The mountain gorilla population is small and inbreeding levels are high – and now there’s evidence this inbreeding may explain why some gorillas have distorted facial features
23 February 2022
The degree of distortion in facial features is on the rise in certain endangered gorilla species, along with their level of inbreeding.
Facial asymmetry in primates – including humans – is marked by a sort of spiraling of the facial features around a central point just above the jaw. Once thought to be a consequence of early life challenges, a study in gorillas suggests that the phenomenon may result from inbreeding, says Kate McGrath at the University of Bordeaux in France.
“It’s either that inbreeding is somehow directly affecting their facial development, or that [being] inbred is making them more susceptible to… illness or other things that pop up in early life,” she says. Or it may be a combination of both factors, she adds.
Scientists have recognised facial asymmetry – which is “like there’s a hinge on one side of the face, and compression, which twists the face” – in mountain gorillas since at least the 1970s, says McGrath. At the time, researchers assumed this developed because the animals preferred to chew on one side of the mouth.
Later, other research groups suggested that facial asymmetry might arise from difficult early life experiences, she says. But because mountain gorillas – with a population that, today, only includes 1000 individuals in two distinct groups – have such significant facial asymmetry, McGrath wondered if the trend were related instead to high levels of inbreeding.
To investigate, she and her colleagues ran 3D geometric morphometrics on the skulls of 40 Virunga mountain gorillas (Gorilla beringei beringei) and compared them with those of 40 eastern lowland gorillas (Gorilla beringei graueri) and 34 western lowland gorillas (Gorilla gorilla gorilla). All gorillas were adults and represented both males and females that had died between 1880 and 2008.
They found that the mountain gorillas had nearly twice as much facial asymmetry compared to eastern lowland gorillas, and nearly three times as much compared to western lowland gorillas. The results line up with the level of inbreeding in each population, says McGrath, since western lowland gorillas have been the least inbred of the three subspecies over the past 130 years, and mountain gorillas are “exceptionally inbred” – more so than any other ape, including human groups like the Neanderthals that are known to have been inbred. Eastern lowland gorillas, meanwhile, fall somewhere in the middle.
Environment didn’t seem connected to facial distortion, she says. In fact, mountain gorillas live in protected natural areas with abundant grass, leaves and shoots to eat. And as for chewing side preferences, the scientists found no link between facial asymmetry and tooth wear, thereby overturning the earlier hypothesis.
Primates may even have evolved to recognise facial asymmetry as a sign of a potentially unhealthy mate, says McGrath. In previous studies, both people and animals have shown more attraction to symmetrical faces. “I think it’s a really interesting possibility,” she says. “Basically, is that symmetry a sort of reliable indicator of the genetic fitness of the [individual]? I think our work supports that idea.”
Even so, when populations are small – as in the case of mountain gorillas – mate choices are limited, she says.
While mountain gorillas are the most inbred, western lowland gorillas and eastern lowland gorillas have experienced dramatic drops in population levels in recent decades, mainly due to human diseases and poaching, McGrath says.
This probably affects how much inbreeding occurs, which seems to be reflected in the study’s preliminary time-related findings. “We see that facial symmetries are increasing in all three groups throughout time,” she says. “That fits with what we know from the genetic data, that inbreeding is an ongoing problem.”
Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2021.2564
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