Testing Behavioral Ecology Models with Historical Individual-Based Human Demographic Data from Norway

Abstract

Behavior varies widely among animals, both within and between species. An evolutionary perspective may help us understand the origin of this diversity. Behavioral ecology models seek to explain much of variation in behavior as adaptations formed by natural selection. In addition to provide answers why behavior varies under different ecological conditions, behavioral ecology also provides insight into the underlying proximate mechanisms. Although theory has made enormous progress in explaining the diversity of animal behavior, it has often been difficult to empirically test theory on long-living wild mammals. This is because generations times are typically long so that adequate transgenerational data is difficult to obtain. Humans, however, have recorded our own history for hundreds of years and represent a unique model species for testing behavioral ecology theories. In this thesis I use individual-based demographic data from two populations (inland and coastal) in historical Norway (1700-1900) to test predictions from such theories. I focus on life history traits like brood size (twins or singletons) and menopause, and explore how environmental factors (wealth, ecology, maternal parity, and solar activity) can explain variation in human behavior. First, I tested if parent-offspring conflict (POC) could explain why twins are so rare in humans (Paper I). One prediction from POC theory is that twins increase inclusive fitness by the death of a co-twin (‘offspring win’), while twin mothers benefit from the survival of both twins (‘parent win’). However, the results indicated that ‘parent win’ but not ‘offspring win’. Thus, POC could not be a selective force shaping brood size in humans. Next, I studied the interactional effects of status and ecology on women’s fitness (Paper II). The positive effect of wealth on fitness was stronger in the inland agricultural than in the coastal fishing society, indicating that the fitness consequence of wealth may depend on local ecology. The variation in diet may be the proximate explanation for the differences. Prenatal levels of testosterone decreases with maternal parity. I predicted that maternal parity may be important for offspring’s subsequent life histories (Paper III). I found that within mated pairs, the difference between the parity of their mothers was a strong predictor of each couple’s fitness. Sons born to mothers of low parity married to daughters from mothers with high parity had a much higher number of grandchildren than e.g. sons of mothers with high parity married to daughters of mothers with low parity. The results suggest that the environmental conditions before birth can have fitness effects that span generations. Why do women stop reproduce long before they die (Paper IV)? I tested the ‘Reproductive Conflict Hypothesis’ (RCH) that predicts that both younger and older generations should experience fitness costs with co-breeding. Contrary to predictions, younger and older women who co-breed had more grandchildren than those who did not co-breed. I conclude that menopause might be understood in the light of both ageing and increased lifespan. Theory predicts that sexual selection is thought to act more strongly on males than on females. This is poorly studied in humans. I found that wealth/status was an important factor for the probability of obtaining a new partner, ultimately influencing the strength of sexual selection (quantified with the Bateman gradient) on both men and women in these populations (Paper V). Finally, I tested the hypothesis if high solar activity during early development affects human life history (survival, fertility and lifetime reproductive success (LRS)). The results show that a high solar activity (large number of sunspots) at birth decreased the individual’s survival to adulthood (in both sexes) and fertility (in low status women). On average, the lifespan of individuals born in a solar maximum period were 5.2 years shorter, than those born in a solar minimum period. To sum up, in this thesis I have focused on fundamental paradigms within behavioral ecology and demonstrated how their theoretical predictions can be tested using two human populations. As this procedure was successful, it may inspire future research in other human populations to be carried out with a similar procedure.