Around 35 million years ago in our evolution, we diverged from the group of non-human primates we call marmosets. There are 11 known species of marmosets alive today, and since the 1960s, one of these species has been used more than the others in medical experiments in the U.S. That species is the common marmoset (callithrix jacchus). It’s a New World primate species native to northeast Brazil.

Physical features

Catlike size:
These small primates are about the size of a small cat. They’re just 9 ounces in weight and 8 inches in length on average. Over time, they evolved to be smaller, possibly to take advantage of an insect diet, but their size is also part of why they’re suffering in experiments; they need less space for housing so they’re ultimately less expensive to use in research.

Coloring:
Their soft fur is mottled in shades of brown, grey, and yellow.

Tail:
Even though most New World monkeys like marmosets have “prehensile” tails (meaning the tails can be used to grab things), common marmosets use their tails only to aid in balance as they move through the trees.

Like us

Social, vocal, emotional:
Like all primates (including humans), marmosets are highly intelligent and emotional creatures with complex social lives. They’re extremely vocal with one another using high-pitched calls that often sound like birds. Interestingly, mature marmosets don’t often interrupt one another in conversation and instead take turns during vocal exchanges. Common marmosets make at least 13 known vocal calls to one another. Of course, we now know that the ability to feel emotion did not evolve solely in humans, and common marmosets, like all primates (human or not), have this ability.

Family features:
Some of their family relations remind us of our own, too. For example, mature daughters are generally subordinate to their mothers, which experts compare to the relationships between human sons and fathers (Abbott 1984, Rothe 1975). Also, in common marmoset groups, there are adults who help raise the infants but who themselves don’t reproduce. That family trait is found in human families of course but it’s actually rare among non-human primates in general.

Family, Diet

Small groups living in a defined territory in the trees:
These social animals live in small groups called troops. The troops often have 4-15 relatives and, in their natural setting, have a territory that’s somewhere between 1-16 acres. They have an arboreal lifestyle, meaning they spend the majority of their lives (5-16 years in total) in trees.

Unique in having twins:
Single births are the norm for most primates. The common marmoset is an exception in the primate world for giving birth to twins over 60% of the time. Triplets are possible but rare.

It takes a village:
Taking care of twins isn’t easy work, and female marmosets don’t have to do it alone. Marmoset groups cooperate to raise their young; all troop (family) members, including males and siblings, help the mother in raising the infants.

Diet:
In the wild, these “specialist omnivores” eat mostly insects supplemented by tree gum, sap, and resin. They usually spend half of their time foraging in treetops for food.

Next, read why these unique yet familiar beings are used in cruel and unnecessary experimentation.