Life History Theory: Why bigger brains for Primate and How it came to be? Introduction
Complex behaviors and intelligence are the trademark of the primate order and this has been attributed in part to relative large brain to body mass ratio (Reader and Laland, 2002). Defining intelligence, however, is a highly problematic issue; an operational definition used points to the primary component of intelligence to flexible problem solving and the ability to cope with any difficult situations (Jerison, 1973). Arriving at an agreement for the driving factors favoring primate intelligence is very difficult. Many theories have been posed as selective mechanisms for the trends toward increased intelligence in primate evolution. Some emphasize complex ecological pressures, mainly foraging strategies as the primary forces driving an evolutionary increases in cognitive abilities, and other suggest that increased social complexity favored the evolution of primate intelligence (Tomasello and Call, 1997). Across the animal kingdom, brain size increases with increasing body size, however, despite the common scaling principle, brain size to body weight ratios differ from one taxonomic group to another (Jerison, 1973; Gould, 1975). In primates, for example, the brains of apes are generally larger relative to body weight than the brains of monkeys, whereas the brains of monkeys are larger than those of prosimians (Jerison, 1973). Structural differences are also apparent. In chimpanzees, a larger proportion of the brain is devoted to neocortex than in monkeys, who in turn have proportionately more neocortex than prosimians (Martin, 1990; Passingham, 1982). Despite the fact that it is metabolically costly, there has been increase in primate brain size (Harvey et al, 1987). What selective pressures have overcome these costs? When the question is applied to humans, answers typically refer to the adaptive advantages of technology – initially stone tools and language. But monkeys and apes use only rudimentary tools and lack language entirely, yet their brains are significantly larger than those of similar-sized mammals (Jerison, 1973). Thus it can be concluded that, some other selective pressures must be at work. This paper will discuss how the possible selective mechanisms such as complex ecological pressures and increased social complexity have favored the primate cognitive evolution and increased the brain size in the primate order. As the large brain correlates with greater intelligence, we have to take in consideration that the primates would not have been able to evolve larger brains if there was no selective advantage in doing so (Finlay et. al., 2007). Thus in this paper it will discuss how both social and ecological pressures simultaneously trigger the bigger brain in primates therefore evolving the complex behavior and intelligence in them throughout their course of evolution. Life History Strategies
Primates are relatively intelligent compared to other animals, as seen earlier, primates tend to have larger brains relative to their body size, humans are an extreme case, even among primates. Among primates, relative brain size is greater in species with larger home ranges and greater in species that are frugivorous or omnivorous than in species that eat leaves (Clutton-Brock and Harvey, 1980). Species that feed on fruit may face special problems in learning and memory because they depend on widely spaced food that is temporary in both space and time (Milton, 1988; Tomasello and Call, 1997). There have always been speculations as to why are primates have larger brain to body size and are intelligent than other mammals. Defining intelligence is a very slippery issue as it is more than just complex behavior. Many complex behavior among animals do not impress us as particularly intelligent for example, we don’t think that homing pigeons are particularly smart in spite of their ability to find their way home from distant, unknown places...
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