by Richard Pallardy
The comedy hot spot at any given zoo is always the primate house. Though the other animal inmates aren’t necessarily slouches in the laughs department (who hasn’t giggled at a deftly timed bowel movement in the pachyderm house or the slap-stick copulations in the chicken coop?), in looking back into the funhouse mirror of evolution, the primates provide the most discernible reflections of ourselves. (Of course: We’re primates, too.)
As a result, observing them might be said to push some of the same buttons relentlessly hammered by reality television. Like the cast of Jersey Shore, monkeys and apes exhibit qualities that suggest humanity while simultaneously behaving in ways that make that designation problematic.
The result in the observer is a combination of discomfiture and superiority, with the end result more often than not being laughter. This feedback between voyeurism and vanity, however, may lead the viewer to ignore the sophisticated social motivations behind such eyebrow-raising activities as public urination and the use of feces as projectiles.
How now, beetle brow?: Comparing chimps and bonobos
Primatologists are making increasing inroads into understanding the evolutionary drivers of these and many more subtle aspects of ape and monkey behavior.
In fact, recent research suggests that launching excrement is correlated to intelligence in chimpanzees (Pan troglodytes, formerly Pan satyrus) and, in evolutionary terms, represented a step forward for primates in general. While it may seem the most primitive and vulgar of weapons to a contemporary human, the primal piece of poo launched by some ancestral monkey millions of years ago actually represented a major evolutionary leap. Not only was it a form of tool use, it may also have been a milestone on the way to the development of language. A study of captive chimpanzees determined that those who were more inclined to chuck their number 2 at unwitting visitors were also those who communicated best with their cohort.
Of particular interest in sussing out the environmental factors that may have urged the evolution of ever more complex forms of intelligence—arguably culminating in Homo sapiens—has been the comparison between the social structures of the chimpanzee and its close relative, the bonobo (Pan paniscus). Both species share roughly 99% of their DNA with humans—bonobos are a little closer to us by some accounts—and they are roughly similar physiologically (bonobos are smaller and slighter). Their societies, however, could not be more different. While chimpanzee groups are ruled by violent patriarchs, bonobo clans are run by pacifist matriarchs. These radically different regimes have profound implications across all of chimpanzee and bonobo life, from food to travel to, of course, sex.
Moveable feasts: Pan manners, or lack thereof
As with all living things, sustenance is primary among the drivers of chimpanzee and bonobo behavior. Both species are largely frugivorous (fruit-eating) and when a plentiful source of fruit or plant matter is discovered, both species usually disperse and eat without competition.
Bonobo repasts, though, are always preceded by sexual arousal in both sexes and a subsequent flurry of sexual contact prior to eating. (Males and females solicit sexual contact from each other, as do females and other females.) The sexual exchanges in bonobos appear to mitigate the aggression that might otherwise define the scramble for edibles.
However, chimpanzees supplement their diets by hunting small mammals (a predilection seen only rarely in bonobos). Hunting is conducted largely by males and consumption of these bloody treats is often dictated by dominance and alliance. While sometimes dominant males will allow the actual captor of the prize—a monkey or small deer—to eat unmolested, they may also claim the meat for themselves, by force if need be. Sometimes, the males will share willingly with their male allies as a means of reinforcing their bonds or offer meat to females in estrus in exchange for sex. Other, less dominant animals are reduced to begging.
Studies in captive apes determined that male chimpanzees experience an uptick in testosterone, a hormone known to trigger aggression, when presented with food, while male bonobos are deluged with cortisol, a stress hormone that usually elicits passivity.
Break up, make up: Chimp and bonobo dominance
While traversing their territories in search of food, the two ape species organize along different lines as well. Both travel in loose aggregations that convene and disperse at will; hence, so-called fission-fusion societies.
However, female chimpanzees often forage alone while the males move together and patrol the edges of the groups’ territory, on the lookout for intruders (who may be killed). Bonobos, though, travel in mixed-sex groups comprising females, their mates, and their off-spring of both sexes. The boundaries between the territories of different groups are permeable and sex is more likely to occur than murder when groups encounter each other.
