Evolution's Rainbow. Joan Roughgarden
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sexed body.
In conclusion, families with multiple genders can be explained using the concepts developed for two-gender families. The idea of helping at the nest in return for reproductive access that was devised for social insects and applied to extended families of birds and mammals also works for how multiple genders are integrated into a social system. Extending kin selection theory now leads to a theory for a labor market that trades access to reproductive opportunity for service, with genetic relationship merely affecting the worth of a unit of reproductive access. The different genders represent different sectors within this economy. While some sectors, like the end-runners, clearly compete with the controllers, others (like the cooperators) are service providers working under contract. Understanding this complex and interesting social dynamic, an animal political economy, I believe is the next step for evolutionary social theory. The part of Darwin’s theory of sexual selection that predicts universal male and female templates may be false, but an evolutionary approach to social behavior is alive and well.
7
Female Choice
As further evidence of the difficulties with sexual selection theory, let’s consider how real-life female choice differs from female choice in Darwin’s sexual selection theory. Darwin focuses on mating only. A female is supposed to select males according to their attractiveness and prowess. Males are supposed to compete among themselves for mating opportunities and to advertise their good looks to females. This peculiar emphasis on the mating act alone is simply not supported by actual female choices, which are more concerned with the totality of reproduction, including the growth and protection of the young.
“Darwinian fitness” is a technical term that refers to production of the young who will partake in the next generation’s reproduction; in mathematical terms, fitness is the product of fertility and probability of survival. Evolution depends on this overall measure of reproductive success. Mating is one component of fitness, but a preoccupation with “mating success” has led to an emphasis on mating to the exclusion of other components of fitness. In reality, female choice considers the overall production of offspring, keeping mating in perspective. Darwin is incorrect in almost all details concerning female choice, although he must be credited with recognizing that female choice among animals exists in the first place.
What, then, are the preferences of female animals, and how do females vary in their preferences? What do females want from a male, how many times do females want to mate, how many males do females want to mate with, how does a female find Mr. Right, and how do females decide how many eggs to produce?
DEADBEATS NEED NOT APPLY
Is a male’s true mettle tested in combat with other males? Does the best male surface as the winner and assume dominance over a hierarchy of wannabes? Shouldn’t a female yearn to shack up with a proven winner? Shouldn’t a female respect the winner of male-male competition as the best father for her baby, a stud with the best genes? Does mating with him guarantee the best and brightest child?
Let’s see what female gobies think about male dominance. Sand gobies (Pomatoschistus minutus) are small fish (5 to 6 centimeters) common along European coasts. To see what a female goby wants in a male goby, specimens were collected from a shallow sandy bay near the Klubban Biological Station in Sweden and housed in seawater tanks for observation.1 After the experiment, they were released back to the sea.
Sand gobies live for one or two years and experience one breeding season. Both males and females reproduce often during the breeding season, which is two months long (May and June). Males build nests under empty mussel shells by covering the shells with sand and excavating a cavity underneath. They attract females with a courtship display that includes showing their colorful fins. During spawning, a female attaches her eggs to the nest in a single layer.
In an experiment, two goby males were allowed to compete for a clay pottery fragment to use as a nest in order to determine the dominant male. The winner was usually slightly larger than the loser, although only by 3 millimeters. They were then placed in chambers at opposite ends of a tank. The tank was divided into thirds using transparent partitions. The middle chamber was left empty. The winner and loser were given new pottery fragments and allowed to build nests by themselves.
Next, a female was introduced into the middle chamber. The female could choose which of the males she preferred, indicated by the side of the chamber where she spent her time. After the female’s preference was determined, she was placed with one of the males, either the one she preferred or not, by flipping a coin, and then the time needed for spawning to occur was noted. Another female was placed with the remaining male, and the time they took to spawn was also noted. Thus both males were able to spawn.
Finally, after spawning, the females were removed, as were the partitions separating the males, leaving two males, each with a nest containing eggs, at opposite ends of the tank. A small crab was introduced, which is an egg predator. Observers counted the number of eggs lost to the predator in order to determine how good the males were at protecting the eggs.
The results are striking. Whether a male was dominant in competition for nests did not correlate with whether he was a good father in protecting the eggs. Also, female preference didn’t correlate with dominance in male-male competition. The females didn’t care if the male they preferred won his fight with another male. The females did care whether the male would protect the eggs. Somehow females were able to predict who would or wouldn’t be a good father, and decidedly preferred mating with males who later turned out to be good egg protectors. A female could somehow look a male in the eye and tell if he was a deadbeat.
Now, let’s take a look at the peacock wrasse (Symphodus tinca) that lives off the coast of Corsica in the Mediterranean in a shallow rocky habitat.2 The female peacock wrasses have a choice of whether to lay eggs in a male’s nest or to broadcast their eggs over the sea floor. Which they do depends on how they assess the offer of male parental care.
Large controller males construct guarded areas of a meter in diameter and place pieces of algae in the middle, to which the eggs attach. Nest construction takes one or two days, followed by two or three days during which females visit the nests and deposit about fifty eggs at a time, leading to as many as fifty thousand eggs in a nest. Thereafter, the male may guard the egg mass until hatching, which varies from twelve days in the cold water of mid April to six days in the warm water of mid June.
Smaller males take on two roles. They may be “followers,” who swim at a distance behind gravid females and fertilize eggs broadcast on the open sea floor. Or they may hang out as end-runners around the territories of controllers and fertilize eggs laid in the territory. During the first half of the reproductive season, however, small males are absent. The small males arrive only for the second half, presumably when the ability of the large male to shoo them away is constrained by the need to guard the eggs that have already been deposited.
Males defending eggs lose weight and appear to have a higher mortality during this period, so they abandon nests that haven’t accumulated enough eggs to be worth their while. Abandoned eggs are hung out to dry, so to speak. Because abandoned eggs are concentrated in one spot, they quickly attract predators. Thus the best chance of an egg surviving is to be laid in a nest that is not abandoned, the next best chance is for an egg to be broadcast on the sea floor, and the worst is to be laid in a nest that is subsequently abandoned.
The males stay with only 20 percent of the nests early in the season, remain with 8 5 percent of the nests at midseason, and drop off to 20 percent again by the end of the season. Thus laying eggs in a male’s nest is a good bet only in midseason. Indeed, only 15 percent of the females lay their eggs in nests at the beginning of the season, rising to 85 percent at midseason, and falling back to 15 percent as the season ends.
What does a female peacock wrasse want of a male? A male who isn’t a deadbeat, who won’t abandon her eggs. And she can tell. The investigators write, “If a female chooses to lay her eggs outside a nest, she tends to do so only after visiting several nests.”
INVITING FEMALE COMMITMENT
How does a guy convince his gal that he isn’t a deadbeat? Fish offer some advice on this ancient question too. Females