that’s a whole lotta geladas. 

Life in the Reproductive and Bachelor Units

For most geladas, social life typically revolves around what’s called the “reproductive unit” or “harem”. Even the fiery adolescent geladas from the bachelor units have their lusty eyes on these groups. The most consistent members of the “reproductive unit” are the females. A dominant female holds the most sway over the social decisions of the unit. The other females and young look to her when it comes time to move the unit to grazing grounds. She tends to be the eldest and has no doubt seen the rise and fall of a few dominant males in the unit in her lifetime. 

While the dominant male will mate with multiple females in the harem, the dominant female will frequently monopolize sexual access to him. This means that she, and occasionally other high-ranking females (closely related to her), will bear the most offspring with him. To be sure, there’s a pecking order. Geladas interact at most with only three other members of the unit. While aggression within a unit tends to be rare, when it does occur, it typically happens between females. 

This dominant male or “harem master”, as some call him, has the reddest chest patch of all the males. The more females he has in his harem the redder his chest becomes so that harem masters with many females (~12) have redder chests than those with less. This colored trait advertises his virility to females and may also serve as a warning signal to rival males. 

It’s thought this red patch serves as a signal to ward off rival males and predators. One day the resident male will be supplanted by a rival male. Until then he must stay vigilant, keeping an eye on those all-male units patrolling the outskirts of his territory. In this zero-sum game of mating, you’re either leading a harem or struggling for access to reproductive females. But this struggle for access can occur in a variety of ways, and gelada males employ a range of strategies to accomplish this. 

Occasionally, unrelated males, known as “follower” or “subordinate” males, will join reproductive units, though this means they’ll abstain from mating for some time (or so it appears) since only the dominant male may mate with the females in the harem. These males act as the group’s babysitters or coalitionary backups should the unit be invaded by rivals. 

Some of these subordinates are unsuccessful rivals; others have left their reproductive unit only to return; others, still, prefer to employ a different reproductive strategy other than taking over a harem. While subordinate males are not supposed to mate with any females in the harem, they often do when the dominant male’s back is turned. In fact, about 17 percent of a unit’s offspring are fathered by subordinate males. Not such a bad strategy after all. Interestingly, when this occurs, both the female and the subordinate male suppress their typically loud mating cries, suggesting they don’t want to be overheard. This is one example of cheating and fear of discovery being observed in wild animals. 

While female geladas stay in a reproductive unit for life, young male geladas typically only remain for four or five years on average and then venture out to start their own unit. This happens in a few different ways: a few simply return to the reproductive unit in which they were raised; others will go off and join another reproductive unit as subordinate males with the dream of breaking off into their own reproductive unit when the females number more than twelve; and, lastly, some will gather their peers and create an all-male unit with the intention of toppling a dominant male leader from his harem. As it turns out, which path an individual takes depends a lot on the social environment in which he was raised.

Juvenile males that form these all-male units are typically the offspring of nondominant mothers. Their blood is surging with androgens, hormones that are associated with male secondary sexual characteristics. While, in pop wisdom, androgens, like testosterone, are associated with aggression, it’s important to note that they don’t actually cause aggression. Rather they amplify pre-existing aggression. They also make the hard things—such as hand-to-hand or tooth-to-tooth combat— easier, even pleasurable, to do. These males typically leave their units earlier than the others, often bringing with them male peers of their own age. That said, these all-male units will have just one dominant leader. In contrast to this group, juvenile males with high-ranking mothers tend to have lower androgen levels. Interestingly, it’s thought that the resources available to the growing juvenile male via its mother’s rank are what determines the androgen levels and pace of their development.

When a rival male topples a dominant male he then looks to the dominant female for acceptance into the group. That’s because when it comes to reproductive units, sure the male draws attention with his flashy chest and tufted cape, but it’s the dominant female who is actually calling the shots. The others in the troop, mostly her close relatives, look to her for decisions, including the acceptance of a new harem master. Should the residing male falter or show weakness in a fight, she will most likely accept the stronger rival. After all, she wants the best for her family, and a male must be able to fend off predators and dangerous intruders. Sometimes the residing male will even fight to his death. With a female leader, this powerful diplomacy takes a gentle touch. Rather than rule by force (as is the case with the gelada’s close relative, the hamadryas baboon), the dominant male grooms the dominant female, making sure she’s content enough to support him when a rival male comes roaming around.