The male-male interactions between chimpanzees thus define the hierarchy of their groups (with young males successively dominating each female before entering the male cohort). In bonobo society, male dominance is determined by affiliation to the dominant females. The male offspring of dominant females are accorded greater deference than those of lower-ranking females.
Female dominance is established first by seniority and then by affiliation to the senior female. Female bonobos constantly engage in tribidation, genital-on-genital contact. This sexual activity appears to be a means of reinforcing social bonds and of climbing the social ladder. (Dominant females were found to engage in these activities least frequently, indicating the stability of their status and the desirability of sexual exchanges with them.) Females leave their family groups as they mature, forcing them to court the affections of the dominant females of the new groups they hope to join.
Lest you think that homosexual exchanges are exclusively sapphic, male bonobos frequently engage in mutual masturbation for a variety of social reasons as well. Interestingly, bonobos are the only species aside from humans that engage in face-to-face intercourse and tongue kissing.
This constant sexual activity of bonobos’ may account for another major difference between chimp and bonobo societies. Reproductive behavior in chimps can turn into a violent affair; females are often taken by force sexually, and young offspring may be killed by males competing for sexual dominance. Mating is allowed according to male hierarchy; subordinate males are violently discouraged from obtaining sexual access to females. Bonobo females, however, are approached respectfully by prospective partners and males abide by their refusals. Infanticide has never been recorded in the species.
This startling difference can be accounted for by the fact that female chimps are sexually receptive during only a short period of their cycle, meaning that sexual encounters with them are at a premium. Female bonobos constantly mate, regardless of where they are in their cycle; this has the added advantage of making paternity impossible to determine and thus making violence against their offspring unlikely. Why would a male bonobo kill a juvenile who might be his son or daughter?
It is worth noting that violent behaviors are not exclusive to chimps, nor are sexually lubricated exchanges native only to bonobo populations. Chimps do sometimes share and play, and bonobos do fight (though teams of females frequently emerge the victors when a male is the aggressor).
No rain, good for the brain?: Drought, speciation, arrested development
In attempting to account for the seemingly strange disparities between the two species, primatologists have had to follow the ancestry of the species back 2 million years to when they diverged from the chimpanzees. (The chimpanzee and human lines diverged some 8 million years ago.)
This direction in research is a relatively new one. Bonobos weren’t taxonomically distinguished from chimpanzees until 1928, when Harvard researcher Harold Coolidge noticed that a smaller skull in a museum collection that had been classified as a juvenile chimpanzee exhibited fused sutures, indicating its maturity. (His find was scooped by anatomist Ernst Schwarz at the Belgian museum where he discovered the specimen.) Even then, it was classified as a subspecies (Pan satyrus paniscus) until 1933, when Coolidge published further analysis designating it a full species (Pan paniscus). Studies of bonobos in the wild did not commence until 1973 and reliable characterizations of their societies did not emerge until the 1980s. Chimpanzees, however, had been studied in captivity since the early 20th century and in the wild beginning in 1960 with Jane Goodall’s pioneering research at Gombe Stream Game Reserve.
Bonobos are found only in a small area of the Democratic Republic of the Congo, south of the Congo River. North of that divide are chimpanzees, which share their much larger range and resources with another great ape, the gorilla. It is thought that a drought several million years ago led to the local extinction of gorillas south of the river. When the drought abated, bonobos were left with a realm devoid of competition for resources. Because food and territory were more plentiful, one compelling theory holds that less conflict was required for subsistence, aggression became less evolutionarily advantageous and a more passive society developed.
Some primatologists contend that this represented a form of “self-domestication.” Among the hallmarks of domestication are reduced aggression and retention of juvenile traits into adulthood. The features that initially led scientists to classify bonobos as juvenile chimps—foreshortened faces, less noticeable sexual dimorphism—are also features frequently associated with domesticated animals (for example, lap dogs).
Interestingly, bonobos also most closely resemble humans. They’re capable of walking upright and when they do so bear a striking resemblance to the australopithecines (such as the famed 3.2-million-year-old fossil nicknamed Lucy). This begs the question: has the evolution of our species—considered by most of us to be the most intelligent of all—actually been the result of evolutionary arrested development?
That’d certainly go a ways in explaining the existence of certain New Jerseyites.
Image: Bonobo and baby—Sergey Uryadnikov/Shutterstock.com.