I was talking to a woman friend of mine the other night over a few gin and tonics. Tina isn't an ardent feminist or anything-she's just a regular person who also happens to be a woman. She and her husband are remodeling their house, and Tina and I were discussing the differing physical abilities of men and women.
"Well, it's not fair that he's stronger than me without putting any effort into it," she complained, staring into her glass. "If I wanted to be that strong, I'd really have to work at it. He ripped those walls out as though he were just unscrewing a lightbulb."
"You know," I replied, "no matter how much you worked out, you could never be as strong as he is. He's got testosterone in his favor, and you know how that is." I squeezed a little more juice from my lime.
"Yeah," she sighed, "It builds strong bodies twelve ways and fools you into thinking you're god's gift to women."
"A normal woman just can't develop the muscle mass that a so-called normal man can," I said, trying to sound like the rational scientist I am. "Of course, there are always exceptions to the norm, but that's why we call them 'abnormal.'"
"Well, you don't have to be so snotty about it," Tina muttered. "And keep your voice down. Those guys at the end of the bar are staring at us. They think we're talking about them."
Alone at home later that night, I thought about our conversation. Here we are, about to leave the 20th century behind us, and it's still controversial to acknowledge that there are gender differences in behavior and physical abilities. We humans have no difficulty acknowledging gender differences in other animals. In the so-called animal kingdom, for example, we know that males are generally more aggressive than females, and we know this differences is caused by genetics, not learning. Take deer, for example.
Male deer have antlers; females don't. During rutting season, bucks fight with one another, engaging in powerful displays of rippling flanks, flashing eyes, and the loud cracks of antlers crashing together. Chests heave as the bucks snort for air and surely, it seems, one of the combatants will be mortally wounded. In reality, there is rarely any serious harm done, as this is a display and mostly for show. Such displays are of crucial importance, though, as it is usually the victor who has the most success in mating. "Mating success," defined in biological and evolutionary terms, is the ability to obtain a mate, copulate successfully with her, and produce viable offspring.
Until about twenty years ago, there was a consensus among biologists that male deer fought with one another over mating "rights" and that the victor "won" the female. Since males with bigger antlers were observed to win the most fights and also to mate more frequently, this seemed a fairly reasonable conclusion, especially from a human-male-oriented scientific point of view. Now, however, there are other theories addressing the significance of antlers and these theories are based on the concept of female mate choice.
Darwin was the first to ponder this question. In 1971, in his The Descent of Man, And Selection in Relation to Sex, Darwin coined the term "sexual selection" in an attempt to rationalize those characters which didn't fit his earlier scheme of "natural selection" or "survival of the fittest." We see deer antlers as natural and don't question whether they serve a purpose for the deer of not. But at second glance, antlers seem to be non-adaptive characteristics (i.e., characteristics that reduce rather than enhance an animal's evolutionary fitness).
Antlers are heavy and unwieldy and require a rich blood supply and good nutrition in order to be maintained. They get in the way of even the simplest of the male deer's daily tasks-eating, for example, or even walking or running. Big antlers make a buck highly visible to predators (and to hunters!) even within a densely forested area. With all of this against him, it's surprising the buck can survive. With this perspective, antlers hardly seem to have evolved as an adaptive trait.
It was a biological puzzle for Darwin and one not confined to deer. Males of many different species have seemingly non-adaptive characteristics. Male birds are usually more brightly-colored than females, and male songbirds do most of the loud singing. Male prairie chickens and sage grouse stage elaborate and lengthy courtship displays, and male crickets and mud frogs call through the night in risky attempts to attract females. All of these characteristics make the males of these species more noticeable to predators and put them at greater risk. Ordinarily, you'd expect these risk-takers to die off before they could reproduce, and these risky behaviors to become extinct, following Darwin's principle of "survival of the fittest." But, since the opposite occurs and the loudest, most colorful and noticeable males have the most successful mating outcomes, Darwin decided that there most be some redeeming (i.e., adaptive) value to these risky characteristics.
