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The figures are based on a multiple regression (through the origin) carried out on independent contrasts in brain weight, body weight, percentage of fruit in the diet and social group size. The contrasts were generated using the CAIC programme (Purvis and Rambaut, 1995) and primate phylogeny (including branch lengths) in Purvis (1995), applied to the data in Appendix 7.1. All variables were log-transformed prior to analysis. For overall regression, adjusted r2 = 0.86, n = 67, F = 141.2, p < 0.0001.

The figures are based on a multiple regression (through the origin) carried out on independent contrasts in brain weight, body weight, percentage of fruit in the diet and social group size. The contrasts were generated using the CAIC programme (Purvis and Rambaut, 1995) and primate phylogeny (including branch lengths) in Purvis (1995), applied to the data in Appendix 7.1. All variables were log-transformed prior to analysis. For overall regression, adjusted r2 = 0.86, n = 67, F = 141.2, p < 0.0001.

mals. Within primates, using Purvis's Comparative Analysis by Independent Contrasts (CAIC) programme and his composite primate phylogeny (see Chapter 3) gives a regression slope of 0.55 (r2 = 0.81, n = 106, 95% confidence intervals = 0.49-0.60). Because error variance has a relatively large effect on contrasts calculated between closely related species (Purvis and Rambaut, 1995; Chapter 3), the author repeated the analysis excluding nodes dated in Purvis' composite phylogeny at younger than 5 million years (approximately half the nodes). This gave a higher slope of 0.64 (r2 = 0.84, n = 50, 95% confidence intervals = 0.57-0.72). In fact, it is well known that, even using species as data points, brain-body weight slopes increase with taxonomic level, a phenomenon for which various biological and statistical explanations have been advanced (e.g. Lande, 1979; Pagel and Harvey, 1988a, 1989). Whilst this variability suggests we should not attach too much biological significance to any particular brain size scaling exponent, however calculated, analyses that take phylogeny into account appear to give values closer to 0.67 than to 0.75. The 0.75 scaling exponent for basal metabolic rate is apparently more robust, in that it survives independent contrasts analysis and does not vary significantly with tax-onomic level (Harvey and Pagel, 1991; Martin, 1996). Hence, the claim that similar scaling exponents for brain size and for metabolic rate imply some kind of causal relationship is questionable.

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Gaining Weight 101

Gaining Weight 101

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