Monkeys, much like humans, could be engaged in the process of self-domesticating themselves, altering the course of their own evolution and physiology through the way they behave with one another, new research suggests.
It's long been recognised that domestication in animals promotes certain physical features that aren't observed in their wild counterparts. This phenomenon – called domestication syndrome – has been noted since the era of Charles Darwin and is thought to lie behind all sorts of physical traits and characteristics.
While the term 'domestication' is perhaps most often used in the context of humans domesticating animals, it doesn't always mean that. Scientists also hypothesise that humans unwittingly self-domesticated ourselves – opting for partners exhibiting less aggressive and more social behaviours.
The thinking is that, over generations, those choices may have bred out some of the more wild and animalistic aspects of our ancient demeanour, promoting tolerance and prosocial conduct instead, which in turn could have led to the development of human civilisation as we know it.
It seems a convincing argument, but scientists acknowledge that evidence for this hypothesis remains largely circumstantial.
"It's really a popular and exciting idea but one that lacks direct evidence, a link between friendly behaviour and other features of domestication," says neuroscientist Asif Ghazanfar from Princeton University.
Thanks to Ghazanfar and his team's new research, though, we might have identified such a link.
In experiments with marmoset monkeys (Callithrix jacchus), the researchers found what they claim to be the first 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 one another by taking turns vocalising. In previous research, Ghazanfar and his team showed that infant monkeys learn these vocalisations in a similar way to how babies learn to speak, via social reinforcement from their parents.
This kind of parental feedback has effects on more than just vocalisation technique, however. One of the known markers of domestication in marmosets is a depigmentation trait: a prominent white patch of fur on the animals' foreheads.
The researchers wanted to investigate whether there was a link between the vocal exchanges and this particular morphological feature, which, if it were found, could be taken as evidence of a form 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 of the twins in each pair was given 10 times more vocal feedback than its sibling. The researchers found that the amount of vocal training the animals received was linked to the size 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 change the rate of the marmosets' vocal development, then you change the rate of fur colouration," Ghazanfar says. "It's both a fascinating and strange set of results!"
The researchers suggest that this phenomenon is due to neural crest cells, a form of stem cell that migrates throughout the body during early development.
One of the derivatives of neural crest cells is melanocytes that contribute to pigmentation, and the researchers contend that the simple act of experiencing more vocal training acts as a kind of self-domestication conditioning that affects the young marmoset's developing body.
There's much to still explore about how neural crest cells may be involved in these processes, and the researchers acknowledge their study is small and in want of future replication in separate research.
Nonetheless, it is that rare thing: experimental evidence of how interaction within a species looks to be associated with self-domestication – a breakthrough that might help spur additional discoveries.
"The potential involvement of neural crest cells provides a mechanism by which behavioural experience can be linked to the emergence of morphological phenotypes associated with domestication," the researchers write.
"This in turn 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."
The findings are reported in Current Biology.