A basic goal of biologists is to explain observed variation at many levels, including observed differences between and within the sexes.1 The biological underpinnings of sex differences are considerably more complex than it might at first seem. In no species are "males" and "females" fully identical, despite huge variation in the logistics of sexual reproduction. What, then, is meant by "biological causes"? It is not simply "genetic" or "hormonal" or a "difference in chromosomes." Rather, sex differences, however mediated, arise from past evolutionary and ecological pressures. Specific environmental pressures favor particular complexes of behavioral, physical, and physiological traits—and these evolved phenomena are the proximate triggers of differences. Under most conditions, these selective pressures lead to a (sometimes striking) divergence of the traits shown by each sex.

Begin with the evolution of sexual reproduction itself. There are evolutionary costs to sexual reproduction (loss of genetic representation) (Maynard Smith, 1978; Williams, 1975). Biologists recognize that sexual reproduction has evolved when there are counterbalancing evolutionary advantages to sexual reproduction. These include the production of variable offspring in unpredictable environments (a sort of bet-hedging): (Maynard Smith, 1978; Williams, 1975; review in Ridley, 1993). The specific mechanisms can vary greatly.

Sexual reproduction is not always achieved by the fusion of two haploid gametes (eggs and sperm), nor is it even always genetic (XY or XO chromosomes). In humans the 23rd pair of chromosomes is either homoga-metic (XX) or heterogametic (XY); XX individuals develop as females and XY individuals become males. In contrast, in birds, for example, males rather than females are the heterogametic sex. In some species, sexual reproduction is accomplished simply by exchange of genetic material; in these species there may be more than two sexes (e.g., 13 sexes are described for slime molds; see reviews by Ridley [1993] and Low [2000]).

Even in species in which sex is determined by gametes, and there are clearly active sex chromosomes, there is chromosomal information, some of which influences sex, in autosomes (Wizemann & Pardue, 2001, pp. 51-55). In many species, sex is not genetically determined. In crocodilians and many turtles, the temperature at which the egg develops determines an individual's sex (Shine, 1999). In some fish, such as the coral-reef-dwelling blue-headed wrasse, sex is determined by the social environment. Individuals change sex depending on the local population sex ratio: the largest individual becomes a male (Warner et al., 1975). The social environment changes the costs and benefits of being male or female, setting in motion a series of hormonal and physical changes (Lee et al., 2001).

Whether sex is mediated by the physical or social environment, whether there are chromosomal differences between the sexes—these are not the crucial biological bases of sex differences. The specifics of sexual reproduction can differ, but sex differences are common and also predictably patterned. Males and females in most species behave differently, in predictable ways, regardless of how sex is mediated; the number of "sex-role reversed" species is very small. The real keys to the evolution of sex differences are the interplay among environmental influences, genes, and expressed traits, and how these mediate the costs and benefits of similar, versus differing, traits for the sexes.

Sex differences are likely to be particularly striking in gametic-sex species. When sex is accomplished by the joining of haploid gametes to make a diploid zygote (as in humans), anisogamy (unlike gametes) evolves with ecological cost-benefit implications. What we call disruptive selection means that the traits that make a small-gamete maker (male) successful (moving about to seek mates, and making small mobile gametes that travel well) are incompatible with the traits that make a large-gamete maker (female) successful (being risk averse, committing considerable nutrition to the fertilized zygote). Further, the fact that there is information in the cytoplasm of each gamete means that conflicts can arise; in part, sperm become smaller and smaller by eliminating cytoplasm to avoid this conflict (Hurst, 1991, 1992; Hurst & Hamilton, 1992).

This means that the ecology of succeeding as a male, versus as a female, differs. Costs and benefits differ for the sexes: roaming or staying home, seeking versus avoiding risk. Note, too, that among the wrasses and some turtles (see above), in which sex is mediated by the social or physical environment, the ecology of succeeding as a male, versus a female, nonetheless still differs— and males and females behave differently, look different, and so forth. The ecological pressure at the heart of all these differences is: Can males be more successful repro-ductively through seeking many matings (and leaving offspring care to females), or through investing in the offspring in ways that preclude additional matings? No matter whether the sexes are mediated chromosomally, or change with the environment, this consideration is central to sex differences.

Natural selection has shaped sex differences in all species, including humans. The important consideration is always: In the evolutionary history of each species, what were the reproductive costs and benefits of behaving in particular ways? These trade-offs give rise to the complex interplay that we see: systematic behavioral differences (Geary, 1998; Low, 2000; Maccoby, 1998; Mealey, 2000) correlated with prevalence of particular alleles, X and Y chromosomes in some species, and production of hormones like testosterone that affect behavior.

In genetically sex determined species like humans, the sex chromosomes are clear proximate influences on many traits. For example, although both sexes produce both androgens and estrogens, they do so in different proportions. There is still variation, of course, and the distributions of most traits overlap when the two sexes are compared. These are always interacting with environmental pressures: the resulting hormonal profiles are clearly associated with consistent behavioral differences, which are differentially profitable to mate seekers and parental investors. All this reflects the ecological and evolutionary costs and benefits.

There is, then, a complex interactive causal mediation of sex differences: external conditions—physical, biotic, and even social—affect the costs and benefits of different genetic, physical, physiological, and behavioral traits for the sexes. Over time, these trade-offs result in systematic differences between the sexes, mediated in a variety of ways. When males and females profit repro-ductively from doing similar things (e.g., when males gain enough from offspring-specific true parental investment, like feeding, that precludes additional matings), males and females will be similar in size, appearance, and behavior (e.g., Canada geese). When males profit from seeking matings rather than investing in offspring, as in most mammals, the two sexes will differ, sometimes profoundly, in size, appearance, and behavior (e.g., elephant seal males are several times larger than females).

Among mammals in general, the sexes tend to differ strikingly, because females are specialized to nurse offspring, giving expensive post-natal nutritional care, while males tend to specialize in mating effort. As a result, male mammals tend to have traits that aid in sexual competition: to be larger, to move about more, and to be more aggressive and risk prone than females. In contrast, females tend to be risk averse and more cooperative than males (Low, 2000). In many mammals, the maximum harem size is a good predictor of the degree of physical sexual dimorphism.

Thus the "typical" suite of human sex differences reflects a mammalian evolutionary history. Among mammals, humans are moderately sexually dimorphic in genes, physiology, physical appearance, and behavior, reflecting a past in which moderate polygyny was probably the rule (review in Low, 2000): The following examples of human sex differences reflect the evolved selective underpinnings of sex differences in humans.

Pregnancy And Childbirth

Pregnancy And Childbirth

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