In science, there is a term for the notion that breeding out aggression in animals affects their evolution - "domestication syndrome" - and might explain some of the marmoset monkeys' physical traits.
Scientists use the domestication syndrome to explain why dogs appear friendlier than wolves, or why domesticated goats have notably shorter horns and more docile behavior compared to their wild counterparts.
A study focused on marmoset monkeys, led by researchers from Princeton University and New York University, is published in the journal Current Biology.
Self-Domesticating Marmosets
The link for these changes is possibly attributed to specific neural crest cells, present before birth and in newborns. These cells were found to be as versatile as stem cells, later changing into a number of specialized cells like adrenal cells - which affects a species' "fight or flight" response - and some physical traits.
Researchers described domestication syndrome as animal traits created by artificial, or man-made, selection "for increased tolerance humans." They also noted that there were species, like humans and bonobos, that have "self-domesticated." Researchers also noted that these phenomena are only observed, supported by largely circumstantial evidence. In the new study, the size of a domestication phenotype among marmoset monkeys - which is a white fur patch in their faces - is correlated to degrees of their affiliative vocal responding.
"It's really a popular and exciting idea but one that lacks direct evidence, a link between friendly behavior and other features of domestication," said Asif Ghazanfar, an author of the study and a neuroscience and psychology professor at Princeton, in a release from the university.
Among primates, marmosets are known to be extremely social and cooperative. Additionally, they also exhibit physical markers associated with domestication. Ghazanfar notes that marmosets express cooperation through vocal exchanges, taking care of another's young, and sharing food.
Establishing a Causal Link
To examine the connection between the white fur patch and vocal traits, researchers observed infant twins. A pair of twins in the study were subjected to a "simulated parent" - computer programmed with the calls common to adult marmosets. One of those twins responded to 100 percent of these vocalizations while the other one only elicited response to 10 percent of their sounds. These brief sessions only lasted for 40 minutes, taken every other day for the first 60 days of the marmosets. The rest of the time, the infants were with their families.
This work follows a previous study, also from Ghazanfar and his team, that demonstrated how infants with more feedback learned to "speak," developing adult-sounding calls, much faster than their siblings exposed to less feedback. In the recent study, researchers then measured white fur patches on the young marmosets' forehead. It revealed that those introduced to increased parental response grew their white patches faster. Their findings strongly suggest a connection between vocal development and facial fur was influenced by the infants' interaction with their parents.
"Domestication in other species is linked both empirically and theoretically to changes in vocal behavior and vocal learning," researchers cited in their paper. A previous study has demonstrated the difference in vocal qualities between domesticated and wild silver foxes. Similar patterns were also observed with the Bengalese finch, which learns more complex songs in its adulthood compared to its wild counterpart, the white-rumped munia.
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