The aim of most of our current research is to understand variation in reproductive success, longevity and the strategies applied by individuals to maximise their evolutionary success in all kinds of human populations. In our research we use large multi-generational data sets recording reproductive performance and survival of recognizable individuals over their whole lifespan, combined with relevant background information on the local environmental conditions and resource availability for each family. At the moment, our research involves work on long-term studies with individually-based records on:
- historical rural Finns
- historical nomadic Sami of Northern Scandinavia
- historical rural Canadians
- contemporary rural Gambians
We are also keen to understand how our findings on these natural mortality and fertility human populations apply to patterns seen in current developing world countries and modern western populations.
The main questions currently asked involve:
- Constructing heritability estimates for demographic life-history traits in different human populations and measuring selection on them
- Estimating inbreeding and outbreeding effects on fitness in humans
- Studying trade-offs in reproductive strategies
- Examining the causes and consequences of family-living in humans
- Studying the effects of early conditions on reproductive performance and survival
Examples of some of our recent findings include:
(1) Why do women live so long after cease of their reproduction?
In these studies we use the theory developed for understanding family living and cooperative breeding in birds and mammals and examine whether it can be used to understand the evolution and dissolution of family-living and cooperative breeding in humans. First, what are the ecological factors that promote family living? Second, does the presence of potential helpers (pre-reproductive offspring and post-reproductive grandparents) influence lifetime reproductive success and overall fitness of breeders? We have so far shown that women that survived long after menopause gave rise to more grandchildren that those that died early. We examined the family histories of women in Finland and Canada during the 18th and 19th centuries, to determine why humans, unlike other animals, survive long after they are unable to reproduce. In the animal kingdom it is usual for both males and females to continue their reproductive life until they die. We found that the longer a woman lived after the end of her reproductive years, the more successfully her childrens reproductive lives would be. These children tended to begin their families earlier, have a shorter gap between children, have a longer reproductive life and produce offspring that were more likely to survive into adulthood if their mother was alive to help them. We examined the lives of almost three thousand women and took into account different ages, socio-economic status, and social and cultural differences between Finland and Canada. We consistently found that women gained, on average, two extra grandchildren for every ten years that they lived past their reproductive life. In evolutionary terms this gives a benefit as it makes it more likely women who survive long after stopping reproduction will forward more genes to the next generation. Read more in Nature 428: 178-181 (PDF).
Effect of females lifespan on the total number of grandchildren she contributed to the following generation. Graphs show predicted means after controlling for social class, birth cohort and population, and the analyses only include women surviving to post-reproductive age. Adapted from Lahdenpera et al. 2004, Nature 428: 178-181 (PDF).
(2) How do the differential costs of producing sons and daughters affect the parents?
In humans, sons are physiologically more demanding to produce than daughters due to their faster intrauterine growth rate, heavier birth weight, and the longer time it takes mothers producing sons to reproduce again. If raising sons incurs larger reproductive effort for mothers than raising daughters, a male-biased reproductive investment is predicted to cost more. In line with this, we have shown that producing sons and daughters have different consequences for the success of the mothers next reproductive attempt as well as her survival. Every son produced by reindeer herding historical Sami women shortened their post-reproductive lifespan by 34wks, whereas daughters had a positive effect. This implies that the costs of reproduction for maternal longevity may be sex-specific, and that daughters may provide mothers with significant benefits. We show that the differing effects of sons and daughters on their mothers longevity followed from the daughters, but not sons, capacity to compensate for the costs incurred on their mother if they survived to adulthood. This may be due to the human family system where especially daughters help their mothers in their everyday tasks. No effect on the fathers was observed. Read more about sex-specific reproductive costs in Science 296:1085 (PDF) and Proc R Soc Lond B 268:1977-1983 (PDF).
Longevity of Sami women (n=375) with respect to the gender of offspring born to them. Adapted from Helle, Lummaa & Jokela 2002, Science 296:1085 (PDF).
(3) How does increased reproductive effort affect maternal reproductive success and longevity?
We have also investigated the consequences of producing twins vs. singleton children for maternal future reproductive success and longevity. We have shown that the fitness benefits (number of children raised to adulthood) from twinning vary between populations according to local ecological conditions (food availability), and that the population fraternal twinning rates (a heritable trait) correspond with the fitness benefits that mothers gain from producing twins in that population. However, if the mothers increase their reproductive effort by producing twins, this will fasten their immunosenescence and reduce their lifespan due to increased risk of dying of an infectious disease. Read more in Nature 394: 533-534 (PDF), Evolution 52: 430-436 (PDF), PNAS 101:12391-12396 (PDF), J Anim Ecol 70:739-746 (PDF).
(4) Can natural selection affect life-history traits in humans?
Our phenotypic correlation studies have shown that natural selection can favour early age at first and late age at last reproduction, and prolonged post-menopausal survival of the mother in historical humans. These studies have suggested possible phenotypic selection on human life-history traits which has consequences for longevity, but whether such selection could have lead to evolutionary responses depends on the heritability of the traits under selection and genetic constraints of trait evolution. To answer this, we have used our pedigree data to estimate heritabilities and genetic correlations of life-history traits and measures of fitness.
This work is only at its start, but so far we have found relatively high heritabilities of most life-history traits and measures of fitness for females, but not for males, which suggest that male fitness is mainly dependent on reproductive quality of spouse. Our preliminary analyses have also detected significant genetic correlations between age at first reproduction and longevity in females which suggests a genetic trade-off between reproducing at young age, pointing that senescence rate is higher when starting reproducing early. Mean inter-birth intervals also have significant genetic correlation with longevity, and therefore selection on shorter inter-birth intervals would also be associated with shorter lifespan. Read more in PNAS 108: 2838-2843 (PDF), Proc R Soc Lond B 272:29-37, J Evol Biol 15: 803-807.
(5) How do conditions experienced early in life affect adulthood survival and reproduction in humans?
Growth, survival and breeding success of individuals in populations of wild mammals are influenced by the climatic and nutritional conditions that individuals experience during their early development. Recent findings have shown that early conditions also have consequences for subsequent survival and reproductive performance in humans. Environmental conditions which affect early development of individuals, such as the quality and quantity of nutrition received in utero and infancy, predict the onset of many chronic diseases in adulthood and longevity and may also influence a range of measures of reproductive performance in both food-limited and contemporary Western human populations. We have reviewed studies showing how birth weight, season of birth or exposure to pre-natal starvation affect different aspects of an individuals subsequent reproductive success in humans, and the growth, survival and reproductive performance of the offspring produced. We have shown that early maternal and environmental conditions can have a large impact on human reproductive strategies and fitness that can span across generations. For example, we investigated the effects of birth-month on reproductive parameters in pre-modern Saguenay women living in a seasonal environment in Canada. We showed that month of birth can have long-term consequences for the length of a womans reproductive life-span, fertility, number of offspring raised to adulthood, as well as her number of grandchildren born to the population. These differences in lifetime reproductive success and fitness between individuals born in different months were primarily mediated through the effects of individuals birth-month on their subsequent mortality rates in adulthood and on the reproductive output of their offspring. Read more in TREE 17:141-147 (PDF), Am J Hum Biol 15: 370-379 (PDF), Proc R Soc Lond B 270: 2355-2361 (PDF).