Sizing the grey matter
MUCH speculation has been devoted to the question of what determines the size of animal brains. The reason: relative brain size is the quickest rule-of-the- thumb estimator of mental capacity.
However, before comparing brains of different animals, it is important to make an adjustment to the measurements. This is because there are some variables that 'go with' brain size, but are not in any way indicative of braininess per se. For example, sizes of the brain and the body are strongly correlated; before deciding elephants are more intelligent than dogs, one has to take into account the fact that elephants are 500 times bigger too. Curiously, dietary specialists (obligate fruit-caters or insect-eaters) tend to have larger brains than generalists (omnivores). Also, the larger the duration of pregnancy and the fewer the number of young born at a time, the larger the brain at birth; that in turn leads to an increase in the size of the adult brain.
Until recently there had been no indication that sex or behavioral patterns might influence as gross a variable as the total size of the brain. But not any longer. In a recent study dealing with museum skull specimens (Proceedings of the us National Academy of Sciences), J L Gittleman of the University of Tinnessee classified species into the following categories:
• those which practice solitary female parental care of infants from birth onwards, as raccoons and small cats;
• those which practice biparental care, as wolves and hyenas; and,
• those which practice communal care, in which members of the community help the mother, as African lions and dwarf mongooses.
These categories of maternal behavior span a wide range in terms of the investment - or time and effort - that a mother makes in her offspring. For example, it is estimated that when an animal mother brings up children single-handedly, up to 80 per cent of her total life can be spent attending on her Young.
Uittleman found that relative to their body we@ght- females of species ih have infants whose survival depends exclusively on their mothers, tend to have larger brains. Gittleman's finding raises the possibility that the explosive growth of the brain during the course of human evolution may have been ftielled, at least in part, by what one tends to pass off as the mundane demands of maternity and its attendant tasks. A recent World Health Organization report states that between the ages of 15 and 45, the average woman in Bangladesh and Pakistan spends no less than 50 per cent of her time in pregnancy or breast feeding. The percentage would have been much higher, 80 per cent or more, when lifespans were much shorter than they are today.