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Some guys feel like martyrs when their wives ask them take out the trash. Before complaining about what men need do for the love of women, the grudging trash-haulers should consider the plight of male redback spiders (Latrodectus hasselti). If Romeo and Juliet had been L. hasselti instead of H. sapiens, Shakespeare's tragic suicide story would have ended very asymmetrically, with only Romeo dead, and Juliet happily gorging herself on his remains. In the act of love-making, the star-crossed arachnid Romeos somersault right into their lovers' waiting jaws. Besides being less than demure in courtship, female redback spiders are also far from petite, weighing over 50 times more than their male dining companions. Thus, the meal, from the perspective of a human female, would be the equivalent of a large steak and a couple of potatoes, though the spidery sperm-donor probably tastes more like lobster.
For her, a little loving and a satisfying dinner. But what's in it for him?
At one time, biologists might have explained the male's behavior as "good for the species." In 1962, V.C. Wynne-Edwards argued that apparently self-sacrificial behaviors can serve the good of the larger population. Wynne-Edwards offered many examples, from celibate birds helping other birds raise their young, to suicidal mass migrations of lemmings. Though migrating lemmings might die, and non-mating birds might not reproduce, the remaining population profits from lessened demand on local resources. Most biologists now reject this argument, believing that individual selection always trumps group selection. On closer examination, apparent self-sacrifice usually reveals the selfish rules of evolutionary economics. Ornithologist Stephen Emlen found that birds only help at the nest, for example, when nest-mates are close genetic relatives of theirs, and when solo mating efforts are likely to fail.
Because apparent self-sacrifice often boils down to kin selection, one possibility is that red-backed spiders offer themselves as meals because it helps their offspring survive. David Geary amassed a great deal of evidence that resources invested by human males dramatically affect their children's survival. But when zoologist Maydianne Andrade conducted careful analyses of red-backed spider matings, it did not appear that paternal investment was the key to explaining male self-sacrifice. Cannibalistic matings did not result in significantly more egg production, or heavier eggs, as would be expected if the male's bodily resources translated into nutrition for his offspring. Instead, Andrade's research led her to attribute male self-sacrifice to sexual selection, the same process that brought us peacock's feathers, the song of the meadowlark, and teenage rock bands with loud amplifiers.
Sexual selection: Why it pays to buy dinner
Darwin developed the idea of sexual selection to explain characteristics, such as peacock's feathers, that exact survival costs on their owners. A peacock's display is like a neon advertisement to predators: Eat here! While reducing the male's probability of survival, though, such characteristics increase his chances of producing offspring. Differential reproduction is, after all, the bottom line of evolution. Animals that live long celibate lives do not produce offspring, those that live shorter, but sexier, lives do. Darwin distinguished between two types of sexually selected characteristics: Those that help an animal attract mates (as in the beautiful feathers on a male bird of paradise), and those help an animal compete with other members of its own sex (as in the rapier tusks of the male elephant seal).
Andrade noted that the webs of individual female redback spiders contained up to 6 males at once, suggesting the males were competing for reproductive opportunities. Dissections of females suggested that the females rarely mated with more than one or two of their suitors. To further examine the possible reproductive advantages of self-sacrifice, Andrade used a "sterile male technique." She made half her sample of male spiders sterile by exposing them to 11 krad of gamma radiation. She then induced females to mate with a normal male and an irradiated male. Stopwatch in hand, she calculated the length of copulation. She also recorded whether the female consumed the male, and estimated how many eggs each male's sperm reached (eggs exposed to sperm of irradiated males would not be fertilized). By allowing themselves to be eaten, males increased the average time of contact from 11 to 25 minutes. Consequently, sperm from males who were also meals reached twice as many of her eggs as did sperm from males who did not provide dinner.
How does this relate to human beings?
Humans don't usually take it this far, but are there any parallels? Courting men are likely, if not to be dinner, at least to buy dinner. This may reflect a process of sexual selection, in which women choose men who demonstrate better abilities to provide than their competitors. When John Marshall Townsend dressed up his research assistants to look like either successful young stockbrokers or Burger King employees, females were attracted to the guys who looked wealthy; males tended to ignore the outfits. Edward Sadalla and his colleagues found that college women were attracted to men who demonstrated social dominance in their nonverbal behaviors or in the way they played tennis; men were again oblivious to dominance cues in women.
Researchers who study sexual selection often have a hard time distinguishing whether a given characteristic (such as bird song) is designed to attract the opposite sex or to warn off other members of the same sex. In humans, many of the signs of sexual selection appear as intra-male competitions for status. Margo Wilson and Martin Daly reviewed evidence for what they call the "young male syndrome" - a tendency to engage in risky behaviors of all sorts, from ski-jumping to fist-fighting to homicidal shoot-outs. The ancestral version of modern bungee-jumping was invented in the Pentecost Islands in the South Pacific. There, young males built towers 100 feet high and jump off the top with nothing but stretchy vines tied to their feet. The goal is to drop as close to the ground as possible, shy of that fatal extra inch. Females do not play this game.
Why is it that females do so much of the selecting, whereas males take so many of the risks? The answer is connected to parental investment. When one sex invests more in the offspring (by producing eggs, or in the case of mammals, growing the eggs inside their bodies and nursing them afterwards), that sex will be more careful about taking just any mating opportunity. As a consequence, the members of the other sex will compete among themselves to be chosen.
From a parental investment perspective, humans are relatively egalitarian. In over 95 percent of mammals, males donate only sperm to the offspring. When males do contribute, as in the case of humans, they do some of the selecting as well. In exceptional cases, like Wilson's phalarope, males care for the eggs while females go off to seek other mates. In those species, males are more selective, and females are more colorful and more competitive. Though there is no known case in which female animals offer themselves as meals, human females do at least cook up the occasional tasty dinner to curry prolonged investments from human males. When the meal is over, the human male is well-advised to be thankful for the egalitarian balance of investments and to whistle on his way out to the garbage can.
Doug Kenrick is author of the recent book: Sex, Murder, and the Meaning of Life: A psychologist investigates how evolution, cognition, and complexity are revolutionizing our view of human nature.
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References:
Andrade, M. C. B. (2003). Risky mate search and male self-sacrifice in redback spiders. Behavioral Ecology, 14, 531-538.
Darwin, C. (1871). The descent of man and selection in relation to sex. London: Murray.
Geary, D. (2000). Evolution and proximate expression of human paternal investment. Psychological Bulletin, 126, 55-77.
Sadalla, E.K., Kenrick, D. T., & Vershure, B. (1987). Dominance and heterosexual attraction. Journal of Personality and Social Psychology, 52, 730-738.
Townsend, J. M., & Levy, G. D. (1990a). Effects of potential partner’s costume and physical attractiveness on sexuality and partner selection: Sex differences in reported preferences of university students. Journal of Psychology, 124, 371–376.
Wilson, M., & Daly, M. (1992). Competitiveness, risk-taking, and violence: The young male syndrome. Ethology and Sociobiology, 6, 59-73.
Wynne-Edwards, V. C. (1962). Animal dispersion in relation to social behaviour. Edinburgh: Oliver & Boyd.
