October 29, 2010

Do health and reproductive drive cancel each other?

This seems to be the case in a population of feral sheep from a remote Scottish island. Individuals with better immunity, and therefore longer-lived, do not manage to reproduce as successfully as their less healthy peers. However, in the long run they compensate that by participating in more reproductive seasons over their lives.

The two tendencies in dynamic equilibrium run along families.

Full story at Science Daily.

Ref. Andrea L. Graham et al. Fitness Correlates of Heritable Variation in Antibody Responsiveness in a Wild Mammal. Science, 2010. Pay per view.

5 comments:

  1. I actually thought about something related today: namely, whether the advent of agriculture favored lowish fertility in some men.

    Agriculture over almost all time (and in some regions, til today) was hard work. It provided steady food resources - but only for a very limited number of people in addition to those working the fields/ tending the animals. Throughout history and prehistory, the work force was supplemented via unwed males and females --- or worse, slaves.

    With steady and reasonably high-quality food, most farmers would have had somewhere around ten children - something difficult to care for once new farming space has run out, unless war, disease, famine, or "offerings to the gods" diminished those numbers.

    A lower rate of fertility would have resolved parents from ethically questionable measures - making better functioning family units.

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  2. What you say is speculative.

    In my experience 10 kids is quite a lot. My great-granfather was the son of a farmer and only had one sister for instance. He himself only had two children as well (although he lived at semi-urban settings by then). To take another more recent example, the real Tasio, a coal-maker who got his life dramatized in the film of the same name, included seven children (though only one appears in the film). Seven of more was a 1st category "large family" in fascist settings and over 4 it was a 2nd category "large family", entitled to lesser benefits. Contraception was strictly forbidden back then, still people had not so many children normally.

    Typically historians estimate five or so people per "fire" (home), what means a couple and three children on average. Though maybe is an underestimation.

    Anyhow, for what I recall of Economic History classes, in the Middle Ages, there were two demographic modes: the "Mediterranean" one, with large growth spurts followed by large declines (epidemics, famine and such), and the "Northern" one, in which marriage was only at reach of men who had some property or business, this retarded the marriage age of sons till their 30s (when they inherited the farm or got established otherwise) and kept demographic growth at bay. However when more land became available (or more productive), men could marry younger and have more children.

    I'm not sure how this system worked in detail in NW Europe but I know how it did in the Basque Country: only one son (father's choice, often the youngest one) inherited the farm, the rest had to find their place in life in other ways (priests, soldiers, traders, fishermen...), unless the father was wealthy enough to buy other farms. Women either got married or became nuns or remained bachelor at the parents' home. It was by restricting marriage the way reproduction was controlled... where it was at all.

    In principle it's something cultural and economical rather than biological.

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  3. "My great-granfather was the son of a farmer and only had one sister for instance. He himself only had two children as well"

    Perhaps that proves my point? ;)

    Of course it is speculation, and for example, late weaning ages of their kids also reduce female infertility. It's not perfect contraception -- so, if kids are weaned at age ~3, a woman may still have close to ten children. And the vast number of miscarriages happen so early that they hardly delay the next child.

    Not sure about statistics, but in the families of the three of my grandparents that had farms, 5-7 kids were quite typical - and that was with some (modest) form of contraception, mostly to spread the timing a bit. On the third grandparent's side, on the other hand, 0 to 4 kids was the norm through all male relatives.

    Lowish fertility seems incredibly counterproductive in societies in which males have many mating opportunities and should be strongly selected against - it seems to me there must be some advantage, somewhere.

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  4. "Perhaps that proves my point?"

    Are you still talking about human fertility? I don't see it that way.

    What happens among sheep is that they have less children per season but overall the same (they live longer).

    But sheep can have more than one lamb each season and that makes up. Women almost invariably have one single child per pregnancy.

    All your figures seem speculative. People having 0-4 surviving children makes total sense, people having 7 children seems a product of the "Green Revolution", not normal. That's why they are called the baby-boom, even for people born in the 19th century so many children were not normal.

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  5. IIRC, the evidence tends to support larger families in farming communities than in hunter-gatherer communities, mostly attributable to longer nursing times (3-4 years) in hunter-gatherer communities.

    The least developed countries have 7-8 births per child per woman on average, but also have very high rates of infant mortality, child mortality and maternal mortality in childbirth.

    This high rate of mortality was not resticted to the poor. For example, one of the reasons that the United States did not become a monarchy is because George Washington had no living descendants, and very few close living male blood relatives at the time of his death, despite having many siblings who in turn had many children.

    Many men at the time had multiple spouses because their wives died in childbirth. By the standards of royal succession familiar to Americans in the Revolutionary War era, at his death, George Washington's successor would have been George Steptoe Washington (1773–1808), the fourth son (by the third wife) of his oldest younger brother to have a son (George Steptoe's three older brothers predeceased George Washington). All closer male relatives predeceased him, despite the fact that he died at the not particularly old age of sixty-seven.

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