Thesis #19 – Such adaptive life-history plasticity will sometimes produce detectable trade-offs between survival and reproduction in the range of environmental conditions that prevailed when natural selection established such life-history plasticity.
It is a general, though not universal, rule that, if you give adult animals abundant nutrition, their fertility will increase. This is true in female insects and in female rodents. Among severely undernourished women, ovulation stops. Male mammals with inadequate nutrition have reduced fertility.
The interesting thing about such situations is that, in some species, this reduced reproduction is associated with increased capacities to survive, both under acute stress and over prolonged periods. This pattern has two interpretations: (i) the inevitable side-effect of reduced costs of reproduction; and (ii) an evolved life-history plasticity which enhances average fitness. Note, however, that these two explanations are not entirely opposed to each other. In some cases, animals may face unavoidable reductions in reproduction that then benefit survival, and yet they may evolve physiological mechanisms that strengthen this enhancement of survival under conditions of moderate deprivation.
But as with other instances of adaptive plasticity, adaptive life-history plasticity will be based on contingencies and constraints that were built in ancestral environment, not necessarily current ones. Thus the patterns of phenotypic plasticity that are found among laboratory animals living in environments that are evolutionarily novel to them will not necessarily reveal their benefits.
In exactly the same way, most humans now live in industrial environments that are evolutionarily novel to an extreme degree, particularly from the standpoints of nutrition and activity. Like most, but not all, animals, we are generally selected to economize on effort. Lions will sleep for many hours in a day, hunting only intermittently. On the other hand, many herbivores graze almost relentlessly during daylight. Apparently, bonobos in the wild also seek out food rather relentlessly. As omnivores, our behavioral inclinations toward activity are probably intermediate. But I wonder about the extent to which the slothful inactivity of the middle-aged on agricultural diets arises from inappropriate signaling. The middle-aged in hunter-gatherer tribes do not seem to be comparably lethargic.