Expectant Fathers

Lesson 7: Pregnancy and Nature

What Jane Learned About Tarazan from Nature

Entomologists and behavioral ecologists in Arizona studied the courtship behavior of the water bug Abedus herbertii and found that it is the males that are careful and coy in mating. A female would often initiate courtship but the male carefully insists on copulation before taking on the burden of her eggs. Actually, he only allows her to lay two or three eggs on his back at a time before re-mating with her. These copulations are called “insurance copulations.” Since a female may store the sperm from previous mates, a male repeatedly copulates to raise the probability that he will brood his own genes and not those of some other male.

We could dismiss the giant water bug as an aberration were it not for the fact that similar reversals occur in other organisms - in various birds, for example. The most famous of the role-reversed birds is the American Jacanas, Jacana spinosa, otherwise known as the Jesus bird for its ability to walk on water. (Actually, it spreads its great wide toes on floating water weeds for support.) A familiar sight on marshes and ponds throughout the American Tropics, the Jacana is an attractive bird with a rich chestnut-brown body and wings that expose a flash of lime-green or yellow when the bird takes flight.

Jacanas are polyandrous which means a female may mate with as many as four males. Each male broods a separate clutch of eggs. It is the female Jacana that is the more assertive, and her territory encompasses the territories of several males. Being larger than the male she uses her physical might to repel other females and it’s been reported that female Jacanas may oust another female, destroy her eggs and replace them with her own, an act equivalent to infanticide. (Infanticide is also practiced by male lions and monkeys when they take over a harem. They kill the infants to bring the female back into estrus quickly.)

Female Jacanas fight to enlarge the size of their male harems. Their reproductive success is limited only by the availability of males in their territory to incubate their eggs. Marshes are extremely productive habitats with plenty of insect food, so females are less limited by the availability of nutrients to make eggs than by finding attendants to guard and warm them.

Other birds can be monogamous but still show maternal-paternal role reversal. Moorhens, Gallinula chloropus, form breeding pairs before they establish nesting territories. Females fight among themselves, especially if one female approaches a courting pair. These fights are reminiscent of rooster fights, in which the antagonists lean back and rake each other with their clawed feet. Again, the heavier females win most fights. But what are they fighting for? If they form pairs, there must be a male for every female. The hens fight for fat males! The fatter the male, the better he will be at nest initiation and incubation. That extra layer of fat provides perfect insulation to a large brood of eggs.

Avian role reversals, also found in birds such as phalaropes, sandpipers and emus, were well known to 19th-century naturalists. But the explanation of the phenomenon was lacking, and role-reversed species were viewed as bizarre exceptions. Charles Darwin wrote of emus that “have a complete reversal not only of the parental and incubating instincts but of the usual moral qualities of the two sexes, the female being savage, quarrelsome and noisy, the male gentle and good.”

Another interesting example of mom and dad role reversal is presented in the shocking account of the sexual roles of the cricket Decticus albifrons as recorded by entomologist Jean Henri Fabre.

“The male is underneath lying flat on the sand and towered over by his powerful spouse, who, with her saber exposed, standing high on her hind legs, overwhelms him with her embrace. In this posture, Master Decticus has nothing to say about the date. Have not the roles been reversed? She who is usually provoked is now the provoker, employing her rude caresses. She has not yielded to him; she has thrust herself upon him, disturbingly, in a commanding way. Master Decticus is on the ground, tumbled over on his back. Hoisted to the full height of her shanks, the other, holding her saber almost perpendicular, covers her prostrate mate from a distance. The two neutral extremities curve in a hook, seek each other and meet, and soon, from the male’s convulsive loins is seen to issue, in painful labor, something monstrous and unheard of, as though the creature were expelling its entrails in a lump.” (Natural History of the Sexes, p. 139)

That monstrous lump has proved to be a valuable experimental tool and source of insight. It was a male spermatophore that Fabre saw, a bundle of sperm and nutrients.

It is often said that sperm are cheap, which is true (unless you’re paying for an expensive dinner and evening out) compared with the cost of an egg. But sperm that come as a packaged deal with an ejaculate can be an expensive gift. Spermatophores of the sort Master Decticus produced contain not only sperm but proteinaceous nutrients that females can eat or absorb. In various crickets and katydids, the female may bend around and eat most of the spermatophore; in butterflies, the female may absorb the nutrients through her internal organs. In either case, the female is acquiring a male investment for use in her egg-manufacturing endeavor.

