As any adult knows, interest in potential mating partners is heavily influenced by sensory cues. A glimpse of lustrous hair or of piercing eyes can suddenly cause a man to be attracted with a woman, or she with him. The detection of a provocative scent or a sensuous touch may also kindle desire.
Recent experimental work with butterflies has borne out Darwin’s suspicions of more than a century ago that species tend to evolve attributes and behaviors that enhance courtship — and thus reproductive success. Some traits might render an individual more attractive to the opposite sex. Color is now known to spark sexual interest for some species in the butterfly world, as do other sensory signals that were beyond Darwin’s human perception. But the creatures are more discerning than this observation might suggest. Ostentatious coloration or scent may do more than attract attention. Appearance and aroma may be shorthand notations of their bearer’s health and heartiness.
The clearest evidence for the role of color in sexual attraction among butterflies comes from studies of species in which males and females have distinctly different appearances. Obviously, to mate successfully, individuals must be able to determine whether other conspecific butterflies are of their own or of the opposite sex. The rest, it can be argued, is fine-tuning.
A gorgeous butterfly species whose males and females differ in color is the Little Yellow. Both sexes appear an identical yellow to the human eye. The shade being produced by pigments in the females looks quite different to butterflies, however, which perceive light at wavelengths beyond the human visible range and into the ultraviolet. Yellow wing scales on the upper surface of the males’ wings reflect ultraviolet light, and those of females do not. On encountering a female, a Little Yellow male flutters about her briefly before landing and attempting to copulate. On confronting another male, he speeds away and continues his search. These simple behaviors allowed me to develop a test for the cues males use to recognize females. I first glued Little Yellow wings to cards and presented them to males. Males landed on, and even attempted to copulate with, female wings. But male study subjects paid scant attention to male wings similarly mounted.
The next phase of the experiment showed that color was responsible for this choice. I prepared a card with two sets of male wings. A quartz slide that transmits both visible and ultraviolet light covered one set of wings, and a filter that blocks ultraviolet wavelengths overlaid the other. Males now attempted to mate with the male wings under the filter — wings that appeared to be female. This species displays a sexual difference in ultraviolet reflectance*, and after a male’s ultraviolet reflectance is deleted other males treat him like a female.
Once a male and a female butterfly have noticed one another, courtship begins in earnest. The male’s goal is to induce the female to alight and remain still for mating, which sometimes lasts an hour or more. In some species the female must also move her abdomen out from between her hindwings to grant the male access. Butterfly biologists have studied the ritual that precedes actual copulation in only a few dozen of the roughly 12,000 species of butterfly, but it seems clear that, for butterflies, what humans might think of as scent can be a language of love. The vocabulary of this language is chemical.
The best-understood case of nonvisual butterfly communication involves the Queen butterfly. Males of this species produce pheromones, compounds designed to elicit specific reactions —of sexual interest in this case from other butterflies. These pheromones disseminate from brush like structures, called hair pencils, found at the end of the abdomen in males only. Hair pencils have a particularly large surface area for their small volume and are thus highly efficient at distributing chemicals. As a male flies up and down in front of a female, he touches her antennae with his protruding hair pencils, thereby depositing pheromones. The female responds to this chemical signal by alighting and remaining still while the male copulates with her.
Many species of butterfly probably use pheromones in courtship. Males often possess features reminiscent of the Queens Hair pencils, such as patches of unusual scales on the wings and brush like structures on the chest. Like hair pencils, these scales and hairs have relatively large surface areas that would presumably enhance pheromone distribution. And for the family of butterflies classified as Sulphurs, special scales on the male’s generally bright yellow orange wings do indeed emit compounds that may affect female behavior.
Gaudy wings, smooth moves and pheromones do a male butterfly no good if he cannot find a female butterfly on whom to practice his seduction. Males of many butterfly species adopt a search-onthefly strategy, wandering the landscape looking for mates. Often they investigate likely areas, such as plants where females tend to lay their eggs or sites where virgin butterflies emerge from their cocoons.
Males of the Empress Leila species, however, use a highly systematic approach. Because that species’ larvae* feed and pupate* on desert hackberry trees and because the females mate but once in their short lives, the males actually seek out that vegetation in search of young virgins. A few hours after dawn, just when the females emerge from their cocoons and become ready to fly for the first time, the males begin their watch.
Early in the day the males perch on the ground in open, sunny spaces near the trees. This early-morning sunbathing probably allows them to keep an eye out for other butterflies while keeping their bodies warm enough to give chase. Because they cannot regulate body temperature internally, butterflies grow sluggish if the environment is too cold. Later in the morning the males move up into the trees to exactly the average plane of flight of females, about one meter above the ground. My students and I have observed that even when the male butterflies are perched at a tilt, they hold their heads so that their eyes are looking horizontally out of the tree. This orientation seems to ensure that their area of greatest visual clearness — which lies in a band at the equator of the visual field coincides with the plane of likeliest female flight.
The male impulse to send their genetic material into the next generation causes them to try to prevent their mate from mating again. Male butterflies actually make a substantial contribution to females during copulation, passing along a large quantity of nutrients. This nutrient store can be as much as 6 to 10 percent of the males body weight; a male cannot afford such an investment in a female who will use his competitors sperm to fertilize her eggs. In fact, evolution has come up with a mechanism that favors the male that has succeeded in mating first. The presence of the nutrient store in the female’s reproductive tract* causes her to be unresponsive to further sexual advances. Experimental evidence supports this conclusion: artificially filling a virgin’s reproductive tract renders her uninterested in mating, while cutting the nerves to this area in a mated female restores her sexual interest. Another male technique for barring other suitors from his mate is less elegant — he leaves a plug that obstructs the reproductive tract.
Females face different evolutionary pressures. They often get but one chance to mate and must therefore be highly selective. By accepting only the fittest male, a female can assure her own offspring a quality genetic endowment, and she might also secure for herself a more generous nutrient store — which most likely helps her live longer and, in turn, lay more of her eggs. Male colors, pheromones and displays may allow females to judge a suitor’s overall fitness and success in life. We suspect that chemical signals indicate the quality of a male’s diet: the crucial mating pheromone of male Queen Butterflies, for instance, is produced only when the males have fed at certain plants. And vibrant colors can signal younger, healthier individuals.
As with human beings, some of the attributes and behaviors of butterfly courtship are quite elaborate, whereas others are fairly straightforward. Intricate or simple, courtship and mating remain the mechanism by which survival and evolution take place. Whether a butterfly watcher catches a glimpse of a swarming colony of Monarchs mating in the mountains of central Mexico or the mating of two alfalfa butterflies in a backyard, the observer is fortunate enough to be watching the results of, and the continuing course of, evolution.
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