Darwin came up with the idea that female choice was the answer. He believed that females must choose the most beautiful males of their species with which to mate; that bright coloration or large antlers were standards upon which the females based their choices. If this were the case, the biggest/brightest males would produce the most offspring and their physical/behavioral characteristics would carry on to the next generation.
Darwin was laughed at-as he frequently was. Females weren't capable of such an ability to discriminate. Imagine the survival of an entire species resting on a female's ability to make a good mate choice. Ridiculous! That females of all species tend to be choosy about with whom they mate was not disputed. However, in Darwin's time (and even in our own), females' coyness was and still is considered just a peculiarity of their sex. So, for the next hundred years, male aggression was continued to be viewed as the primary mechanism of who mated with whom. Caveman-style, the strongest male just clubbed the female over the head and dragged her into the bushes.
Scientists finally realized that they hadn't seen any big-antlered bucks raping any females lately - in fact, had never seen it happen. Maybe Darwin was right? Indeed now, after much study, scientific consensus is that males compete-whether via bright coloration, mating displays or aggression - and females choose. Males with larger antlers are known to have a greater ability to sequester calcium-an advantage to their future female offspring. A male bird's bright-colored plumage is an advertisement to a female that he is healthy and parasite-free. Are females choosing males based on the appearance of healthiness? Is that what the current American fitness craze is all about?
I was in my favorite bar again--no, I'm not there all the time!--listening to the latest male pick-up lines. In this age of the "sensitive" male, these lines have taken on a peculiar lilt. "I'm fragile right now," is one of the latest, the man in question having been through some kind of painful break-up--even if it occurred at his senior prom twenty years ago! "I mourn the fact that I'll never know the joys of childbirth," another stud declares, twenty minutes after telling me that his wife "let herself go" after bearing his three children, leaving him no alternative but to divorce her for someone with a fresher body.
My thoughts drift to the ocean, where some male seahorses do bear the children for their wives. In some species of the seahorse family Syngnathidae, the female deposits her fertilized eggs in the male's brood pouch, where he carries and nurtures them until they burst from his contracting belly in rhythmic waves.
Interestingly, in these species, the females are more brightly-colored than the males and develop their coloration only at the time of courtship. The females are more aggressive and compete with each other for mating access to males, while the males are discriminating and choosy about which females' eggs they'll accept in their brood pouches. This case of sex-role reversal brings up an interesting question: What makes one sex choosier than the other, when picking a mate? Furthermore, what characteristics does the chooser look for when making her/his choice?
Robert Trivers proposed the theory of "parental investment," which asserts that the parent who contributes the most to the rearing and survival of the offspring should be the choosier of the two mates. "Investment" includes such things as feeding and rearing the young, protecting them from predators and even such things as pregnancy and lactation. Parental investment benefits the offspring while disadvantaging the parent in some way.
For example, a female deer, who expends energy on pregnancy and milk production and who protects her young from predators while at risk herself, is choosy when selecting her mate. The male deer contributes little to the offspring other than his sperm and is indiscriminate about his mating partners.
Speaking of sperm, Trivers also proposes that there is a different parental investment when it comes to producing gametes (eggs and sperm). Eggs are larger than sperm and require more energy to produce-especially when you add in the costs of yolk, shell or, in the case of mammals, a placenta. A small investment by a male produces millions of sperm while a female has limitations to the number of young she can produce.
A human female, for example, is limited in the number of offspring she can produce and rear in a lifetime. Even if she has a baby a year, she has a limited number of reproductive years. A man, on the other hand, produces an average of 250 million sperm per ejaculation and is not required to incubate and rear the offspring he produces. There is also, theoretically, no limit to the number of ejaculations he can have in a lifetime, especially now, with Viagra available.
