Sex on Six Legs Page 15
Fruit flies and the other insects I have been discussing do not have elaborate social systems in which such subtleties are important. Yet they still exhibit same-sex courtship and even mating. The conclusion, though, is not that bugs are stupid, but that sex is hard. Figuring out how to do it involves a complex interplay between genes and the environment. Featherstone and many of the other researchers using genetically modified flies measure their behavior by observing how the mutants respond, not to a living, breathing companion, but to a male or female with its head cut off. The fly's body still emits the same odor cues and provides the same audience for the displays, but it obviously cannot interact with its partner. The decapitation controls for the inevitable interaction between individuals that could alter the results. The genes don't just issue commands that make the flies behave in a certain zombielike way regardless of circumstances. Instead, the genes, and the chemicals they deploy, affect the way that experiences such as being rejected or accepted by another fly are interpreted.
The flies and other insects may also use some of the same-sex interactions as a way to practice their technique. Young male Drosophila are often courted by older ones, and Scott McRobert and Laurie Tompkins showed that males that had been the recipient of such courtship in the lab were more successful in wooing females later in life. The difference was not huge, but in evolution, every little bit counts. In a different species of Drosophila than the one used for most of the genetic research described above, males that are isolated from other flies during development have a hard time telling their own species from similar ones, a crucial skill, since hybrid offspring are not fertile and, hence, an evolutionary disaster. It takes experience with one to know one, it seems, even though Drosophila lack a true social organization like that of wolves, bonobos, or even bees. These flies also exhibit male-male courtship, with the socially ignorant males showing it much more frequently than males reared in the company of other flies.
Part of what seemed to get the public so excited about Featherstone's work was the idea that it wasn't a gene, but "a chemical" that altered the sexual behavior of the flies, which led to the speculation about popping a pill to change one's sexual orientation. If what's sauce for the fly is sauce for the human, this could mean that chemicals in our own nervous systems are involved with sexual orientation, too. But I don't find this in the least alarming, or indicative of some dystopian possibility of transforming people from gay to straight and back again. The truth is that chemicals no more control who we are sexually attracted to than they do anything else. Which is to say, everything and nothing.
Chemicals are where the body's rubber meets its road. They are how our genes exert their influence. It's fine to say that a gene controls eye color, or digestive speed, or whether we like mangoes, but what does that mean? Somewhere, a chemical is involved. Better living through chemistry? It's more like living through chemistry, period. Featherstone's lab has winkled the details out of the devil by connecting the gene to the proteins it codes for and the action of the substances those proteins control. None of that alters the crucial role of experience, even in a creature such as the fruit fly.
What does Featherstone himself want to do with this information? He doesn't seem motivated to get in the pocket of Big Pharma and develop a drug to enable people to go from straight to gay. Presumably he recognizes that this isn't possible. But he has some ideas. Going back to his website, "An understanding of Drosophila neuroscience raises the possibility that we may be able to engineer a ruthless bionic insect army, and use it to take over the world. From our despotic biotechnological throne, we can seek revenge on everyone who ever wronged us. What's that? A buzzing in your ear? I hope you're on OUR side."
All I can say is that I assume he is saying this with tongue in cheek. I really do.
Chapter 7
Parenting and the Rotten Corpse
I HAVE never understood why nature shows on animal families are always filled with images of doting monkeys nursing their infants, or diligent songbirds delivering a beak full of worms to the nest, when much more tender sacrifice takes place under leaf litter in the garden. If you want an ideal example of a good animal mother, for my money you can't do better than an earwig. Now there's a devoted parent for you. After they lay their eggs, earwig mothers stand protectively over the clutch, scrubbing them clean of fungus and other nasty contaminants and keeping predators at bay. Once the eggs hatch into minuscule copies of their parent, mama earwig goes resolutely out into the world to catch prey in the form of aphids and other tiny invertebrates for her brood. In some species, the female digests the food first and then regurgitates it to her begging offspring, as if offering a squalling infant a bottle. If the young earwigs signal their distress, she responds to the solicitation with eager defensiveness. Oh, and that business about them climbing onto people's heads and into their ears? Utter nonsense. According to entomologist James Costa, the name was probably originally ear-wing, after the resemblance of the insect's hind wing to a human ear (honestly, I don't see it, but that's an urban legend for you). How that got transmogrified into an auricular horror story is anyone's guess.
I happen to have an admittedly unpopular fondness for earwigs, but there are a lot of other good insect parents out there. The real champions, of course, are the social insects such as bees and ants, in which the mother goes into the hive or nest after a brief mating flight, never to emerge again. Day in and day out for months, sometimes years, the queen mother produces egg after glistening egg, like chocolates on an assembly line, forswearing any other activity. After the first batch, of course, the maternal care itself—the feeding, the cleaning, the guarding—is foisted off onto the queen's other offspring, but she is still at it, using the sperm she has saved from a dimly recalled frolic in the open air. She can never wait "until the children are grown" to do something for herself; her nest will never be an empty one, and her sacrifice is lifelong. I suppose it's true that not a lot of alternative careers are out there for older female ants and bees, but then many of the human mothers who dream that if only it weren't for the children they would be the CEO of a Fortune 500 company are not exactly being realistic either.
