The maternal energy hypothesis, as elaborated by Martin (1996), suggests that the need for a mother to sustain brain growth lies at the heart of the time course of a reproductive event, from conception to weaning. Thus, while the mother's body size and condition determine some of her ability to allocate milk resources for growth, the growth requirements are twofold and possibly separate: firstly, that of somatic growth to a metabolic weaning weight (as noted above); and, secondly, that of brain growth during gestation and lactation.
Assuming that the majority of growth from neonate brain mass to adult brain mass occurs during lactation - a model that fits most of the nonhuman primates (Martin, 1983) and may apply to humans when an average duration of lactation is four years (Dettwyler, 1995) - then the change in brain mass should be a function of maternal energy capacity via the mother's metabolic rate and body mass relations (Martin, 1996). A relationship between brain mass and maternal mass has been noted for a variety of taxa (Martin, 1981; Gittleman, 1994), with further effects of brain mass on other life history parameters (e.g. age at first reproduction; Harvey et al., 1987) for primates.
As predicted, adult brain mass scales with the duration of a reproductive event from conception to weaning among the primates in this sample. Larger-brained primates have longer reproductive events (r = 0.77, n = 38, p < 0.001). However, when the effects of maternal body mass are removed from the duration of a reproductive event and from the change in brain mass between birth and adult mass, the residuals show only a weak relationship (r = 0.42, n = 14, p = 0.13)! The underlying factor in the relationship appears to be simply that of maternal mass. In part, this is to be expected because mothers must ensure brain growth through lactation, and milk energy is itself a function of maternal mass (Martin, 1984; Oftedal, 1984). But, as noted above, the residual variation in interbirth intervals does correlate with residual brain mass change. Thus, primates with greater change in brain mass than predicted for their maternal mass also tend to have more potential to prolong interbirth intervals than is expected for size alone. This finding provides additional support for both the maternal energy hypothesis and the metabolic constraints hypothesis, in that expensive growth processes can be more effectively supported when time variables are labile and thus sensitive to local ecology, individual condition or social status.
As noted above, more expensive brain growth is associated with relatively higher somatic growth costs (relative neonate mass and relative wean mass), suggesting some concordance or rate constraints in growth. A further trend appears in relation to brain growth, which might be unrelated to its metabolic costs. The strong association between relative M1 eruption and relative brain growth (e.g. Smith, 1992), also noted here, suggests that, despite the tiny sample size, when maternal size is removed, brain and tooth growth appear to covary. I would suggest that one expense of growth is that of maintaining a suite of rate-limited traits that all contribute to the energy burden on the mother.
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