The cost of a spermatophore to its male bearer can be considerable. It may weigh half as much as his body. (Thinking of this in human terms gives new meaning to the terms “big and tall”. And ads placed in the Personal Section of the newspaper would definitely have some interesting descriptions of “lonely hearts looking for love.”) Males contributing a spermatophore are very selective when it comes to its contribution. In some butterflies with spermatophores, it appears that males discriminate against older females, whose worn, dull wings reveal them as a poor, high-risk venture in which to sink a large amount of capital. Male fruit flies, following a similar logic, prefer to court virgins because they will have more eggs to yield.

Males of any species don’t have unlimited supplies of sperm so many mammals, including goats, sheep, bulls, rodents and humans, as well as various finch, newts and insects, suffer sperm depletion. An ejaculation is followed by a period of reduced male fertility, and a recovery period is necessary for the sperm count to rebuild. In the case of various crickets and katydids, where the spermatophore is a huge proportion of male body weight, it is unlikely that males can ejaculate many times before they die. This sets up the perfect conditions for role reversal.

Let’s look at this resulting role reversal pattern or the Morman cricket, Anabrus Simplex. The males provide a large spermatophore and so male choice exists. Females fight with each other if two of them approach a male at the same time; each desiring to be his mate. But the final choice is made by the male. When a male gets ready to mate, he weighs the female. Males seem to favor heavier females over the “Twiggy” type.

Role reversal and a male preference for heavier females is also evident in many fish. For example, the male three-spined stickleback, Gasterosteus aculeatus, and the male mottled sculpin, Cottus bairdi, if given a choice, prefer fat females over small ones. In each case, males’ role reverses in mating and they invest heavily in parental care. The male’s ability to choose the female acts as a limiting resource for females. This explains why females behave in a “male-like” manner and fight for and become extreme in their solicitation of males. Males have acquired “scarcity value” for females and are worth competing for. But there are other reasons for males to be choosy, even when they provide no parental care.

Males are choosy about females whenever females vary in quality and males must invest heavily, not only in providing parental care but in the task of acquiring a mate in the first place. The best example of this is the snout weevil, Brentus anchorago. The snout weevil performs one of the most spectacular mating tournaments in nature. They are found in the dry forests of Costa Rica where certain trees can be alive with large, decorative weevils -long, thin black creatures striped with bright yellow. They are usually clustered in groups, some males using their long snouts to lever and bat other males away from the stout, lumbering females. Males copulate whenever they find time away from battle but must also wait on a particular female as she drills her into wood. Since male weevils offer no parental care, it wouldn’t seem that males should be in any sense choosy. But they are.

Males of this species vary tremendously in size, as do females. Some adults may be more than 20 times heavier than the other. In the large male, much of the weight is due to his tremendously long snout, actually an elongated head, which is used for flipping and swatting rival males away from females. The largest males have a mating advantage, but it is not enough to give them free rein. They are not able or inclined to inseminate just any female indiscriminately. Given a choice, large males select large females, a choice that makes sense because large females seem more fertile and more resistant to sporozoan parasites that attach to wood-boring beetles.

All of these biological studies are valuable in several ways. They dispel the notion that there is such a thing as a masculine or feminine behavior pattern inherent to the genders and they show that the sex roles assumed during courtship depend on the relative investments of time and resources made by the two sexes. It is this theory, choice and behavior based on the costs and benefits of investments, that makes sense of masculine and feminine behaviors.

Many of the most interesting insights have come from the study of insects, and this is no accident. Insects are the richest group by classification and probably outweigh any other animal group in sheer biomass. It is to be expected that they will have the richest repertoire of roles. Some people may feel that they are, nonetheless, irrelevant. Not so, as indicated by the stories of John Steinbeck.

In a scene in John Steinbeck’s novel Cannery Row, Doc and Hazel are watching a patch of black stinkbugs waving their tails in the air. Doc, the biologist, comments to Hazel: “The remarkable thing isn’t that they put their tails up in the air - the really incredibly remarkable thing is that we find it incredible. We can only use ourselves as a yardstick.” The reverse is also true. To understand what it is to be human is to appreciate the diversity of nature and use other species only as yardsticks.”

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