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Biomedical subjects

V Lummaa

Publications and source records attributed to V Lummaa.

5 recordsLinked to original sources
PubMed2004

Menopause: why does fertility end before life?

Menopause is associated with an ultimate cessation of child-bearing potential. Medical research on menopause focuses mostly on the underlying physiological changes associated with menopause. By contrast, evolutionary biologists are interested in understanding why women lose their potential to reproduce before the end of their lives. Evolution by natural selection predicts that the behaviors that we observe today are products of generations of selection on the genes that govern those behaviors. Since one would expect an individual reproducing throughout its life to produce more offspring than an individual stopping early, one would seldom expect genes for menopause to be selected for during our evolutionary past. This article discusses how menopause and prolonged lifespan might be explained by evolutionary theory, and highlights some angles for future research.

Aging↗
PubMed2001

Reproductive investment in pre-industrial humans: the consequences of offspring number, gender and survival.

The number and gender of offspring produced in a current reproductive event can affect a mother's future reproductive investment and success. I studied the subsequent reproductive outcome of pre-industrial (1752-1850) Finnish mothers producing twins versus singletons of differing gender. I predicted that giving birth to and raising twins instead of singletons, and males instead of females, would incur a greater reproductive effort and, hence, lead to larger future reproductive costs for mothers. I compared the mothers' likelihood of reproducing again in the future, their time to next reproduction and the gender and survival of their next offspring. I found that mothers who produced twins were more likely to stop breeding or breed unsuccessfully in the future as compared with women of a similar age and reproductive history who produced a same-gender singleton child. As predicted, the survival and gender of the offspring produced modified the costs of reproduction for the mothers. Giving birth to and raising males generally appeared to be the most expensive strategy, but this effect was only detected in mothers who produced twins and, thus, suffering from higher overall costs of reproduction.

Birth Intervals↗
PubMed2001

Induced abortion ratio in modern Sweden falls with age, but rises again before menopause.

A woman's reproductive value decreases over her reproductive life span and it is therefore predicted that the likelihood of termination of investment in a child decreases with increasing age. An eventual increase in termination ratio in the oldest age groups, as is often found in abortion statistics, could depend on older women on average having larger families rather than on age per se. We used data on abortions and births in Sweden during 1994 to investigate how abortion ratio is related to age and parity of women. We found that age-specific abortion ratio is U-shaped (i.e. that it is highest for the youngest and for the oldest age groups) in each parity class from zero to four children but that age-dependence breaks down in higher parity classes (5, >/=6). Thus, for each of the parity classes 0-4, the incidence of abortion decreases with age up to a point, but increases again as women approach menopause. This late increase in induced abortion ratio seems to depend on age per se. The data indicate that abortion ratio is an inverse function of fertility, and that investment in new reproduction gradually decreases as a woman approaches menopause. Assuming grandmothering as an important driving force in human life history evolution, such a pattern might indicate that the transition from behavioural investment in one's own children to one's grandchildren is a gradual process similar to the decline in ovarian function.

Journal Article↗
PubMed1998

Adaptive sex ratio variation in pre-industrial human (Homo sapiens) populations?

Sex allocation theory predicts that in a population with a biased operational sex ratio (OSR), parents will increase their fitness by adjusting the sex ratio of their progeny towards the rarer sex, until OSR has reached a level where the overproduction of either sex no longer increases a parent's probability of having grandchildren. Furthermore, in a monogamous mating system, a biased OSR is expected to lead to lowered mean fecundity among individuals of the more abundant sex. We studied the influence of OSR on the sex ratio of newborns and on the population birth rate using an extensive data set (n = 14,420 births) from pre-industrial (1775-1850) Finland. The overall effect of current OSR on sex ratio at birth was significant, and in the majority of the 21 parishes included in this study, more sons were produced when males were rarer than females. This suggests that humans adjusted the sex ratio of their offspring in response to the local OSR to maximize the reproductive success of their progeny. Birth rate and, presumably, also population growth rate increased when the sex ratio (males:females) among reproductive age classes approached equality. However, the strength of these patterns varied across the parishes, suggesting that factors other than OSR (e.g. socioeconomic or environmental factors may also have influenced the sex ratio at birth and the birth rate.

Adaptation, Physiological↗