If primates indeed exhibit generally high levels of encephalization because their early relative brain size was translated up to larger sizes during subsequent size and adaptive diversification, do we see comparable trends in other groups of mammals? It is very significant that no other mammalian groups seem to qualify for direct comparison. None of the other candidate groups of small, precocial mammals—such as elephant shrews or hyraxes or selected precocial rodents—ever gave rise to broad and long-term radiations characterized by size increase and adaptive diversification. Other mammalian groups where small body size, moderately large brain size, and reduced litter size (if not full-blown precociality—Shea, 1987) combined are the tree shrews, elephant shrews, bats, and flying lemurs. These specialized lineages have enjoyed variable evolutionary success, but none have further diversified through broad ranges of body size and adaptive zones. The primates stand alone as the only eutherian mammals to fit this pattern—no other mammals started small, lived slowly, and diversified broadly in terms of body size and adaptive strategies.
This may be a fundamental key to comparative studies of mammalian brain evolution. While many groups have produced one or multiple taxa which fall with primates in the higher range of relative brain size for mammals, these are generally somewhat atypical for the general pattern of their ordinal relatives. Examples include the seals and otters among the carnivores and dolphins and other taxa among the cetaceans. The fact that we find no other groups of mammals as uniformly above-average in relative brain size as the primates relates to these basal, ordinal adaptations involving the synergistic relationship between precociality and relative brain size in early primates. If we were to engage in a type of theoretical life history analysis and hypothetically select another mammalian order in which comparable potential for generally high encephalization resides, the best candidate would probably be the Macroscelidea. The bizarre and generally nocturnal elephant shrews might not be the first group which springs to mind when considering high encephal-ization in the mammals, but the evidence of primate evolution suggests there may be tremendous evolutionary potential for such development resident within these small, precocial creatures, which scurry through leaf litter searching for insects and other foods in dry African environments. The chiropterans represent another possibility (Jones and MacLarnon, 2001), though flight has obviously set an adaptive constraint on body size evolution in this order. The earliest cetaceans were likely precocial, and their generally high levels of encephalization are probably related to this in part (Pagel and Harvey, 1988; Shea, 1987), but the order did not originate at small body sizes, and the evolution of extreme size has had a depressing effect on EQ values in this group (Jerison, 1973) and other aquatic mammals (O'Shea and Reep, 1990).
Superordinal relationships within Mammalia are of great interest to many, and attempts to reconstruct these are currently in a state of high activity. I stressed in a previous discussion (Shea, 1987) of encephalization and reproductive strategy in primate evolution that the Archonta of Szalay and Drawhorn (1980), combining primates, bats, tree shrews, and dermopterans, might be linked by some degree of precociality, arboreality, and encephalization as an ancestral adaptive suite of features. Accumulating molecular data indicate that these similarities in reproductive strategy are likely not homologous for the Archonta as thus constituted, since chiropterans consistently root distantly (e.g., Madsen et al., 2001; Murphy et al., 2001). Their life history similarities to primates (Jones and MacLarnon, 2001) would therefore likely not be homologous. However, the grouping of primates with tree shrews and flying lemurs in these and other investigations does suggest the possibility that the common ancestor of these three orders of mammals was characterized by reduced litter size, relatively small adult body size, moderate encephalization, and an arboreal habitus. This is not to blur the real distinctions between primate reproductive strategies and those of the tree shrews (e.g., Martin, 1968, 1969, 1975a,b) and little-known dermopterans (e.g., Wharton, 1950), which obviously manifest their own specializations. But the selective pressures favoring production of a few relatively well-developed offspring in the arboreal environment were possibly operating not only on primates but also on their closest relatives.
The diversity and polarity change of reproductive strategies observed within and across mammalian groups suggests that this character is subject to considerable homoplasy, as might be expected. Nevertheless, it is of some interest to examine what independent and molecular analyses of mammalian higher-level relationships might tell us about the evolution of such life history complexes. The phylogeny of living placental mammals suggested by Madsen et al. (2001) and Murphy et al. (2001) corroborates a close relationship between primates, tree shrews, and flying lemurs. This fits well with a reconstruction of a basal adaptive complex for this triad characterized by a significant degree of precociality, relatively small body size, and moderately high encephalization within an arboreal environment, as noted. Early primates likely further specialized along this trajectory, increasing the precociality and high brain size at small body size.
These molecular phylogenies also support the higher grouping known as Afrotheria, composed of Afrosoricids (tenrecs and African golden moles), macroscelids (elephant shrews), aardvarks (Tubulidentates), and a subgroup comprising sirenians, hyracoids, and proboscideans (Madsen et al., 2001; Murphy et al., 2001). This phylogenetic grouping raises the possibility that the ancestral life history adaptation in the Afrosoricids was one of significant precociality and small litter size (along with moderate-to-high degrees of encephalization), with an important reversal seen within the tenrec (plus golden mole?) component of the clade. It is of interest here that the tenrec radiation runs the gamut from relatively precocial forms (Microgale talazaci, with a litter size of 1-2) to highly altricial ones (Tenrec ecaudatus with a maximum reported litter size of over 30; see Louwman, 1973). African golden moles generally have small litter sizes (1-2), though data on developmental state of the neonates are unknown to me. Additional evidence is clearly needed, but the generally shared precocial reproductive strategy of most of the Afrotherian species raises interesting issues regarding the evolution of life history strategies in these and other mammalian assemblages. Recent published analyses by Symonds (2005) may greatly clarify these issues.
Was this article helpful?