These are the world's smallest monkeys. The marmosets (22 species are found in Brazil and few in adjacent tropical countries) live in both dry and wet forests. Most of them have a body 25 cm (10 in) long, with a 35 cm (14 in) long tail, while the weight is around 250 grams (0.55 pounds). The long tail, used for balance when jumping from one branch to another, is ornamented with 20 white rings. The pygmy marmoset (Callithrix pymaea) of western Amazonia is the world's smallest monkey: just 140 grams (0.3 pounds) for males and 120 grams for females.
The common marmoset (Callithrix jacchus) has a very fine silky fur, hence its scientific name ("splendid fur"). Its ears are covered by large white tufts. They also have white hairs over the eyes, on the forehead. Despite their cute look, the French called them "marmosets", which means "grotesque faces". In fact, the main threat for these monkeys is the pet trade, but also the loss of the habitat, due to deforestation, road construction and industries.
The marmosets are part of a primitive family of monkeys of the New World, including tamarins too. These monkeys have claws, not nails. The claws enable them to climb the trees in the style of the squirrels, with an amazing skill and speed. Marmosets rarely descend on the ground and, in resting position, they stay lain on their belly, with the tail hanging.
These monkeys live in groups, in a well-established hierarchy. Not only males fight for supremacy, but also females engage in aggressive disputes for the social rank. The winner is the female who manages to deliver more slaps and scratches. By the night, marmosets retreat in tree hollows.
Insects make the preferred food, but they also eat fruits, tree sap and leaves, and pillage bird nests, consuming eggs and chicks.
The individuals keep contact via mono or bisyllabic whistles. Affection is manifested by combing the partner's fur with the claws, removing dandruff and other little things (like thistles).
After a 4-month pregnancy, the female gives birth usually to two non-identical twins, which may weigh about 30 grams together. Scientists have discovered in marmosets one of the oddest biological phenomena: "germline chimerism".
In chimerism, some of the sperm or eggs in the individual's gonads carry the genes of another sibling, not their own DNA. 33% of the marmoset offspring were found to represent chimeras. Genetic investigation of these twins revealed that all tissues, from skin, hair, brain, lung and muscle may harbor cells derived from the other twin.
This is possible as in marmosets the placentas of the twin fetuses fuse early in development, and stem cells cross from one to another, before starting their development in different positions. The stem cells are also transferred in utero, and can enter the reproductive organs of the other twin.
As a result, chimeric monkeys may not produce their own genetic offspring (their testicles and ovaries produce the sperm and the eggs of the brother or sister). It would also be interesting to see in marmosets the sperm competition, as the semen contains two lines of sperm cells by origin. The little marmosets can carry the genes of their uncle, not of their apparent father, for example.
In an evolutionary sense, chimerism may promote the system of social care by increasing relatedness between fraternal twins just as the increased relatedness in social insects, like ants and bees, is thought to underpin their social system.
Normally, full brothers and sisters share on average 50% of their genes (theoretically from 1 to 99 %). But in the case of the chimeric siblings, the percentage of the common genes is much higher, as some of their cells bear the full DNA package of their sibling.
Chimerism cases have been signaled in humans, but usually they are connected to sterility. However, twins that 'vanish' in utero may not be that uncommon. Even if we do not have a living twin, we may be carrying chimeric cells from our 'vanished' twin.