Jonathan MatthewsNaturally, the lion’s share of discourse regarding the ongoing Ebola epidemic has focused on the risks faced by humans. But we aren’t the only species of primate affected by the virus. Ebola has a long history of ravaging gorilla populations that reside throughout Africa; causing thousands of deaths and exacerbating the human impacts that have led to gorillas attaining that foreboding status of ‘critically endangered’.
The obvious consequence of widespread infection is increased mortality and risk of extinction. However, disease also has impact on social dynamics. A recent study published in the Journal of Animal Ecology explored precisely how Ebola impacts the population structure of western lowland gorillas. The study looked at gorillas in the Republic of Congo; who as yet remain untouched by the current outbreak. Surprisingly, it seems that gorilla populations can return to their original social structure relatively quickly after an outbreak of Ebola. The virus led to distinct behavioural changes which helped to compensate for such dramatic population decline. This provides some hope that even after a population is almost decimated, normality can eventually be restored. The process essentially hinges on a reduction of breeding during the outbreak, and increased breeding afterwards. The study may act as a looking glass for observing how animal populations cope with such disastrous viruses, and provides cautious optimism that previous conditions can be restored.
Within a population of western lowland gorillas, there are breeding groups and non-breeding groups. The breeding groups are typically led by a physically imposing patriarch, the silverback male, joined by a small harem of adult females and their young, as well as sometimes young non-breeding adult males called blackbacks. The non-breeding groups contain juvenile members of both sexes and blackbacks. Females are never solitary, while males can be. For example, sometimes during maturation they join so-called ‘bachelor groups’. Upon the death of the silverback, a breeding group disbands. The joining of an adult female with a solitary silverback leads to the formation of a new breeding group.
An Ebola outbreak occurred in 2003-2004 in the Odzala-Kokoua National Park. The outbreak reduced overall survival of the gorillas and lowered their ability to reproduce. During the outbreak, the likelihood of males becoming solitary increased, mainly due to troops disbanding following the death of the silverback. These solitary gorillas had a higher chance of survival than their grouped counterparts, as sociality increases the likelihood of disease transmission. In addition, females were shown to transfer from breeding to non-breeding groups. Immigration of breeders into the affected population greatly reduced during the Ebola outbreak, and new breeding groups were much less likely to form during, as gorillas became solitary or joined non-breeding groups.
However, after the outbreak has passed immigration rates return to those of the pre-epidemic population. In addition, the formation of new breeding groups shot up, before eventually returning to ordinary levels. This formation of new breeding groups and immigration following an outbreak is a compensation for the decreased reproduction and mortality during the outbreak. In fact, it is thought that 6 years after an outbreak, original social dynamics and survival rates were restored – demonstrating how gorillas can fight environmental change.
Unfortunately, abandoning the vacuum packed arrangement of human society for distinct breeding and non-breeding units seems an unlikely strategy for coping with the Ebola outbreak. These findings do however bring the discussion of other animals into what has so far been an anthropocentric concern; reminding us that we’re not the only ones affected by Ebola. We should remember that the surviving gorilla population is still very low, and human activity currently only looks to decrease it. Nevertheless, the survival of the western lowland gorilla in the face of adversity provides a glimmer of positivity in what has been an understandably morbid conversation. It presents us with a reminder of the strangely oxymoronic state in which nature exists; simultaneously possessing a kind of porcelain fragility and astounding resilience to change.