The Flight of the Iguana (9 page)

Well, yes and no. The reality is more complicated and more interesting.

The flytrap's anatomy includes a few ingenious features that allow it to
measure
and
taste
its potential prey before committing itself to the meal. This plant is no heedless glutton. On the contrary, its behavior is forbearing and judiciously economical.

First the question of taste. Those reddish glands crowded onto the flytrap's palate secrete a digestive fluid, a mixture containing weak acid and an enzyme called proteinase, which dismantles animal protein. Once the animal protein has been broken down into soluble fragments, that nutritious solution can be reabsorbed by the plant. But unlike the pitcher plants (which hold a permanent reservoir of digestive fluid, into which victims fall), the flytrap remains dry-mouthed until a morsel of prey has been caught. Furthermore, it isn't to be fooled by poor substitutes. It responds only to real food. The lobes close on a cricket or a fly—and the proteinaceous saliva begins flowing. The lobes close on a small gobbet of raw beef—here also the saliva begins flowing. The lobes close in reaction to the touch of a glass rod, or the tip of a pencil, or the weight of a pebble fed to it just like the beef—and nothing at all happens. The plant does not waste its time or its juices.

It isn't receiving the right signals of chemical feedback. In other words, the pebble tastes wrong. Still dry-mouthed, the flytrap opens its lobes again as soon as possible, resuming the wait for a genuine meal.

After a false alarm, the plant is ready again in less than twenty-four hours. If the chemical signals are positive and the chamber floods with digestive fluid, on the other hand, five to ten days will pass before the flytrap can reopen. That difference in the expenditure of resources (time and digestive fluid) seems to be why the plant also
measures
its prospective victims, and proceeds or refrains accordingly. It simply refuses to bother with insects that are too small to be worthwhile.

The measuring is done in two ways. One is inherent in the structure of the triggering mechanism. On the inner surface of each lobe, among the digestive glands, are three sensitive hairs that serve as trip wires for the trap. Merely touching one hair, though, is not sufficient to spring the trap. At least two distinct touches (upon the same hair or different ones) are required, and those touches must occur no less than about one second nor more than about twenty seconds apart. The hairs themselves are spaced just far enough from each other—as well as from the nectar glands that attract an insect's attention—that a small insect cannot bump any two in close succession.

The second method of measuring was discovered by Charles Darwin himself. He had noticed an odd fact about how the flytrap closes: that the closing movement occurs in two discrete phases. Upon triggering, the lobes swing together quickly (in less than half a second) to a position where the long spines have crossed but the lobe edges haven't quite met, leaving a row of narrow, short gaps like the spaces between bars in a jail window. For the lobes to close completely, sealing off that row of gaps, takes another half hour. Why the hesitation? wondered Darwin.

He guessed that the Venus's flytrap was saving itself the trouble of digesting insignificant meals. “Now it would manifestly be a great disadvantage to the plant,” he wrote in
Insectivorous Plants,
“to waste many days in remaining clasped over a minute insect, and several additional days or weeks in afterwards recovering its sensitivity; inasmuch as a minute insect would afford but little nutriment. It would be far better for the plant to wait for a time until a moderately large insect was captured, and to allow all the little ones to escape; and this advantage is secured by the slowly intercrossing marginal spikes, which act like the large meshes of a fishing-net, allowing the small and useless fry to escape.” So the Venus's flytrap, terror of large insects, is benignly indifferent to little ones.

The most basic question remains: Why do they eat meat?

Not only the Venus's flytrap but also the sundews, the pitcher plants, and a still more elaborate genus of animal-trapping plants called the bladderworts—why do these species share a hunger for fresh flesh? Why must they feast on animal protein while other species of plant are content with sunshine, water, air, and a bit of decent soil? It seems not only presumptuous but greedy.

The truth is exactly opposite. Carnivorous plants have been driven to this extremity not by boldness and gluttony, but by shyness and starvation.

In the matter of habitat, evolution has awarded them hind tit. But like a determined runt that will grow into a proud hog, they make the best of it. They have developed strategies for collecting animal protein because, in the nutrient-poor habitats to which they are exiled, on soils so inhospitable that few other plants deign to invade, without some dietary supplement they could scarcely survive.