The result of Trivers' proposal is that, in theory, males should try to avoid pair-bonding and, instead, try to inseminate as many different females as possible. That way, they'll leave the maximum number of offspring for the smallest parental investment. Females, on the other hand, should choose mates carefully, picking the best possible genes for their offspring and should try to wring as much parental investment from the males as they can.
This theory has been borne out in observations of a variety of species. Females are more particular when choosing mates and have fewer partners in their lifetimes than do males. Males are eager to copulate with almost anyone, and some male insects have been observed to attempt matings with anything even vaguely resembling females of their species--including rocks and leaves. Males have more partners and, frequently, even after previous copulations have exhausted them, are ready to copulate again if they're presented with a new partner. This tendency of males to exhibit renewed copulatory vigor if presented with a new female, is known as the "Coolidge Effect."
The story goes that President and Mrs. Coolidge were given separate tours of a chicken farm. Mrs. Coolidge noticed a large group of hens and remarked, "Your rooster must be kept quite busy. You might mention that to Mr. Coolidge when he comes by." Later, when the President arrived, the guide said, "Mrs. Coolidge asked me to point out that our single rooster must copulate many times each day." "Always with the same hen?" the President asked. "No, sir," came the answer. "Be sure you point that out to Mrs. Coolidge," her husband replied.
When sex roles are reversed and males make a greater parental investment, you'd expect that males would also become choosy about their mates. Male Mormon Crickets encase their sperm in protein-rich packets called spermatophores, which are an important food resource for the females who receive them. When the cricket population is dense and food is scarce, females compete for mating access to males. A male will frequently terminate copulation halfway-through and move on to another female, trying several before finding one female--usually the largest--upon whom he finally bestows his precious spermatophore. However, when food is no longer scarce, females no longer compete, and males stop being choosy about with whom they mate. Versatility in mating strategies allows Mormon Crickets to adjust their behavior to environmental changes.
Versatility is the name of the mating game, especially if you want to be successful. During copulation a male damselfly uses his modified penis to scoop out another male's sperm before depositing his own, thus ensuring his own success at parenting the female's offspring. In some dragonfly species the male has two inflatable lobes on his penis. These lobes compress a rival's sperm to a far corner within the female's spermatheca, where the sperm are unlikely to fertilize any eggs. Some insects (and some mollusks and worms) simply pierce the females skin and inject sperm directly into her body cavity, bypassing other sperm enroute to her eggs.
Male chimpanzees compete by producing vast quantities of sperm. Their testicles make up about three-tenths of a percent of their total body weight - about ten times that of human males. Since female chimpanzees mate with many different males, the male who produces the most sperm has the best statistical chance of fathering her offspring, and an average ejaculation contains about 600 million sperm (compared to 250 million in humans). A male chimpanzee can also sustain his erection for several hours, thus monopolizing the female and preventing her from mating with other males.
Males don't have a monopoly on competition, however. In some vespid wasp species, all females-not just the queen-are born with ovaries (in honeybees, only the queen has ovaries). The other females represent a reproductive threat to the wasp queen, who must ensure that only her offspring benefit from the colony's resources. So the wasp queen, to protective her exclusive mating privilege, stings and intimidates the other females to the point that their ovaries atrophy and become useless.
Males and females have different natures. They have different physiologies, different anatomies and different genetic mechanisms to ensure their survival in the world. We may not like it and political-correctness may dictate that we not discuss it, but that's the way it is.
My favorite bar just isn't the same anymore, or maybe it's me who's changed. I look around and see males showing off - strutting, preening and flexing their muscles. There's the occasional bar fight, but the fights are mostly for show and real injuries are rare. Females ignore and rebuff repeated male advances, waiting to see if something better comes along.
A man approaches me and says, "Hi, what's your sign?" "I'm a Darwinian," I reply, staring pointedly at a blank spot on the wall. "Hey, cool. That's in the end of February, isn't it? I'm a Taurus and I bet we're real compatible." He doesn't have a chance.
(Copyright 1998 by Susan Smith - No reproduction without express permission from the author)