Why are marmosets and humans the only known primates that have names?

Names are a rarity in nature. Until now, we only knew of four animal species that use names to call other individuals: humans, dolphins, parrots and elephants. But according to a study published in the journal Science, there is another primate that has to be included in the list. It is not a great ape, but the common marmoset (Callithrix jacchus), a little monkey from Brazil that could fit in the palm of our hand.

I must confess that this discovery did not surprise me too much. In 2019, I attended a Primatology conference in Oxford. I remember that, of the nine talks dedicated to communication, three of them were about the marmoset. They all emphasized that these animals had unusual vocalizations, different from those of other primates.

When it comes to vocal communication, humans are more like parrots, elephants or cetaceans than other primates. We are born unable to speak and we acquire this ability through learning. In contrast, primate vocalizations are generally innate and hardly change throughout their lives. Despite having tried on numerous occasions, we have only managed to get some chimpanzees to learn very simple words like mama. It cannot be said that our primate cousins are completely devoid of vocal learning, but it is very limited.

Even vervet monkeys cannot be considered an exception. These animals have gained a certain fame because they produce an alarm call when they see a snake, another one when they see a leopard and another one when they see an eagle. They sound like words, as if they symbolize the different predators, but we already know that they are not. Vervets are born knowing how to make these calls. Predators simply provoke in them different types of aversive emotions that lead them to produce the different alarm calls.

Naming each of the different individuals in a group requires an advanced vocal learning ability, as vocalizations need to be flexible, capable of being modified and generating a different sound for each individual in the group. So it's no surprise that we didn't know of any non-human primates that had names... until now.

Marmosets have been studied extensively, both in the wild and in the laboratory, because they have a remarkably complex vocal repertoire. Wild populations use at least 13 different vocalizations. Using squeals, whistles and clicks, they mark their territory and warn that they have found food or that predators are in sight. In fact, like vervets, they also differentiate between terrestrial and aerial predators.

One of their most common vocalizations is the phee call, which is used to locate other individuals. It turns out that marmosets live in small, cohesive groups and are extremely social. In the jungle, undergrowth sometimes makes it difficult for the group to come together, so they use phee calls to establish contact dialogues with other members so they don’t get lost. They are considered dialogues because these calls are short and they take turns.

The most striking thing about the phee calls is that they do require some learning, which makes them more flexible. When the babies are born, they begin to emit an immature version of these calls and, only thanks to feedback from their parents, they learn to pronounce them correctly. The learning time required varies greatly between individuals and depends largely on the time that the parents dedicate to them.

Where learning occurs, there is often also diversity. Several studies indicate that different populations of marmosets have dialects, and that these variations are not due to living in different environments or genetics. On the contrary, everything indicates that individuals learn from each other. If you move a marmoset from one population to another, it will end up speaking like its new companions.

The so-called phee

Given all this evidence, it makes sense to ask whether phee calls vary depending on the individual they address — in other words, whether they resemble human names. To find out, a research team at the University of Jerusalem analyzed the phee calls of 10 captive marmosets in several experiments.

The basic procedure was as follows: two monkeys were placed in two adjacent transparent enclosures and allowed to see each other briefly before a curtain was placed between them. The phee calls emitted by the marmosets were then fed into an artificial intelligence system (Random Forest Classifiers). This detected differences between the calls and was able to predict which individual they were directed at.

In the next step, they played the recorded calls back to the marmosets to see if they responded to their names, and they did. However, each marmoset used a different name to refer to the same individual, even though those in the same group looked very similar. Again, this suggests that marmosets learn from each other.

Why are humans and marmosets the only known primates with vocal learning? Since it’s been 40 million years since our lineage split from that of marmosets, it seems we’re dealing with a case of convergent evolution. This occurs when the same trait appears in two groups of organisms independently, such as the wings of a butterfly and the wings of a bat. The interesting thing is to understand what causes these traits to appear.

Some authors suggest that vocal learning is favored in a cooperative breeding system. Among primates, marmoset society is the only one, apart from humans, in which relatives and unrelated individuals actively and systematically help parents in raising their offspring. This is necessary because marmosets always have twins, a situation that is very demanding from an energy point of view. Single mothers cannot take care of the twin pair, so all members of the group help with transportation and feeding.

In our case, humans engage in cooperative parenting because babies are altricial, meaning they are born underdeveloped and extremely dependent. For example, the brain of a human baby is only 25% the size of an adult brain, while the brain of a baby macaque is already 65%. This also means that we are born without the ability to control our voices and that we develop it through the influence of our social environment.

Some studies indicate that human infants who produce vocalizations that more closely resemble human language receive more attention from their caregivers. So there is selective pressure. In this scenario, where there are many potential caregivers, infants compete for more attention. The sooner an infant learns to speak, the better it will do.

Interestingly, although marmosets develop twelve times faster than humans, their offspring are also altricial. Because of twin pregnancy, organ formation is delayed during gestation and they are born less developed than most primates. Among other traits, this can be seen in the poor locomotor skills of newborn marmosets.

We therefore find a parallel between humans and marmosets that may have fostered the emergence of vocal learning: both are born altricial in a cooperative rearing system where babies compete for attention. This is a hypothesis and more research is needed for it to be accepted, but it makes it very clear why it is essential to study animal behavior in order to understand human beings.

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