Comparing Sociality and Mating in Primates

In the following Blog Post I will be comparing the mating patterns and sociality of representatives of five different categories of primates: Lemur, Spider Monkey, Baboon, Gibbon, and Chimpanzee.  

a.       Lemurs are primates from the suborder Strepsirhini (prosimians.  Their geographic habitat is limited to Madagascar and the nearby Comoro Islands.  Because of Madagascar’s climate extremes due to both tropical rain forests and dry desert topography, Lemurs have developed adaptive traits such as hypo-metabolisms (the ability to hibernate), cathemerality (they are both nocturnal and diurnal), and rigid breeding periods (1).  Spider Monkeys are New World Monkeys (Platyrrhini) found in the tropical rain forests of Central America and northern regions of South America.  Their habitat is the very top reaches of the trees and they use their extremely long arms and prehensile tails to grab tree limbs and move from tree top to tree top(2).  Baboons are Old World Monkeys (Cercopithecoidea) who can be found in Africa. Baboons are extremely adaptable to the environments that they live in, so much so that Baboons living in the savannah region of southern Africa are dubbed ‘savannah baboons’, those living in the mountainous regions are called ‘mountain baboons’ and those living in desert areas of Africa are termed ‘desert baboons’.  They also adapt their diet and their ability to digest and process food to whatever food is available within the habitat, even eating human garbage when necessary (3).  

A Gibbon is a Lesser Ape (Hylobatidae) who can be found in Southeast Asia.  Like the Spider Monkey they make their home up in the trees where they move about from tree to tree.  Unlike the Spider Monkey, they do not have a prehensile tail, instead using their unusually long arms (longer than their legs) and lightweight bodies to move rapidly through and across the trees.  In fact, they are considered the fastest “non-flying tree-dwelling mammals in the world”(4). Chimpanzees are Great Apes (Hominidae) who can be found in Central and West Africa.  Through genetic science we now know that Chimpanzees are our closest living relative, sharing 98% of our human DNA. However, they still often walk on all fours, and enjoy eating and sleeping in trees like some other primates.  Sadly, many species of great apes, including Chimpanzees, are in serious danger of extinction at the hands of humans, due to deforestation and other encroachments upon their natural habitats, as well as exposure to disease and killing for profit. (5)

    b. & c.  A common social pattern found in Lemurs is that while they often roam alone during the day, foraging for food, they sleep in organized groups at night.  In fact, even if the location of the sleeping group changes, the Lemur will still find, recognize, and rejoin his group to sleep with.  Another socialization feature common to Lemurs is that males more often form groups with other males while females group with females. Over time, this has led to the formation of more mother-daughter dyads, or strong pair-bondings between females.  As a result, as pairs of females have joined with other females there has been a shift in Lemur socialization to one of females in the dominate position, which differs from other primates (1).  As mentioned previously, Lemurs have a truncated mating season which is driven by environmental factors such as length of daylight. During this time the female vagina, which is normally closed, will open to allow for mating to occur.  Females are often aggressive during mating, and studies have suggested this is due to the female having more to gain from the reproductive success of the mating session, given the limited resources.  

Unlike Lemurs, Spider Monkeys mate year round with the female selecting a male to mate with.  The process includes both the male and the female sniffing each other before copulation. Spider Monkeys form small, loose social groups of 15-25, and during the day these groups break off into smaller groups of 2-8, only to come back together at night to sleep. This social system is called fusion-fission and is unique to only a few primate categories, including the Chimpanzee.  Another unique social trait of the Spider Monkey is that males stick together and tend to form life-long bonds, whereas females leave the group at puberty.(6)  

The social habits of baboons are very much driven by their environment.  They can vary in group size from just a few to a couple hundred and this is in direct correlation to how much food is available and how hard it is for them to get it.  The harder it is to obtain food, the smaller their groups tend to be.  In addition, some Baboons migrate to higher ground during the wetter months at which point their groups may dwindle in size, only to increase when they move to lower ground in drier months (3). In terms of mating, the female presents her swollen rump to the male as an invitation to mate.  Although males often can mate with whichever female they choose, often they attempt to almost ‘woo’ the female by bringing her food and helping her care for her offspring. 

Gibbons have long been thought to be monogamous, with the male and female pairing up for life and the female giving birth to a single offspring at a time which the parents raise together for the first year, although the child remains under the mother’s care for another 6-7 years.  This pair and their offspring are called a troop. However, recent research has shown that this is not necessarily the case.  According to a study by Brockelman, et al., they observed young adult males of one “troop” moving to a neighboring troop and dispersing the adult male, to pair up with the female to form a new troop. In most instances, these dispersals were not done with force and the new troop adapted fairly quickly to the new structure (7). 

Chimpanzees form fusion-fission social groups much like Spider Monkeys do, however their groups begin with a large community that socializes together as a group and can include as many as a few hundred.  The small sub groups (also called parties) that break off are very fluid, with members of the group changing quickly and they can stay away for multiple days before returning to the group.  The size and make-up of these break-away groups varies and is driven by several factors including mating and resource availability.  For instance, if more food is available the party size will increase dramatically.  While parties can sometimes contain all males groups, all female groups, and parent-child only groups, the party size also increases dramatically if ovulating females are present in the group (8). Another similarity between the Spider Monkey and the Chimpanzee is that it is the female who leaves the natal home upon reaching adolescence (9-14 yrs) as opposed to the male. It is interesting to note that the typical transition period for a female who leaves her home to join a new group is about 2 years, and during this time she is often going back and forth between her new home and her natal home as she tries to assimilate. (8)

After evaluating the the sociality and mating behaviors of Lemurs, Spider Monkeys, Baboons, Gibbons, and Chimpanzees there were several insights I gained. First, I was intrigued by the similarities between the Chimpanzee and the Spider Monkey in two key socialization practices: their fusion-fission socialization process and the fact that both of these primates have the female leave the home as opposed to the male. This was especially interesting in light of recent research showing that Gibbons may not be monogamous, but that in fact the young males are being observed leaving home to take over new troops and disperse the adult male. I have to wonder if there are environmental factors at play among Gibbons that is causing this new phenomenon, although the researchers have not found any indication of this as of yet. To some degree, all of these primates roam in groups, or have a community environment.  It was interesting to note that environmental and reproductive factors play such a big role in the size of Baboons' groups. Especially because the biggest thing that resonates throughout all research is that each of these species is facing serious threats due to things like deforestation and other man-made destruction of their natural habitats. The Baboons appear to show the most promise in the face of this with their ability to do things like move to high ground during heavy rain, and change dietary habits to account for changes in the environment, including being able to digest new foods like human garbage.  Maybe this group size adaptation is another example of that.

1. http://en.wikipedia.org/wiki/Lemurs

2. http://en.wikipedia.org/wiki/Spider_monkey

3. http://www.imfene.org/baboon-biology/baboon-ecology 

4. http://a-z-animals.com/animals/gibbon/

5. http://animals.nationalgeographic.com/animals/mammals/chimpanzee/

6. http://www.worldanimalfoundation.net/f/SpiderMonkey.pdf

7. Brockelman, W. Y., Reichard, U., Treesucon, U., Raemaekers, J. J. (1998). Dispersal, pair formation and social structure in gibbons (Hylobates lar). Behav Ecol Sociobiol, 42. 329-339.

8. http://pin.primate.wisc.edu/factsheets/entry/chimpanzee/behav