There are many factors that we need to consider when attempting to reverse this, and some of them also have a bearing on the evolution of our own species. Among these is the question of how gorilla groups are socially constructed and how they interact. It turns out that here, we can't just consider "gorillas" en masse because the two species behave in very different ways. For example, while one species can have multiple silverback males in the same troop, this is rare (but not unheard of) in the other.
The latter species is the more common of the two: the western gorilla (Gorilla gorilla). While the eastern gorilla (G. beringei) is critically endangered because of its tiny surviving population and restricted range, the western species is actually relatively numerous, with as many as 300,000 thought to live across at least four different countries. Unfortunately, however, there used to be at least five times as many just three gorilla-generations ago, and it's that massive ongoing population crash that earns it the "critical" qualifier to its conservation status. You just can't sustain that sort of thing for long.
It's thought that the differences in social behaviour between the two species are heavily influenced by the type of environment in which they live. Eastern gorillas, which include the mountain gorilla subspecies, live in lush forests where food is plentiful, allowing neighbouring troops to share some territory even if silverbacks are aggressive towards one another in defence of their mates. While this will likely become more of a problem if that suitable territory shrinks, things are likely to be different in the more wide-ranging western gorillas, which may have to search harder to find sufficient food of the sort they prefer.
In fact, we'd expect that, if two troops happen to come across the same stand of high-quality fruit-bearing trees, violence is more likely to ensue that it would be among eastern gorillas, who can just move somewhere else. Except... that's apparently not true. Silverback western gorillas are likely to be relatively tolerant of other males - and remember, this is the species that tends to force our rival silverbacks to live on their own for a while. Indeed, when they encounter one another near a good food source, two troops often end up sleeping within 40 metres (120 feet) or so of one another without any apparent awkwardness.
While the social structure of mountain gorilla communities has been studied in some depth, this has been less true of the western gorillas, despite them being more numerous overall. However, such a study was published a few weeks ago, the result of eight years of observations conducted at Nouabalé-Ndoki National Park in the Republic of Congo.
The studied population consisted of four troops, with a total of 51 gorillas, including infants. The largest had the classic structure: a single silverback, eight females, and their offspring. The other three all had at least some non-dominant "blackback" males for some of the study period, mostly because they reached adulthood during it and didn't immediately leave.
The study consisted of almost 6,000 observations of social interactions between the gorillas, recorded by observers following international guidelines to minimise interference and the possibility of spreading disease. 95% of these were either between adults and their infants or between two infants playing together, confirming that gorillas are not, on the whole, very sociable as adults. Nonetheless, that's still around 300 adult-adult interactions, very few of which turned out to be violent.
When groups met at stands of fruit trees, they were rarely aggressive towards one another, with just eight fights between members of different troops being observed across the study period. If anything, such places gave troop members more opportunity to socialise with their neighbours. This suggests that they had an ongoing familiarity with each other, having at least some kind of long-lasting social bond rather than simply seeing one another as rivals.
This reinforces some work from earlier studies that suggested western gorilla society is more complex than we might suppose - or that is apparently the case among eastern gorillas. It is thought that gorilla society is hierarchical, in the sense that multiple troops are gathered together into larger "communities" that share some degree of commonality and tolerance for one another. While these communities have no overall leadership, as human ones might, there is evidence that they are based on ties of kinship, recognising each other as relatives.
Much of this appears to be mediated by juvenile gorillas and blackback males, who are largely playing about and having fun with one another. Western gorillas don't play as much as chimpanzees do, but it seems plausible that, as with chimps, such play helps with social learning and development. It's also possible that these interactions could lead to the multi-troop communities each developing their own "cultural" traditions, albeit at a very basic level. There is some evidence that this happens, although such questions remain controversial.
Significantly, the tolerance between groups even extended to the silverbacks leading the relevant troops, who perhaps had the most reason to fear an intruder who might tempt away their females. A couple of specific events during the study are worth mentioning here. One is that one of the female gorillas did indeed leave the group she was born into on reaching adulthood, moving to one of the other troops. She, however, was probably a daughter of her birth troop's silverback, and so not a potential mate for him anyway. Since this was the only instance of a gorilla changing troops, the silverbacks may indeed have little to fear from rivals who already have their own troop.
Even if that's so, what we would expect is that a silverback would at least be hostile to a rogue silverback living on his own or with an all-male group, since these are exactly the individuals who are likely to move in and take control of a troop. It may be that that's often the case, but in this study, just such a rogue silverback did enter one of the troops, didn't displace its resident leader, and was still there almost a year later, when the study ended.
For at least several months, then, that troop had two silverbacks. This has apparently never been observed before among western gorillas, or at least, not confirmed. Previously, whenever a group has been found with two silverbacks, it proved to be a brief event where a male born within the troop had achieved the status (which happens around the age of 12) and left shortly thereafter. While we don't know what happened in the years after the study ended, that a multi-silverback group persisted for even this long suggests more tolerance than we would have expected.
Such studies, showing that western gorilla troops are more interconnected than we had previously thought, have a bearing on the conservation of this highly charismatic and endangered species. Any understanding of how the animals live in the wild can help improve our ability to monitor groups as they come under continued pressure, and may also help us maintain them in captivity in as natural a manner as possible. But it can also help us to understand how sickness can spread between troops, something that obviously becomes easier if they aren't regularly isolated from one another.
Indeed, in a separate publication last year, the authors of this study revealed how the gorillas in one of the troops caught a respiratory infection from one of the others, first displaying symptoms just days after they had met. When the population is already in rapid decline, any fast-spreading disease could be a threat.
[Photo by Clément Bardot, from Wikimedia Commons.]
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