Marmosets and tamarins (callitrichids) are small platyrrhine monkeys: total lengths range from 40 cm for the Pygmy marmoset Cebuella pygmaea to 75 cm for the Golden lion tamarin Leontopithecus rosalia. The Pygmy marmoset can weigh as little as 120 g. Callitrichids are unique to tropical South [UPDATE: and Central!] America.

About 60 species are recognised, though – as always with extant primates – debate continues as regards which populations should be regarded as ‘species’ or ‘subspecies’. A dazzling array of manes, silky coats, long, tufted tails, ear tufts, moustaches and other visual display structures decorate their heads and bodies; indeed, the group name ‘Callitrichidae’ means ‘beautiful hair’.

Callitrichids have been extensively studied by primatologists interested in their complex social lives. Monogamy is common, (non-identical) twins are typically produced at a time, and males and females are similarly ornamented. All members of a social group assist in caring for babies, and fathers are attentive babysitters, even handling newborns the instant they are born. Sexual maturity is reached quickly – typically at about a year in age.

Because the small body size of callitrichids is unusual compared to the majority of other platyrrhine lineages, it has often been supposed that they are dwarfed relative to the ancestral condition (Ford 1980). The alternative - that they've inherited small size directly from ancestral stem-platyrrhines - has also been suggested (Hershkovitz 1977). Goeldi’s monkey Callimico goeldii is unlike other callitrichids in producing single babies and in having three lower molar teeth (as opposed to two), so has sometimes been regarded as a sort of evolutionary intermediate between callitrichids and other platyrrhines. This view hasn’t been supported by phylogenetic studies, however, since these generally find Goeldi’s monkey to be the sister-taxon to marmosets, not to all other callitrichids (e.g., Canavez et al. 1999, Chaves et al. 1999, Opazo et al. 2006). Tamarins (Saguinus and Leontopithecus) appear to represent a callitrichid ‘grade’ outside the Goeldi’s monkey + marmoset clade (Opazo et al. 2006). While most workers have regarded callitrichids as a distinct ‘family’, close to, but separate from, cebid platyrrhines, others have regarded callitrichids as a cebid ‘subfamily’ (hence, Callitrichinae instead of Callitrichidae).

It’s well known that primates have (mostly) switched claws for nails. Callitrichids are unusual in having claws on all digits except the hallux. This has to be an evolutionary reversal, given the ubiquity of nails elsewhere in primates. Further, callitrichids have apical pads on their digits. These seemingly aren’t used in locomotion and must be leftovers of an original nail-bearing condition (other anatomical features associated with nails are also present, and also seem to be redundant leftovers) (Rosenberger 1977).

These clawed digits are presumably beneficial for a lifestyle that involves speedy, squirrel-like scurrying in the trees as well as vertical clinging. Fruit, plant exudates like gum and latex, and animals including arthropods, snails, frogs and lizards are eaten, with tamarins eating more fruit, and marmosets eating more exudates. Marmoset dentition is specialised for exudate feeding: in some species the lower incisors are huge and curved, being similar in size to the canines, and with a thick outer layer of enamel. While the upper incisors are hooked into the bark, the lower ones repeatedly gouge into it, opening an oval cavity and thereby allowing the gum, sap or latex to seep out.

Has the small size of these monkeys evolved under selection for arboreal locomotion on small branches? Actually, a list of hypotheses have been invoked to explain the evolution of dwarfism in callitrichids, with extensive predation, a reduced availability of resources and/or ecological niche space, and a need to breed fast and breed young all being suggested as possible forces encouraging the dwarfing process (Ford 1980).

And, once again, so much for a 'picture of the day'-type post. More primates soon.

Refs - -

Canavez, F. C., Moreira, M. A. M., Simon, F., Parham, P. & Seuanez, H. N. 1999. Phylogenetic relationships of the callitrichinae (Platyrrhini, Primates) based on 2-microglobulin DNA sequences. American Journal of Primatology 48, 225-236.

Chaves, R., Sampaio, I., Schneider, M. P., Schneider, H., Page, S. L. & Goodman, M. 1999. The place of Callimico goeldii in the callitrichine phylogenetic tree: evidence from von willenbrand factor gene intron II sequences. Molecular Phylogenetics and Evolution 13, 392-404.

Ford, S. M. 1980. Callitrichids as phyletic dwarfs, and the place of the Callitrichidae in Platyrrhini. Primates 21, 31-43.

Hershkovitz, P. 1977. Living New World Monkeys (Platyrrhini) with an Introduction to the Primates. Volume 1. University of Chicago Press, Chicago.

Opazo, J. C., Wildman, D. E., Prychitko, T., Johnson, R. M. & Goodman, M. 2006. Phylogenetic relationships and divergence times among New World monkeys (Platyrrhini, Primates). Molecular Phylogenetics and Evolution 40, 274-280.

Rosenberger, A. L. 1977. Xenothrix and ceboid phylogeny. Journal of Human Evolution 6, 461-481.