The native monkeys of the Americas are generally smaller and frailer than the related species from the other side of the ocean. With anatomically simpler brains, they are often regarded as less intelligent and less capable of carrying out complex cognitive tasks, such as communicating voluntarily with other members of their group. Now, however, new arguments have arisen in favor of revising this line of thought.
Brazilian researchers obtained, for the first time, consistent empirical evidence that monkeys from the so-called New World engage in behaviors that are just as complex, if not more, than those of their cousins from the Old World, said to be more evolved. Experiments involving groups from Rio Grande do Norte, São Paulo and Brasília showed that a type of marmoset from the Mata Atlantica (Atlantic Seaboard Rainforest) and from the Caatinga brushlands makes whistling noises and cries out to communicate elaborate information and not merely basic primordial emotions such as pain, fear or excitement.
At the laboratories of the Edmond and Lily Safra International Institute of Neuroscience of Natal (IINN-ELS), marmosets of the Callithrix jacchus species were put through a battery of fairly simple tests. Cristiano Simões and his collaborators made six marmosets listen to the recorded sounds of other animals of the same species, for 45 minutes. They separated the monkeys into two groups (one that responded to the calls, by crying out and whistling spontaneously, and another that kept quiet) and afterwards analyzed what had occurred in their brains.
Using a protein that accumulates in activated brain cells, the researchers found that the act of vocalizing activates three important regions of the cerebral cortex, the most superficial cell layer of the brain. In primates, it is connected to performing complex tasks such as attention, language and awareness. “These areas undergo a continuous transformation when the animals hear or emit sounds,” explains Koichi Sameshima, a neuroscientist from the University of São Paulo and one of the authors of the study.
“Many researchers believed that New World monkeys only made sounds when they felt emotions such as fear or pain, which involve activities of the subcortical areas, more primitive brain areas,” says Sidarta Ribeiro, the coordinator of the research, published in Frontiers in Integrative Neuroscience.
What was most striking was the activation of the pre-frontal ventrolateral cortex. Located near the temple, this region, described in 1861 by the French anatomist Pierre Broca, is associated in humans with the understanding of language and the control of speech. “People with a lesion in this area make sounds, but are unable to talk in an articulated manner,” explains Luiz Eugenio Mello, from the Federal University of São Paulo and co-author of the article.
“Even though marmosets have a far smaller cortical area than humans and than the Old World monkeys, with whom they share an ancestor some 40 million years ago, marmosets already have a cortical circuit connected to the sounds,” states Ribeiro, an IINN-ELS researcher and a professor at the Federal University of Rio Grande do Norte.
The suspicion that the cerebral cortex might be involved in the sounds from New World monkeys is not new. In 1967, the researcher Uwe Jürgens, from the German Primatology Center, observed that there was activity in the cortex of the Saimiri sciureus marmoset. However, some doubts remained. In these experiments, Jürgens applied electrical stimuli near the cortex areas connected to emitting sounds. It was unclear whether the animals made sounds because of the electrical stimulus or spontaneously. By making the animals listen to the voice of their companions, the Brazilian group cleared up this doubt. “Our work makes it clear that a voluntary vocal control system exists in New World monkeys,” says Ribeiro
It is still too early to find out whether the these little monkeys intend to express themselves – and, for instance, warn others that they are lost, in danger or have found food – when they make their characteristic noises. “We still need to find this out, but perhaps the sounds emitted by these monkeys are intentional,” comments Mello. If this comes to be proven, this intentional character will not surprise many researchers. “Vocal communication makes it possible to overcome visual barriers and is a fundamental need for human beings,” explains Mello. “It probably didn’t appear from one minute to the next in our species, but it must have developed in other monkeys and evolved over the course of millions of years.”
At present, Ribeiro’s group is investigating whether marmosets can learn and use symbols. “If we demonstrate this,” says Ribeiro, “then we’ll try to find out the neurophysiological process underlying this phenomenon.”
SIMÕES, C. S. et al. Activation of frontal neocortical areas by vocal production in marmosets. Frontiers in Integrative Neuroscience. v. 4, p. 1-12. Sep.2010