Comedian Milton Berle said, "If evolution really works, how come mothers only have two hands?" The answer is that at least some of the time, they don't—they have six. The social wasps, ants, and bees are highly specialized in their parenting, but many other kinds of insects, from true bugs to ladybird beetles, tend their young to varying degrees. Giant water bug females glue their eggs to the backs of the males, who carry the eggs around until they hatch. Assassin bug parents stand guard over their eggs and hatchlings. Ladybird beetles (a more entomologically correct name than ladybugs, since they lack the soda straw-like mouth parts of true bugs) produce infertile eggs alongside the more conventional ones, apparently solely to feed the voracious young beetle larvae when they hatch. And in some species of cockroaches, insects even more reviled than the earwigs, parents will remain together after mating, and females feed the young with special secretions, almost like milk. Sometimes females do all the parenting, sometimes males do, and sometimes both parents cooperate to raise their young. Finally, in most insects, no one cares for the young at all—the eggs are plopped unceremoniously in their place to fend for themselves after hatching.
All of this variation means that insects are an ideal place to look for insight into the evolution of family life. Why did parental care evolve in some groups and not others? Why does the amount of care, from a brief cleansing swipe over the eggs to putting the kids through college, differ so much among different kinds of animals? Why does mother do most of the work in many species, both parents in a few, and the father alone in fewer still?
Humans and other social mammals are of little help in answering these questions, because we always invest an enormous amount of energy into raising our offspring; we cannot compare situations where care is and is not given. Scientists often use birds to study the evolution of parenting, but the
se too are rather invariant in their behavior, at least when viewed next to insects. True, some birds, for example, ducks, are ready to waddle at hatching and need only judicious direction to the nearest body of water, while others, for example, robins, spend days or weeks of hard labor ferrying food to the gaping mouths in the nest. But that difference pales in comparison with the contrast between a butterfly who lays her eggs on a plant stem and flits away, and a pair of beetles who collaborate to prepare a ball of carrion for their larvae and then respond to the begging motions of their offspring by feeding them a liquefied meal.
People have dearly held opinions about parenting, of course, which is one reason it is an interesting and worthwhile behavior to understand; for example, we want to know if it is "natural" for fathers to take less of an interest in their daughters than their sons, or in being an attentive parent at all. But beyond questions such as these, understanding how the family evolved can help us understand the evolution of social life itself. At the core of society is the oldest bond between individuals that exists: the bond between a mother and her offspring. Once a female stays with her young, the stage is set for siblings to collude and squabble, for alliances to form and dissolve. That in turn makes more complex social interactions possible. In the wink of an eye—well, maybe in a few million years—presto, you have a society, with insurance companies and a movie industry and supermarket tabloids. Families become labor unions, political parties, and royal dynasties. And it all started with a female bug standing guard over her egg. Looking at insects can help us see how we got there. The true social insects, which include the ants, termites, and many of the bees and wasps, are a situation unto themselves and, as I discuss elsewhere, share unique genetic relationships that make the costs and benefits of their behaviors different than those in other organisms. I will therefore confine myself here to the "other" social groups, where only the first timid steps toward a complex network of interaction have been taken.
Why Should You Care?
SO IF most insects, and indeed most animals, get by with recklessly flinging eggs to the four winds, metaphorically speaking, why has more elaborate parental care evolved where it has? People often assume that human children require as much care as they do because of two characteristics of our species: high intelligence, sometimes seen as a reliance on learning rather than instinct, and being born at a comparatively early and helpless stage of development. Our intelligence, and the accompanying complexity of our lives, supposedly means that parents need to spend a great deal of time teaching their offspring the ins and outs of life in society. If we were simple little automatons, the conventional wisdom goes, we too could dump our babies into the world and let them fend for themselves. But faced with an unopened cereal box and a carton of milk, much less a gazelle or a yam, a child needs someone else to open, kill, or cook the meal, and hopefully that someone will make sure that eventually the child will be able to do the same thing unassisted.
The problem with that argument is obvious once you think about insects. Although they do learn more than had previously been believed, as I discussed earlier, not even I am going to champion their qualifications for Mensa. And yet many groups still show elaborate parental care. Furthermore, the species that have doting parents don't seem any more or less intelligent than those lacking them.
What about the idea that we are stuck with helping our children because they are born at such a nascent state? Our big brains mean that the female pelvis can't accommodate a baby born any later, according to this notion, so it is related to that "we are so smart we have to toddle around for years before we can manage to leave our mothers" idea. But here too the insects make that suggestion look dubious, since egg size doesn't have much to do with the size of the brain of the bug that is hatched from it. Earwig mothers, doting though they are, never labor to bring forth their six-legged progeny.
