The New World Monkeys: The Dwarfing Hypothesis

THE NEW WORLD MONKEYS: THE DWARFING HYPOTHESIS

Introduction

I will consider support for the dwarfing hypothesis in New World monkeys. Since evolution has shown to result in a general increase in body size, the case of reduced body size in the New World primates is quite unusual. To explain the phenomenon, the dwarfing hypothesis has been proposed (Martin, 1992).

The dwarfing hypothesis implies that there must have been selective pressure that favored a reduction in body size. Phyletic dwarfing is then presumed to be fundamental to the further development of the species' characters (Martin, 1992). Callitrichids are not the only primates that are thought to have undergone phyletic dwarfing. For example, there are also dwarf lemurs and dwarf bushbabies (Martin, 1990).

I will consider the dwarfing hypothesis in the context of analyzing whether the morphological features of the callitrichids are primitive or derived. Although the callitrichids have many common features (including twinning, small body size, reduction of the molar tooth row, simplification of the upper molars, and claws) in this paper I will focus on the morphological features: body size, claws and the dental features of the Callitrichidae (twinning is an important common character of the callitrichids, but will not be considered here). One important hindrance to finding support for the dwarfing hypothesis has been gaps in the fossil record. However, using the available fossil data and modern studies of callitrichids I will show why each feature is primitive or derived, and how it applies to the dwarfing hypothesis.

The New World Monkeys

The New World monkeys (Platyrrhines) can be divided into two families: the Cebidae and the Callitrichidae (the Callimico is also a Platyrrhine and shares many of the characteristics of the callichitrids, but the position of the Callimico within the Platyrrhines will not be considered here). The Callitrichidae family includes the marmosets and the tamarins, and will be the main focus of this discussion. Specifically, there has been disagreement as to whether the callitrichids are primitive or whether they are really specialized dwarf forms (Martin, 1992).

Dental Characters

The callitrichids have triangular upper molars without a prominent hypocone (as is characteristic of the larger-bodied New World monkeys). This makes sense, as smaller-bodied primate species (such as the callitrichids) have less developed upper molars than larger-bodied primate species (Martin, 1992). Although it has been claimed that a missing hypocone on the upper molars is a primitive retention, fossil evidence shows that the ancestral anthropoid had a hypocone. It is therefore evident that the New World monkeys have secondarily lost the hypocone (Ford, 1980). Martin (1992) supports this conclusion, formally using the sister group principle. As Martin writes, "According to this principle, when there is a choice between two alternative character states as the ancestral condition for a particular group, the presence of only one of these characters states in the sister group...identifies that as the likely ancestral state" (Martin, 1992). The sister group principle shows that the simple upper molars are a secondary reduction since Old World simian primates and fossil simians from the Fayum have more complex upper molars with a hypocone. Martin suggests that secondary reduction of the upper molars in the callitrichids occurred because of a change in their diet to insectivorous (insectivory is associated with small body size). The more triangular upper molars of the callitrichids are better for insectivory than the rectangular, hypocone-bearing molars of the larger, frugivorous true monkeys (Martin, 1992).

While the ancestral placental mammal dental formula included three upper and lower molars, the Callitrichidae have lost the third molars. The loss of teeth is the general trend for evolution of the primates, and loss of particular teeth has been shown to have occurred independently many times. For example, Ford (1980) suggests that the third molar has been lost at least twice in the South American primates (Ford, 1980) Both Ford (1980) and Martin (1992) agree that the loss of the third molars is clearly a derived feature of the Callitrichidae.

Rosenberger (1977) found that there is a correlation between the loss of the third molars and facial reduction which provides support for the dwarfing hypothesis (Rosenberger, 1977). It has also been suggested that loss of cheek teeth in the Callitrichidae occurred in order to balance the area of the cheek teeth against the reduced body size of the callitrichids (Martin, 1992) Since these findings show that loss of molars occurred as a special adaptation to phyletic dwarfing, the dwarfing hypothesis is again supported.

Claws or Nails

Another feature common to the callitrichids is claws (as opposed to nails) on all digits except the hallux. The claws of the Callitrichidae can be thought of as intermediate in structure between the claws of non-primate mammals and the nails of most primates. While typical claws are flattened bilaterally and have two histological layers, typical nails are flattened dorsoventrally and have only one histological layer (Martin, 1992).

Martin (1992) states two possible evolutionary scenarios: either the Callitrichidae have retained the claws that were ancestrally present on the placental mammals, or the claws are secondarily developed, and the common ancestor of the callitrichids had flat nails on all digits. The most parsimonious analysis points to secondary redevelopment of the claws in the common ancestor of the callitrichids. This result is confirmed by application of the sister group principle, since Old World monkeys have nails (Martin, 1992).

Ford (1980) suggests that the presence of apical pads in the callitrichids cannot be explained unless we assume that the ancestor of the callitrichids had nails. Like Martin (1992) Ford (1980) uses parsimony to determine if claws are a primitive or derived condition within the Platyrrhines. However, she uses the presence of a terminal matrix and a deep stratum (two characteristics of "true claws") in order to determine whether callitrichid claws are primitive or derived. With new evidence discovered by Thorndike in 1968, of a vestigial terminal matrix and deep stratum in callitrichid nail, she suggests that the connection between the terminal matrix and stratum, and the claws is not as strong as was thought previously (Ford, 1980). Rosenberger's (1977) findings are in accord with those of Ford, as he suggested that callitrichid claws are actually a "derived