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ENCEPHALIZATION and gestation in placental MAMMALS JON TOLAAS* Fetal Growth and Brain Size On the basis of data from-a number of species, Huggett and Widdas [1] showed that mammalian fetal growth could be described mathematically as a linear relation of the cube root of fetal body weight to the conception age, t, for t greater than a minimum age, to, H/1/3 = a(f _ to)> t > t0 Extending the cube-root law from ontogeny into phylogeny, they confirmed the existence of a linear relation of cube of birth weight within orders (seven species) and determined the variances of the slope of the relation among orders. However, they were unable to explain very large differences in growth rates between orders. Later work involving the cube-root law has also revealed unexplainable variances within orders [2]· Pointing to the fact that neither genetically determined rates of maximum cell proliferation nor placental type can account for differences in rates of fetal mass growth, Sacher and Staffeldt [2] suggested that fetal growth in mammals might depend on brain weight at birth. Thus, the common limiting factor governing rates of fetal growth would be the rate of growth of neural tissue. This thesis was examined in a study involving multivariate analysis of data on gestation time, neonatal and adult brain and body weight, and litter size for 91 species. The analysis confirmed the hypothesis that the duration of gestation in homeothermic eutherian mammals is much more precisely related to brain size at birth than to neonatal body size or the three other dimensions examined. Gestation time increases with neonatal brain size and with advancement of brain weight at birth, whereas it decreases with * Eid vidg. skule, 6770 Nordfjordeid, Norway.© 1983 by The University of Chicago. AU rights reserved. 0031-5982/84/2701-0373$01.00 Perspectives inBiology andMedicine, 27, 1 ¦ Autumn 1983 | 39 increase of litter size. The authors developed a theory of fetal growth in which the brain is viewed as the pacemaker for growth of other tissues over which it exercises inhibitory control. The underlying mechanism of brain-controlled growth is unexplained, but it is suggested that it may arise from a molecular-genetic upper limit on the readout rate ofgenetic instructions for development. More recent work [3] points to a link between brain size in placental mammals, birds, and reptiles and the maternal metabolic turnover (basic metabolic rate). The purpose of this presentation is to point out the nature of the relationship between adult brain and body weight and the duration of gestation in directly comparable placental mammals, that is, two or more species of the same genus. It is assumed that the brain is the seat of adaptive learning capacities. The Encephalization Quotient Rather than speaking of the "intelligence" of a species, I prefer the term "information-processing capacity," referring to the sum total of adaptive organism-environment transactions that have had to be learned and retained in the species' ecological niche. Like other functional adaptations , this capacity tends to be conserved once it has evolved and is linked to the level ofcephalization or relative brain size ofthe species [4]. Thus, an objective index of cephalization was developed by Dubois [5], who attempted to compare the relative development ofbrains in animals by controlling the body size factor and then determining the residual difference in relative brain development. Following Dubois, several researchers have developed somewhat similar indices (reviewed byJerison [4]). This study is based on the Jerison index [4, 6]. Investigating the evolution of the brain during the emergence of the ungulates and carnivores, Jerison developed a parameter termed encephalization quotient (EQ), which he suggested represents the degree of brain development beyond that required for normal body function. Jerison used gross brain size or volume as a natural biological statistic, assuming its close relationship with such parameters as average neuron density, absolute number of neurons, number of glial cells, size of cell bodies, and fiber proliferation. According to Jerison [4], these measures represent an orderly function of brain size, independent of species, in mammals. Relative brain size is defined as the ratio between actual and expected brain size. Expected brain size in "average" mammals is found by the regression...


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