Sex on Six Legs_ Lessons on Life, Love, and Language From the Insect World - Marlene Zuk [51]
Sperm competition can also occur via the sperm itself and the chemicals that accompany it in the semen. Although they occur in many, perhaps all, insects, these chemicals have been best studied in the fruit fly Drosophila, which produces substances accompanying sperm that can kill the sperm of previous mates. These accessory proteins, as they are called, also influence the female's sexual behavior, sometimes rendering her less receptive to future matings, sometimes decreasing her overall life span but increasing the number of eggs she lays that are sired by her latest mate. Eberhard and his colleague Carlos Cordero call these seminal products chemical genitalia, because they can be seen as extensions of the more conventional physical reproductive organs. We are only just beginning to understand their complexity; Tracey Chapman from the University of East Anglia in the United Kingdom, in an article titled "The Soup in My Fly," referred to the bewildering diversity of seminal proteins as "an embarrassment of riches," surely the first time this phrase has been used in the context of sperm. At least 133 different substances have been identified, with doubtless more to follow. Whether each has a different function remains to be seen.
All else being equal, the more sperm that are present in an ejaculate, the more likely the male is to win at sperm competition, simply by overwhelming the prior male's efforts. In the Pieridae, a family of butterflies that includes the familiar cabbage white butterfly, ejaculates are significantly larger than in a family in which females are less likely to mate with multiple males, the ironically named (at least in this context) Satyridae. In insects, as in humans, sperm are produced in the testes, though insects generally lack external testicles housing the male organs. The larger the testes, the more sperm a male insect can produce, and you would therefore expect that sperm competition would cause the testes of species more likely to mate with multiple partners to evolve to a larger size than those in comparable but more monogamous species. That's exactly what was done by dissecting and weighing the testes in different types of Satyrids, and as expected, the greater the likelihood of females mating with many different males, the larger the testes relative to the size of the body.
Recently, sperm competition was actually experimentally shown to influence the evolution of testes size, not just indirectly via comparisons of species, in some elegant work by my friend Leigh Simmons and Paco Garcia-González, a Spanish scientist working in Leigh's laboratory at the University of Western Australia. Leigh has continued in the manure-inspired vein begun with the dung flies by performing pioneering research on dung beetles, those indefatigable insects that tidy up the world's ecosystems by removing the droppings of large mammals and using them to provide an all-purpose larder and nursery for the offspring. Many types of dung beetles occur all over the world, and in some species the males possess large horns used in combat with other members of their sex to gain access to underground tunnels excavated by females. Larger horns make it easier to win fights, but as with many insects, the sexual competition is not over after the physical battle is won. Some males sneak into the burrows of the winners and mate with the females behind the resident's back, as it were, and the only recourse of the former winner is to mate more frequently with the female.
One of the many