Over the past 150,000 years humans have manifestly been migratory, travelling into vastly different climatic regions of the globe. Darwinian evolution can lead to genetic differences in populations living in different climates, but any mechanism that can protect individuals from relatively short-term changes in living conditions that differ from those in which previous generations lived will also be highly advantageous. If a mother can transmit to her unborn offspring cues that will affect its stature, metabolism and a host of life-history characteristics, she will be at an advantage in fitness terms over a mother who cannot. Paradoxically the reasons for thinking that this has indeed been the case is best found when things go wrong. Strong evidence points to a link between low birth-weight of humans and subsequent ill-health in rapidly developing countries. I have applied a weather forecasting metaphor to the relations between the state of the mother and the phenotypic characteristics of her offspring (Bateson 2001). Forecasts are often more right than wrong, but mistakes can occur and these are revealing.

Some have doubted that that the idea of weather forecasting provides the best explanation for the poor health of people who were born small and then grew up in affluent conditions, suggesting instead that the evidence can be explained in terms of maternal manipulation (Wells 2007). Ultimately the mother is in control. If conditions are dire, she can spontaneously abort her fetus or abandon her infant. At a more subtle level, she can trade off the benefits of fully supporting her current offspring against those of holding herself in readiness to produce another. This famous principle, first developed by Robert Trivers (1974), is widely accepted by evolutionary biologists and led to the idea of a conflict between the interests of parent and offspring. Yet the conflict principle can easily be overstated and often is. Empirical studies of parent-offspring relations in mammals suggest that both parties adjust their behaviour to the state of the other. Mothers both monitor and respond to the progress of their current offspring, and may actively compensate for the slow development of their young (e.g. goats in Klopfer & Klopfer 1977) and delay breeding again if their offspring is developing slowly (rhesus monkeys in Simpson, et al. 1981). If the mother herself is on a relatively low level of nutrition or is pregnant from a post-partum oestrus, the offspring will take the initiative and wean earlier onto solid food (cats in Bateson et al., 1990, rats in Gomendio et al 1995). Two-way communication between mother and offspring benefits both sides (Bateson 1994).

I believe it is possible to distinguish between maternal weather-forecasting and maternal manipulation. The acid test is whether the small baby will be better suited to the poor environment predicted by the mother’s low nutritional level than a big baby. More evidence is needed than is currently available (Bateson 2001, Chali et al. 1998). Nevertheless, it is obvious that a small individual will require less food than a big individual and therefore will be better able to survive when food is in short supply. So I put my money on the idea that human mothers unwittingly provide to their unborn offspring cues that trigger the development of characteristics that are usually well suited to the environment in which they will ultimately live.

References

Bateson, P. (1994) The dynamics of parent-offspring relationships in mammals. Trends Ecol. Evol. 9, 399-403

Bateson, P. (2001) Fetal experience and good adult design. Int. J. Epidemiol. 30, 928-934

Bateson, P., et al. (1990) Play in the domestic cat is enhanced by rationing the mother during lactation. Anim. Behav. 40, 514-525

Chali, D., et al. (1998) A case-control study on determinants of rickets. Ethiop. Med. J. 36, 227-234

Gomendio, M., et al. (1995) Maternal state affects intestinal changes of rat pups at weaning. Behav. Ecol. Sociobiol. 37, 71-80

Klopfer, P., and Klopfer, M. (1977) Compensatory responses of goat mothers to their impaired young. Anim. Behav. 25, 286-291

Simpson, M.J.A., et al. (1981) Infant related influences on birth intervals in rhesus monkeys. Nature 290, 49-51

Trivers, R.L. (1974) Parent-offspring conflict. Amer. Zool. 14, 249-264

Wells, J.C. (2007) Flaws in the theory of predictive adaptive responses. Trends Endocrinol. Metab. 18, 331–337