Costs and consequences of the conflict over infant sleep

Abstract

In the accompanying paper [1], David Haig describes the conflict over the inter-birth interval (IBI) in humans. Evolutionary theory predicts a conflict between a mother (who favors a shorter IBI) and her breastfeeding infant (who favors a longer delay before the mother’s next child). Haig argues that selection on the infant to delay the next pregnancy is likely to have been strong over human history, and that waking to breastfeed at night represents a possible adaptation on the part of the infant, as it will tend to delay the mother’s resumption of ovulation. The argument is further supported by sleep patterns observed in infants with Angelman and Prader-Willi Syndromes, which reveal the extension of this conflict to imprinted genes within the offspring, where alleles favor a shorter IBI when maternally inherited and a longer IBI when paternally inherited. But what are the actual consequences of this conflict for human health? The evolutionary model indicates an arms race between the mother and infant strategies. That arms race will escalate until it reaches some sort of resolution. The cause and nature of that resolution will determine the severity of the fitness and health consequences of the evolutionary conflict. One possible resolution is an outright victory by one of the factions. For example, if a maternal trait were to arise that fixed the IBI, making it completely resistant to manipulation by the infant, the IBI would evolve to the maternal optimum, and the coevolutionary arms race would be diffused. The conflict would still exist, in the sense that genes in the infant would still favor increasing the interval, but mutations lengthening the interval would no longer be available to natural selection. In fact, we would expect the infant to lose any IBI-lengthening traits it had previously acquired. The purifying selection required to maintain those traits would disappear, and the traits would be lost, just as sight, and even eyes, have been lost in certain fish that live entirely in lightless caves [2]. The evidence for adaptations to IBI manipulation in mother and infant (and at imprinted loci) suggests that an outright victory has not occurred. In the absence of such a victory, the arms race will continue until the accelerating fitness costs of side effects outweigh the benefits of further escalation. There are two broad classes of costs that are likely to be associated with negative consequences for both fitness and health.