Sex on Six Legs_ Lessons on Life, Love, and Language From the Insect World - Marlene Zuk [45]
My Sister, Myself
SO HOW do the honeybees get away with having female-dominated societies? The drones comprise less than 5 percent of the total number of bees in the hive, so they clearly violate the equilibrium ratio. In many other insects, although the sex ratio is not always so extreme, a preponderance of females is the rule. The reason the bees can do what we can't has two sources: the way sex is determined, and a special violation of the "all else being equal" clause above.
Unlike the species discussed above, bees, wasps, and ants don't make males and females using combinations of special sex chromosomes. Instead, males are produced from unfertilized eggs, and hence have only one copy of each chromosome, while females have a more normal (to us, anyway) complement of two copies, because the queen produces them by fertilizing eggs with the sperm of the hapless drone or drones she left behind when she entered the hive to start her monarchy. Again unlike humans, queens of social insect colonies can store sperm for years, doling out a son here, a batch of daughters there. The daughters may become workers, in which case they are sterile, with undeveloped ovaries, or future queens, in which case they are cosseted by their sisters and fed exclusively on royal jelly, a nourishing compound (for bees, anyway) that alters hormone levels and facilitates ovary development. (Little or no evidence exists that royal jelly, whether applied topically or eaten, does anything for humans besides diminish their pocketbooks.)
This genetic peculiarity means that in cases where a queen mates with a single drone, her daughters are more related to each other—their sisters—than they are to hypothetical daughters of their own. To understand this, recall that in humans and other species with XX/XY sex determination, offspring get half their genes from their mother and half from their father, making siblings half like each other as well. Because each sperm and egg cell contain only half of each parent's genetic complement, in species such as our own, each sibling has a 50 percent chance of getting any one chromosome from the parent. But because male bees have only one copy to begin with, all the sisters get the same genes from their father. They get the usual 50 percent from their mother, giving them a total of 75 percent of their genes in common. The queen, on the other hand, shares the usual 50 percent of her genes with either her sons or her daughters.
This unusual sisterly closeness is thought to play a role in the extreme altruism exhibited by many of the social insects, but it also is important in the sex ratio. I have been acting as if sex ratio is concerned only with numbers, but in fact, the mere number of males versus females is only part of the story. What really counts is the investment in terms of energy that is made in either sex. Say that females "cost" less to produce—perhaps they are smaller than males, and so fewer calories go into manufacturing them than are required to make the same number of males. Natural selection will favor making more girls than boys, because each girl is cheaper, but the overall investment in each sex is still equal.
For the bees and ants, the asymmetry in how closely related sisters are compared with mothers and offspring means that the evolutionary payoffs for producing different sex ratios are different for the workers than they are for the queen. The workers' only chance at perpetuating their genes is via the future queens and drones, or males, produced by the queen, because workers are sterile. The queen's genetic future lies in the same individuals, but the two differ in what will most benefit each. From the workers' perspective, more of their genes will be passed on if more energy is allocated to the future queens, their sisters, than to their brothers, because they share only