Sex on Six Legs_ Lessons on Life, Love, and Language From the Insect World - Marlene Zuk [70]
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