Summary

These preceding discussions may be summarized through the development of a four-input framework, which accounts for the key elements and variance in relative brain size within the primates, and between primates as a whole and other mammalian groups. This framework is summarized schematically in Figure 10. First, there is the "grade shift" linked to strong precociality and small body size in early primate evolution (Figure 10, arrow #1). Second is the primate-wide phylogenetic trajectory, exceeding typical static interspecific coefficients (Figure 10, line 2). This component of primate brain-body scaling "preserves" the initial grade shift of early primates and transfers it through larger body size ranges during primate evolution. Because the slope of this general phylogenetic trajectory exceeds any reasonable estimate of "functional equivalence," selection on relative brain size is an integral component of this pattern, whatever the specific selective and ecological bases. Among other key developments in primate evolution, features evolved by the anthropoids and associated with diet, habitat, and social structure/complexity were undoubtedly central to this phylogenetic patterning of brain-body scaling.

The third input involves changes in relative brain size that reflect allomet-ric correlates of simple increase or decrease in body size. This case is represented by line #3 in Figure 10. Such cases are typically relevant to more restricted comparisons of phylogenetically linked species, usually on the genus level. Slope values for these groupings are approximately 0.2-0.4, as predicted by quantitative genetic theory (e.g., Lande, 1979) and the observation that such size-variant series of closely related forms typically exhibit interspecific coefficients resembling values for adult intraspecific variation (Gould, 1975a; Pilbeam and Gould, 1974; Shea, 1983). These phylogenetically restricted comparisons result in marked changes in relative brain size (or EQ's)

Encephalization, Body Size, and Life History Strategies Four components in primate brain size evolution

Encephalization, Body Size, and Life History Strategies Four components in primate brain size evolution

Encephalization Coefficient

log Body weight

Figure 10. A schematized summary of four primary inputs to the patterning of brain-body values in primate evolution. The initial increase (arrow #1) in relative brain size in early primates is translated up to larger sizes via phylogenetic scaling (solid line, trajectory #2). Intrageneric scaling (dashed line, trajectory #3) characterizes simple allometric transformations among closely related species differing in body size. The substantial residual variance about all scaling trajectories is represented here by #4 and the individual data points. See text for additional discussion.

log Body weight

Figure 10. A schematized summary of four primary inputs to the patterning of brain-body values in primate evolution. The initial increase (arrow #1) in relative brain size in early primates is translated up to larger sizes via phylogenetic scaling (solid line, trajectory #2). Intrageneric scaling (dashed line, trajectory #3) characterizes simple allometric transformations among closely related species differing in body size. The substantial residual variance about all scaling trajectories is represented here by #4 and the individual data points. See text for additional discussion.

determined in comparison to broad interspecific patterns (i.e., 0.66-0.75 best-fit trajectories). Specific examples within the primates include the elevated EQ for talapoin monkeys, argued by many to represent a dwarfed Cercopithecus (Bauchot and Stephan, 1969; Gould, 1975a; Shea, 1983, 1992b), and the depressed EQ for Gorilla, viewed by most as a phyletically enlarged African ape (Gould, 1975a; Pilbeam and Gould, 1974; Shea, 1983). Attempts to establish increased or decreased brain size relative to the expectations of size diversification alone, or efforts to reconstruct relative brain size in specific fossil taxa, should utilize the 0.2-0.4 baseline criterion and the phylogenetically most relevant sample of near relatives (Shea, 2005). Strong supporting evidence for this claim has been provided by Williams (2002), who demonstrated a much more precise fit between literature-based assessments of learning ability and EQ determined relative to a log-log coefficient of 0.28 for anthropoids, than between the cognitive assessments and EQ values determined using 0.67 or 0.75 slopes.

Finally, the fourth input in this framework is the key variance about any broad interspecific lines of best-fit, whether these are static or phylogenetic scaling trends. These real increases or decreases in total relative brain size or its component parts—as determined against broad slopes of best-fit (Figure 10, lines 4a) or within specific ancestor-descendant comparisons (Figure 10, lines 4b)—reflect direct selection for altered relative brain size and function unrelated to allometric changes associated with body size. Likely bases include changes in diet, habitat use, social organization, etc., as discussed in various important papers on primate evolution too numerous to list here. The fact that the present chapter differentially focuses on grade shifts and scaling patterns in no way denies recognition of this key variance, or the selective and ecological contexts for such substantial changes in relative brain size and its component parts. For many primatologists, these changes are the most interesting in studies of brain-body relationships; indeed, the initial shift argued here to be related to precociality and small body size provides but one important example. Unfortunately, even when such acknowledgments are explicitly offered, some critics may insist on caricaturing allometric studies and chastising researchers for ignoring the existence and biological significance of variance about general scaling trajectories. One such example is the reaction to the papers by Finlay and colleagues (Finlay and Darlington, 1995; Finlay et al., 2001). In spite of the fact that the stated primary emphasis of these workers was on the internal scaling generalities and developmental sequencing of brain size, and proportions during mammalian ontogeny and phylogeny, they did very clearly acknowledge the existence of substantial important residual variance which many previous studies had cogently shown to be correlated with key taxonomic, ecological, and functional inputs in primates and other mammals. Nevertheless, several papers (e.g., Barton and Harvey, 2000; Clark et al. 2001; de Winter and Oxnard, 2001; Rilling and Insel, 1998) have erroneously argued or implied that Finlay and colleagues in fact viewed all meaningful brain size and proportion variance as rigidly linked solely to developmental and allometric constraints. In light of these critiques, we must redundantly stress here that such nonallometric variance is both pervasive and central to any general explanations of relative brain size in primates and other mammals. In the framework presented here, it is represented by the fourth primary input into overall variance in primate brain size. The goals of accounting for both general scaling trends of brain-body associations, as well as key residual variance in brain size from these broad patterns, are in no way contradictory or substantially at odds. This reasonable and synthetic perspective has been emphasized by Kass and Collins (2001) in regard to the flurry of interest and commentary spurred by the Finlay and Darlington's (1995) paper.

In conclusion, we return to Gould's prescient insight quoted at the beginning of this chapter. A central component of the generally high levels of encephalization observed in our order was indeed the high relative brain size of the early primates. This high encephalization is seen here as significantly related to the highly precocial reproductive strategy, and well-developed neonates and young of the early primates. Their adoption of a life history strategy characterized by marked precociality at small body size proved to be a highly successful, if unusual, evolutionary development for mammals. It also served as a key basis for continued selection for high encephalization and complex sociality in subsequent primate evolution, as Vaughan et al. (2000: 356) stress when they note that "the evolution of an early-primate reproductive pattern involving long gestation and precocial young may have been critical in setting the stage for the highly social lives of higher primates."

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