Saturday, December 01, 2012

Delaying crunch time

kw: book reviews, nonfiction, insects, natural selection

We once lived near a field that had much milkweed. One day in early spring I noticed some chrysalises of Monarch butterflies hanging on milkweed stems. I went out the next day with a box and gathered more than twenty of them, by cutting a substantial part of the plant so they would stand upright in the box. I left the box on the front porch, which was enclosed but was nearly as cold as outside. When a warm day came, I took the box out into the sun. Soon I could hear rustling from inside it. I opened the lid and watched as the butterflies struggled out of their pupa cases and stretched their wings. They all rested there, drying and moving slowly. Then they all took flight at once. For a minute or two I was surrounded by their beauty. We saw Monarch butterflies in the area for several weeks before they migrated out. I suppose at least some of them made it to Mexico. Near the end of that time, I saw a few orange butterflies that were a little smaller. I netted one, and thus made the acquaintance of the Viceroy butterfly. My "bug book" informed me it is a Batesian mimic of the Monarch.

Why does the Viceroy look like a Monarch? The Monarch tastes bad to birds, so a bird that has tried to eat one will avoid them. I don't know if birds learn from each other, but even if they don't, there are many more butterflies than there are birds. Once each bird has had a taste of Monarch, both Monarchs, and the Viceroys, that so resemble them, are pretty safe from being eaten.

Later I learned about another kind of mimicry. Many whole genera of tropical butterflies are inedible (or at least taste very bad), and look so much alike that naturalists have a hard time determining which species a particular specimen belongs to. Not only so, but other species, also inedible, look very similar to them. This is Müllerian mimicry. A bird that eats any of these species will avoid them all. By the way, this goes for mice, toads, snakes and other insectivores, as does the former principle.

The fate of nearly all insects is to be eaten. They aren't going to live very long in any case. Even in the tropics, adult insects seldom live a year, and in temperate climates, most overwinter as eggs or pupae. But the most insectivores prey on adults or larvae. For a species to survive, then, at least some individuals need to avoid, hide from, outrun or otherwise fend off predators long enough to reproduce. It must be hard to be a fly or moth in southern New Mexico. The millions of bats that live in the Carlsbad Caverns are estimated to eat a few tons of insects every single night.

Naturalist Gilbert Waldbauer relates the various ways that insects put off the inevitable in How Not to be Eaten: The Insects Fight Back, a highly informative and entertaining book. Of course, the author discusses mimicry of both kinds, and a few others lesser known. He also tells us of the ongoing debates about just how effective mimicry is. It turns out that trying to prove such a thing is close to impossible. How do you catch, mark, release, and re-capture hundreds of insects, so you can figure out how many are left? A few very clever experiments have been performed, with ambiguous results so far. In my view, the existence of mimicry, plus the known mechanism of natural selection, prove that it is effective.

The bulk of the book relates a number of other strategies insects use: hiding, fleeing fast, mimicking twigs and bird droppings and flower parts, using noxious chemicals either in their tissues or as sprays, making startle displays, and even literally fighting back. This last is not just the province of stinging insects, the ants, wasps and bees. One anecdote describes a fight between a sparrow and a praying mantis. The mantis put up a good fight, but was eventually killed and eaten.

Then there is the other side. A great many insectivores are insects. Robber flies consume bees; dragonflies captures bees, flies and moths; and fireflies eat mosquitoes (so don't let your kids catch them all to make light jars!). If a new kind of fly evolves that is twice as fast as the fastest dragonfly, it will multiply until most flies can outrun their predators. Dragonflies will be hard pressed to survive unless they evolve greater speed in return, and perhaps get more crafty also. Certain tropical mantises resemble orchids, only partly to fool birds, but even more to fool pollinating insects that they eat. Some species of moth and other bat pray have ears, not to hear one another but to hear a bat's sonar clicks so they can hide or fly erratically to avoid the bat. If the moths get better at this, the bats have to improve also, or die out. It has been called an arms race.

Of course, the primary reproductive strategy of most insects is to lay many, many eggs. They attempt to overwhelm the opposition. If an average female moth lays enough eggs so that two offspring survive to reproduce (one to replace her and one to replace her mate), the species will live on. If they somehow were to average three surviving offspring each, the population would explode, increasing by 50% per generation. For some insects, the population could easily increase by a factor of ten or more in a single season. Then, they'd be likely to outgrow their food supply, and many would starve. Starving insects are easier prey than healthy ones, so keeping the balance is also a way "not to be eaten."

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