What is essential is the guiding principle behind the evolution of every trait, whether that trait is a behavior, such as offspring care, or the shape of a body part, such as the length of a tail. Doing it has to increase the bearer's fitness, the likelihood of passing on its genes, more than not doing it. When it comes to parental care, that means that even if tending the young takes away valuable time and energy that could theoretically be used to have more offspring, it's worth it if you leave more copies of your genes that way than you would if you deserted your young and went off to have more children. Having offspring is valuable, certainly. But it will win you the evolutionary jackpot only if those offspring survive. Rampant fecundity for its own sake goes unrewarded.
For insects, and maybe for the rest of us, the threat that made it worth giving up future offspring to focus on the current batch seems to be predation. It's a beetle-eat-beetle world out there, so to speak, and eggs are about as vulnerable a stage to be exposed to it as can be imagined. In a small, unassuming black bug called a burrower bug, females guard the mass of eggs they produce in the leaf litter on the forest floor. Once the young hatch, the mother feeds her babies with nutlets, products of mintlike plants that grow nearby. When Japanese researchers Taichi Nakahira and Shin-ichi Kudo removed a female from her eggs on the grounds of Hokkaido University, the eggs succumbed to predators or were attacked by fungus and virtually never survived to hatching. Female shield-backed bugs in the harsh deserts of Baja California crouch protectively over their tiny brood on the stems of croton plants, and if the mother is removed, her young are almost immediately eaten by ants and other insects. If they happen to fall to the desert floor without their mother's watchful presence, they shrivel and die. It looks like love. But it's just a smart investment.
The earwigs demonstrate this trade-off even more clearly, because unlike the burrower bugs, in some species of earwigs an individual female may or may not tend her young. Some broods get more care, others less. In a paper titled "Benefits and Costs of Earwig Family Life" (with just a little bit of jazzing up, can't you see this as a sitcom?), Mathias Kolliker at the University of Basel in Switzerland pointed out that female earwigs sometimes lay two batches of eggs in a season, but if they invested in the work of protecting and provisioning the first set of young, they were less likely to produce a second. Even when they did lay another clutch, they did so later in the season than mothers who had abandoned the first clutch. That delay can be crucial when the cold weather threatens, which means that the benefit of nurturing the first batch of offspring has to be weighed against the cost of having the second batch die in the first storms of autumn.
Similarly, females of a kind of treehopper found in eastern North America will guard their eggs for varying lengths of time, but sometimes desert them right after they are laid. Andrew Zink painstakingly followed 370 female treehoppers over the course of an entire season, dabbing their bodies with colored paints so that he could identify individuals day after day. In terms of the benefits and costs of staying versus going, it turned out to be six of one and half a dozen of the other, or (in the case of the treehoppers) perhaps a hundred of one and five score of the other. Females that stayed longer had more eggs that hatched than the females that left after they laid their eggs, but the protecting females then had fewer and smaller broods later on. The bottom line is that taking care of children has to be selfish in evolutionary terms. A mother who died in the process of keeping one youngster alive under difficult circumstances would leave fewer copies of her genes than one who cut her losses but lived to reproduce again and again.
Sometimes it does pay to go for broke on the first batch of young, if life is so uncertain that survival to produce another brood is unlikely. In such instances, mothers sometimes offer up the ultimate sacrifice, as is detailed in a paper on a social spider by Ted Evans and his colleagues in Australia, delightfully titled "Making a Meal of Mother." Spiders are not insects, of course, but I include this one because it is such a wonderful example of how evolution can produce apparently self-destructive behavior. As any Charlotte's Web aficionado knows, all spiders show some kind of maternal
care, but in this one, the young spiderlings slowly suck blood from the leg joints of their mother while she is still alive. Gradually they consume more and more of her body, until, in Evans's words, "After several weeks, she is decrepit, unable to move, and the offspring eat her entirely." When the scientists weighed the young spiders, they had gained virtually exactly the amount their mother had lost. And being fatter at the outset meant you had more to offer; scrawnier mothers were consumed sooner than their more zaftig counterparts, which then meant that the young spiders were less likely to turn from their parent to an arguably even more unsavory occupation: eating each other. Having your children feed on your still-quivering flesh to keep them from cannibalizing their siblings sounds like something out of a parody of a Philip Roth novel. But for the spiders, it's sensible, since each brother or sister eaten is a genetic investment lost to the parent.
Under what circumstances do we expect these and other less dramatic efforts by parents to show up? In his classic book Sociobiology: The New Synthesis, Harvard biologist (and ant lover) E. O. Wilson lists the "prime movers" that make the evolution of parental care more likely. First is the threat of predators that I mentioned earlier. Other contributors include the kind of environment in which a species finds itself. If food is unpredictable or the climate harsh, offspring will benefit from a parent's buffering them from the vagaries of the world.