Western Lowland Gorillas—New research on their social structure and possible implications for the transmission of ebola virus

It is good to see old friends in print. This time it is the Western Lowland Gorillas we were privileged to visit and watch at Ngaga in the Odzala-Kokoua National Park of the Republic of Congo (Congo, Brazzaville) nearly five years ago (see my post of 18 August 2014).

We visited two groups just a few miles apart and it is the interactions that occur between the three group in the immediate vicinity of Ngaga that is the subject of a paper just published in Proceedings of the Royal Society.

I cannot overemphasise the enormous amount of work that went into this study done between 2013 and 2017. Behaviour of the groups was monitored on approximately 305 days each year. The trackers (who move at speed through the forest) located the groups each morning. Every individual gorilla can be recognised by the trackers and researchers. Notes were made of their behaviour for an average of two hours per day, itself a difficult undertaking because of the dense vegetation. And the authors do not even mentikon the sweat bees.

The behavioural work was done by Magdalena Bermejo (who heads the whole project in the Republic of Congo) and Germán Illera, with help from the same trackers who took us from the lodge to the groups of gorillas.

In addition to the behavioural observations, gorillas over a wide area were genotyped from samples of their faeces collected from the nests they build and occupy at night.

Previous observations on Western Lowland Gorillas (Gorilla gorilla gorilla) at bais (open grassy clearings often surrounding a body of water) suggested a very different behaviour from the Mountain Gorilla (G. beringei beringei). Groups of the former met without aggression—in marked contrast to encounters between group of the latter. In Ngaga, an area without bais, these observations were not only confirmed those but indicated a dynamic community structure. In the words of the authors:

Both approaches revealed a social system much more dynamic than anticipated, with non-aggressive intergroup encounters that involved social play by immature individuals, exchanges of members between groups likely modulated by kinship, and absence of infanticide evidenced by infants not fathered by the silverback of the group where they were found. This resulted in a community composed of groups that interacted frequently and not-aggressively, contrasting with the more fragmented and aggressive mountain gorilla (G. beringei beringei) societies. Such extended sociality can promote the sharing of behavioural and cultural traits, but…

The but refers to the disease that has devastated local gorilla and human populations, including the area studied—ebola. The authors make the point that interaction between groups could promote the rapid transmission of the virus:

Social behaviour may thus have greatly contributed to the massive impact of past Ebola outbreaks that have resulted in an increase of the threat level for the species, raising major conservation concerns about population declines in the future. Understanding group dynamics in social species is of utmost importance when coming to model the transmission of pathogens such as Ebolavirus. However, since the high mortality imposed by outbreaks is likely to select against this social behaviour, its persistence in W[estern] L[owland G[orilla] implies that either such massive die-offs may have been rare in the past, or that the associated benefits outweigh the disadvantages. In any case, the peculiar social behaviour of western lowland gorillas is an outcome of its evolutionary history and will definitively impact its fate.

I do wonder though whether the dynamic social structure and outbreeding may actually ensure a higher degree of immunocompetence of individuals. In other words, and turning the argument round, whether the social structure is actually protective to individuals exposed to potentially lethal infections.

Here is a link to the video included in the paper.

And here  again are my videos of the gorillas at Ngaga:

Forcina G, Vallet D, Le Gouar PJ, Bernardo-Madrid R, Illera G, Molina-Vacas G, Dréano S, Revilla E, Rodríguez-Teijeiro JD, Ménard N, Bermejo M, Vilà C. 2019. From groups to communities in western lowland gorillas. Proceedings of the Royal Society B 286: 20182019. http://dx.doi.org/10.1098/rspb.2018.2019 

Ebola: Gorillas and other wild animals

Female gorilla in the Jupiter Group
Ngaga Camp,
Republic of Congo, May 2014
Her toddler was clambering in the trees
above her

The conversation at the funeral of a former colleague with another former colleague, a microbiologist with wide interests who I had not seen for some years, turned to the outbreak of ebola virus (EBOV) in West Africa and how it may spread between wild, domesticated and human mammals, and the characteristics needed for one or more reservoir species, in other words, those that carry but do not suffer to the point of rapid mortality the infectious agent.

In May we were at the site of the 2003 outbreak in the Republic of Congo and heard of the devastating effect ebola had on the Western Lowland Gorilla population. Since returning I have had the chance of catching up with the literature on that outbreak and how it was established that ebola has such a devastating effect on the survival of gorillas, with a mortality rate of 90-95% and the estimated loss of 5000 gorillas from a relatively small area.

Magda Bermejo who lives at the camp in the Republic of Congo we visited (see post of 18 August 2014) after the effective loss of her study groups at Lossi to the ebola outbreak, established the death of gorillas and chimpanzees during human outbreaks, found ebola virus in a number of the carcasses and showed an apparent passage from group to group of gorillas rather than independent transmission form a putative reservoir species. But then, and apart from the enormous potential effect on survival of the great apes in Africa, all sorts of question arise as to who or what becomes infected first and how does the infection (not that easily transmitted according to the evidence with contact through body fluids being needed) pass between groups.

Bushmeat has been implicated as the initial source of human infection while bats, particularly, but also duikers, pigs and domestic dogs are suspected as possible reservoir species. Recently, I see those sequencing the virus collected during the current outbreak have found rapid changes and are reaching different conclusions on the origins of EBOV. The article by Gretchen Vogel in Science (29 August 2014) reads:

Some researchers theorized, based on early sequencing data, that EBOV had circulated for decades, undetected, in animals in the region. But the new analysis, strengthened by the unprecedented number of genomes, supports another theory: that the virus spread, via animal hosts, from Central Africa within the last decade. Researchers aren’t sure which animal to blame, but fruit bats are their leading suspects (Science, 11 April, p. 140). At least one fruit bat species known to carry ebolavirus has a population range that stretches from Central Africa across to Guinea.

That more research on ebola and much more money to fund that research are necessary are truisms. The dangers to those working in a human outbreak to collect samples in areas where facilities for the safe care of patients and basic hygiene are poor also cannot be overestimated. Five of those who collected samples for sequencing  in the current human outbreak have died. Until really hard evidence is obtained from extensive surveys in wild (and domestic) animals over meaningful periods of time and over a wide geographical area, speculation and soft epidemiological evidence may have to suffice in the meantime. But, in that meantime and beyond, alongside local outbreaks that devastate human populations, the great apes of Africa are at risk.

Tom, our former colleague, would I am sure have been delighted that his funeral was the occasion for several coffee-room type discussions on science and science politics; Scottish politicians were not rated highly in the latter.

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Useful starting points for further reading:

Bermejo M et al. Ebola outbreak killed 5000 gorillas. Science 314, 1564
Vogel G. 2014. Are Bats Spreading Ebola Across Sub-Saharan Africa? Science 344, 140
Vogel G. 2014. Genomes reveal start of ebola outbreak. Science 345, 989-990
Walsh PD et al. 2003. Catastrophic ape decline in western equatorial Africa. Nature 422, 611-614
Wittmann TJ et al. 2007. Isolates of Zaire ebolavirus from wild apes reveal genetic lineage and recombinants. Proceedings of the National Academy of Sciences of the USA 104, 17123-17127