Why males and females look different

Why do males and females look different? In “Odd Couples: Extraordinary Differences Between the Sexes in the Animal Kingdom,” (2013, Princeton University Press, p. 312), Daphne J. Fairbairn provides a scrupulous catalog of possible answers, perhaps the most detailed collection of animal sex differences since Darwin’s 1874 “The Descent of Man and Selection in Relation to Sex.” Sexual dimorphism is the condition in which males and females exhibit distinct phenotypes, beyond physiological traits involved with reproduction. Fairbairn’s volume is supplied with some of the best examples of such differences now known, and the disparate sources of data are pressed into compelling shape by a talented scholar and careful experimentalist. Fairbairn’s manifesto is, “an overtly Darwinian approach to understanding sexual differences” (p. 3). She notes that when selection operates in sex-specific ways, “we expect trait distributions typical for each sex to be adaptive for that sex” (p. 4), and she launches the reader into this view of dimorphism with sweeping taxonomic and gender-modified scope. We learn of lumbering male elephant seals, who in the throes of their ardor may annihilate the offspring of their future mates; of great bustards, giant Eurasian cranes whose males shove rivals and take blows to the face like bear-pit pugilists while females watch nearby; of shellcarrying cichlid fish, whose territorial and satellite males dart and parry for opportunities to ejaculate into snail shells containing females’ unfertilized ova. As vertebrates, we humans are most familiar with mating systems in which males are larger or somehow more elaborate than females. Fairbairn answers this preexisting bias with fascinating contrasts, weaving tales of yellow garden spiders and blanket octopus, whose gigantic females either squat in the center of conspicuous webs, or float huge and enigmatic over tropical reefs, each attended by males of their species as much as 40,000 times smaller in size. We learn of giant seadevil anglerfish, whose enormous, fang-jawed females host tiny males, who cling to female undersides by their own peculiar mouths, absorbing nutrients from a shared bloodstream and dangling close enough to their mate’s genitalia to ensure successful spawning. Fairbairn details the divergent life histories of bizarre, deep-sea bone worms whose females embed their bulbous bodies within decaying skeletons of whales, and maintain hundreds of minute suitors within their translucent skins. Lastly, she describes shell-burrowing barnacles, in which embedded females may (or may not—45% of females fail to mate) host posses of minuscule males with colossal, inflatable penises that may exceed each little fellow’s corporal dimensions by as much as eight times. The density of information and much delightful prose make this a fascinating read for lay and professional readers alike. Footnotes and appendices comprise nearly one-third of the book. One diagram describes the distribution of sexual dimorphism among 73 animal classes, a synthesis across 1.4 million species that is the first I have seen of its kind. Fairbairn glides among the animals with remarkable skill, and does Darwin one better with abundant recent data. In each chapter, she recounts male and female life spans, the fractions of individuals who do and do not mate, and the numbers of offspring that individuals in each sex produce. The raw materials are here to make concise, quantitative statements about how a sex difference in the variance in relative fitness (also known as the opportunity for sexual selection; Wade 1979) can explain the magnitude and direction of sexual dimorphism. But each chapter ends without an attempt. Offspring numbers are what Fairbairn considers “the currency of Darwinian fitness,” yet the trove of such information presented is used mainly for implication. Fairbairn concludes, “the enduring message is that there is no one way of being a male or a female animal . . . there is no “normal” or “typical” pattern of sexual differentiation across the animal kingdom,” and furthermore that the lack of sex differences in externally fertilizing species “remains a mystery” (p. 165). I found such capitulation