Should you be reading this blog, I assume you have a general understanding that primates are highly social mammals. Great apes, like humans, gorillas, and chimpanzees express even more complex social behaviors.
There are many consequences that living in social groups have towards survivability of the individuals of the group. Sociobiologists have often expressed that groups are natural selected upon just as individuals are in traditonal evolutionary theory — this concept is known as group selection which refers to the idea that alleles can become fixed or spread in a population because of the benefits they bestow on groups, regardless of the fitness of individuals within that group. The benefits a group can bestow, regardless of individual fitness, is higher predator awareness, more variation in foraging behaviors, and socio-emotional cohesiveness.
Groups can also be risky to individual survivability because all groups are regulated in size through environment. Groups which exceed in the size that their habitats can support them often crumble and lead to selective bottlenecks. Larger groups also bring about greater probabilities that disease will spread and effect more individuals compared to smaller groups. A brand new paper, published in Current Biology two days ago, documents a probable cause for why gorilla populations are on the decline compared to the rise of mortality due to the Ebola virus.
The paper, “Gorilla susceptibility to Ebola virus: The cost of sociality” is published from a group of French researchers who hypothesize that social contact facillitated the spread of Ebola virus among a population of gorillas in the Democratic Republic of Congo. This population is now only 3% of what it used to be in 2004. They finalize at the end of their paper that human group sizes have been most likely regulated by a similar disease diaster… but let’s read it in their own words,
“In an evolutionary perspective, this study provides direct evidence that, in hominoids other than humans, group individuals face a higher disease risk. This cost has probably been an important constraint to sociality evolution in early humans “
The authors have ulterior motives from this natural history and conservation tone. Instead, they are using this model of gorilla disease as a tangent to understand human socio-cultural evolution. They summarize that pre-humans were slow to live in large social groups because disease outbreaks could wipe out those who did.
I wouldn’t go as far as to say that’s true. The case for the dramatic spike in gorilla deaths due to Ebola has to be due to the devastation of deforresting and habitat destruction that are forcing gorillas to live closer to each other and in immediate contact to human civilizations, where disease runs rampant. Like I said before, ecology is a determining factor in group size and individual survability. I also say this because in the introductory paragraph the authors outline how the disease was transmitted to these gorillas:
“The large population of western lowland gorillas, Gorilla gorilla gorilla, monitored since 2001 at the Lokoué clearing, Odzala-Kokoua National Park, Congo, was affected in 2004, providing us with the opportunity to address both questions using an original statistical approach mixing capture–recapture and epidemiological models. The social structure of gorillas strongly influenced the spread of EBOV. Individuals living in groups appeared to be more susceptible than solitary males, with respective death rates of 97% and 77%. The outbreak lasted for around a year, during which gorilla social units (group or solitaries) got infected either directly from a reservoir or from contaminated individuals.”
Furthermore, other communicable and zoonic virii such as influenza can be transmitted from encroaching ecotourists and impoverished peoples living in Africa. These diseases are more likley to be foreign to gorilla immune systems, and would severly compromise their ability to fight off an Ebola infection.
Lastly, the more a disease is prevalent the more it emphasizes the increase the advantages of sociality, as outlined by Bonds in “Higher disease prevalence can induce greater sociality: a game theoretic coevolutionary model” In this paper, diseases bring about positive selection for more cohesive groups, whereas diseases more impactfully effect individuals living outside of groups because they lack the benefits reaped from social living (better quality & more abundant food, care, and protection from predation and other stressors). In addition, human immune systems are relatively more complex to other great apes, as are human social groups as in the form of culture and civilization — I most likely belive, then, that diseases bring about more incentive for groups to stick together…
…More importantly though, I think this paper documents the devastation, that even through inadvertantly actions, of the issues facing gorilla survival. I always wonder about well intentioned ecotourists who venture out to view wildlife, spefically gorilla populations and bring their illnesses and diseases with them. In their naivity they end up affecting great ape populations just as significantly as would a poacher or logger.