Monkeys May Have Self-Domesticated Just Like Humans Did, Study Suggests

  Monkeys, very similar to humans, might be engaged within the process of self-domesticating themselves, altering the course of their own evolution and physiology through the way they behave with each other, new research suggests.

It's long been recognised that domestication in animals promotes certain physical features that are not observed in their wild counterparts. This phenomenon – called domestication syndrome – has been noted since the time of Darwin and is assumed to lie behind all kinds of physical traits and characteristics.

While the term 'domestication' is maybe most frequently utilized in the context of humans domesticating animals, it doesn't always mean that. Scientists also hypothesise that humans unwittingly self-domesticated ourselves – choosing partners exhibiting less aggressive and more social behaviours.

The thinking is that, over generations, those choices may have bred out a number of the more wild and animalistic aspects of our ancient demeanour, promoting tolerance and prosocial conduct instead, which successively could have led to the event of human civilisation as we all know it.

It seems a convincing argument, but scientists acknowledge that evidence for this hypothesis remains largely circumstantial.

"It's really a well-liked and exciting idea but one that lacks evidence, a link between friendly behaviour and other features of domestication," says neuroscientist Asif Ghazanfar from Princeton.

Thanks to Ghazanfar and his team's new research, though, we would have identified such a link.

In experiments with marmoset monkeys (Callithrix jacchus), the researchers found what they claim to be the primary data showing an association between vocal social behaviour in an animal species and a physical domestication trait in individual animals.

Marmosets exhibit a high degree of social tolerance and pro-sociality and communicate with each other by taking turns vocalising. In previous research, Ghazanfar and his team showed that infant monkeys learn these vocalisations in an exceedingly similar thanks to how babies learn to talk, via social reinforcement from their parents.

This kind of parental feedback has effects on quite just vocalisation technique, however. one in all the known markers of domestication in marmosets could be a depigmentation trait: a prominent white patch of fur on the animals' foreheads.

Common marmoset with a white patch on the forehead. (mb-fotos/Getty Images)

The researchers wanted to research whether there was a link between the vocal exchanges and this particular morphological feature, which, if it were found, may be taken as evidence of a type of self-domestication.

In experiments with three pairs of infant twins from three different marmoset families, each of the infant monkeys was given vocal feedback from a 'simulated parent': a computer designed to sound like an adult responding to their own calls.

However, in these sessions, conducted over two months, one amongst the twins in each pair was given 10 times more vocal feedback than its sibling. The researchers found that the quantity of vocal training the animals received was linked to the dimensions of the white patch of fur on their head, with the marker of domestication appearing bigger and growing quicker if they received more talk time.

"If you alter the speed of the marmosets' vocal development, then you modify the speed of fur colouration," Ghazanfar says. "It's both a desirable and strange set of results!"

The researchers suggest that this phenomenon is because of neural crest cells, a sort of vegetative cell that migrates throughout the body during early development.

One of the derivatives of neural crest cells is melanocytes that contribute to pigmentation, and also the researchers contend that the straightforward act of experiencing more vocal training acts as a form of self-domestication conditioning that affects the young marmoset's developing body.

There's much to still explore how neural crest cells are also involved in these processes, and also the researchers acknowledge their study is little and in want of future replication in separate research.

Nonetheless, it's that rare thing: experimental evidence of how interaction within a species looks to be related to self-domestication – a breakthrough that may help spur additional discoveries.

"The potential involvement of neural crest cells provides a mechanism by which behavioural experience is often linked to the emergence of morphological phenotypes related to domestication," the researchers write.

"This successively provides new insights into how selection on correlated phenotypes may have acted during human evolution, as hominins became increasingly reliant on cooperative networks for survival and reproduction."