The adult sex ratio (ASR) is a key parameter of the demography of human and other animal populations, yet the causes of variation in ASR, how individuals respond to this variation, and how their response feeds back into population dynamics remain poorly understood. A prevalent hypothesis is that ASR is regulated by intrasexual competition, which would cause more mortality or emigration in the sex of increasing frequency. Our experimental manipulation of populations of the common lizard (Lacerta vivipara) shows the opposite effect. Male mortality and emigration are not higher under male-biased ASR. Rather, an excess of adult males begets aggression toward adult females, whose survival and fecundity drop, along with their emigration rate. The ensuing prediction that adult male skew should be amplified and total population size should decline is supported by long-term data. Numerical projections show that this amplifying effect causes a major risk of population extinction. In general, such an "evolutionary trap" toward extinction threatens populations in which there is a substantial mating cost for females, and environmental changes or management practices skew the ASR toward males.
Lizards are interesting.
Interesting, yes, but whether there is a human implication is unclear.
Posted by: Jonathan on December 16, 2005 05:47 AM
Humans, lizards...except for the inability to regrow our tails, what's the difference?
Posted by: Moira on December 16, 2005 06:54 AM
Besides, that's just the abstract — one of the details mentioned in the study is that the males began forcing the remaining female population to wear little tiny burkas.
Posted by: David Fleck on December 16, 2005 07:04 AM
I'm leery of popularizers who extrapolate from results of animal research. Remember the mouse-overcrowding experiment that was used to rationalize ZPG arguments? Sometimes a lizard is just a lizard.
Posted by: Jonathan on December 16, 2005 04:13 PM
No lizard popularizers here. I've been too busy working out my theories about penguins, the nuclear family, and obnoxious misuse of cell phones in public.
Obviously I don't think lizard behavior is directly translatable to human behavior. (You gotta wonder about the little burkas, though.) The abstract sruck me because the researchers did not get the outcome they expected, and this reminded me of the many times I've seen an optimistic "market" outcome for women advanced as the consequence of skewed human sex ratios, without the proposers ever advancing any empirical evidence to support their assertion. My preliminary guess, based, as I said, on my admittedly limited knowledge of history and culture, is that a human sex ratio skewed toward males does not equal higher status/better conditions for human females.
Posted by: Moira on December 16, 2005 06:12 PM
Speaking of sex ratios, it would be interesting to know more about what happened in Paraguay after their defeat in the 1865-1870 war. They took on Argentina, Brazil, and Uruguay and lost, but not after horrendous losses. Here's how my half century old Britannica describes the results:
"The bravery of Paraguayans become a tradition, and from a population of 1,1,337,439 at the outbreak of the war, the country was reduced, according to some estimates, to less than a quarter-million, of whom but 28,746 were men."
The brief accounts I have read say that polygamy was more or less openly accepted for some time after the war, but that Paraguayan society changed less than one would expect otherwise.
Oh, and they got into another big war with Bolivia in the 1930s.
Posted by: Jim Miller on December 18, 2005 08:16 AM
I'm familiar with the Paraguayan example, but don't know anything more about it than what you've stated. I would guess that under conditions of a huge excess of women, things would just trundle along more or less normally, though rather less productively. Now I'm curious to know how long it took for the sex-ratio to re-balance itself. Time for some demographic googling.
Wonder what kind of economy you could maintain, if you had a stable state of only ~11% males?
I recall reading that in Mongolia in the 19th-early 20th century a large proportion of the male population were monks and therefore removed from the productive economy. Can't remember the exact figures, but they were very high. Normal sex-ratio, afaik, and the "celibate" monks had families outside the abbey, only without, it seems, the normal head-of-household obligations. Don't know how well a pastoral economy can support that sort of thing.
Posted by: Moira on December 18, 2005 09:32 AM
Wonder what kind of economy you could maintain, if you had a stable state of only ~11% males?
I don't see how it could be stable unless migration were somehow prevented. And how well could such a society be defended (including from immigration, not that anyone would really want to do that) if there were so few males?
Accepting arguendo the unrealistic premise, I am skeptical that any society with so few males could be very productive. Despite the fantasies of the feminist Left (". . . lots of fat, happy women"), men do most of the economic and physical risk-taking in all societies, and risk-taking is critically important for business formation and economic growth. Perhaps women in the absence of men become less risk-averse (an empirical question), but it seems likely that a society consisting of 9/10ths females would be less dynamic and productive, maybe much less, than would a more sexually balanced society.
Posted by: Jonathan on December 18, 2005 02:51 PM
That's what I'd think, too. But I don't know that it's such an unrealistic premise - you can't naturally sustain such a sexual imbalance, but you can reduce the level of male productivity such that it amounts to the same thing. There's the possible example I gave above, and a common observation about the Third World is that women often seem to do hard physical labor that would be much more efficiently done by men (or machines or draught animals) - while the men enjoy a remarkable amount of leisure or spend all their time trying to kill each other, which kind of cuts into civilization-building time. ("Leisure" in the coffee-drinking and gossiping sense, not in the sense of society's finer minds having the opportunity to think about ways to improve human life.)
Posted by: Moira on December 18, 2005 05:39 PM