The floating islands of peat in the Okefenokee Swamp are a representative outpost, supporting carnivorous plants of three different genera (sundews, bladderworts, pitcher plants) within little more than a canoe's length of one another. The Pine Barrens of New Jersey can claim the same distinction. What these spots have in common with that soggy meadow in Norfolk, where F. W. Oliver saw a million well-fed sundews, is a critical shortage of the basic soil nutrients (like nitrogen and phosphorus) that most flowering plants require. One study has shown that the average patch of bog inhabited by sundews has twenty-seven times less nitrogen than the average patch of pine forest. Around the world, habitats of carnivorous species tend to fit the same pattern—plenty of water, plenty of sun, terrible soil. These are unpromising corners of real estate where, if sundews or pitcher plants weren't growing, almost nothing else would be.

Look at it this way: Meat-eating is the last resort of the shy, uncompetitive plant. Those carnivorous species have removed
themselves evolutionarily from the ruthless competition of the thicket, the forest, from all those fecund and clamorous places where plants flourish in wild vigor and variety, battling each other upon nutritious substrata for position and water and sunlight. The Venus's flytrap and those few others have taken a more gentle path.

In that sense they belong in company with certain other retiring creatures that go to great lengths to avoid gratuitous violence. I'm thinking especially of the rattlesnake and the black widow spider.

Sex Determination as a Mid-Life Experience

The study of biology is such a fine antidote to rigid, normative thinking that perhaps all our televised preachers and tin-whistle moralists should be required occasionally to take a dose of it. The experience couldn't help but be broadening. No general truth emerges more clearly, from even a browser's tour of the intricacies of the natural world, than this: Chances are, there is more than one right way to do it.

Flying is a good example. Birds and reptiles and insects and bats and seeds have all mastered that feat, at different times and in their utterly different ways. The arrangement of anatomical support is another. Who is to say that a skeleton should be worn
inside
the body (as by us vertebrates), when lobsters and other arthropods do so well with their skeletons on the
outside,
and jellyfish get by with none whatsoever? For a further instance, consider the matter of how gender is determined among those creatures showing two distinct sexes. Boy or girl, cow or bull, colt or mare, goose or gander: The interesting question, biologically, is not
which
but
why.
What dictates that a particular individual should turn out to be male or female?

In mammals the point is decided genetically at the moment of conception. That's the most familiar sort of sex determination,
and we humans are likely to think of it as the norm; but such genetic sex determination (GSD) is just a contingent fact, not a logical or biological necessity. Among certain other animals, known as “sequential hermaphrodites,” sexual identity can change as a stage of growth, as routinely as a human might pass through puberty or menopause. These sequential hermaphrodites, including a number of fish species, begin life in one sexual form (say, as males) and function reproductively in that role for a time; then as they grow older and bigger, they transform at some point to the opposite sex (female), in which role their large size may be more advantageous. If physical magnitude happens to be a more crucial advantage for males than for females, in any such species, then the sequence of sexual identities will be reversed, each individual making its transition from small female to big male.

There is also a third option for sex-differentiated species, one which has not gotten much scientific attention until the last few years. This option is called “environmental sex determination,” or ESD. The term means, simply, that in certain species the sex of the offspring is determined at a point sometime after conception, by some environmental influence acting upon those unhatched eggs or those sexless young. That environmental influence might be a matter of chemistry or sunlight or temperature or something else. Theoretical ecologists are still struggling to explain just how ESD might have evolved, and just why it might be useful, but in the meantime field and lab studies have shown that the phenomenon is more common than we might expect.

•   •   •

This ESD business was probably first recognized in a beast named
Bonellia viridis,
a benignly grotesque sea animal belonging to the phylum Echiurida, a group casually known as the “spoon worms” and not remotely related to anything you've ever heard of.
Bonellia
itself looks like some sort of bad party joke made out of latex. The adult female of the species consists of a bulbous
body roughly the size of an avocado and with a similar dappled surface, from which extends a long tube-like proboscis ending in a pair of leafy lobes. It lives amid rocks on the bottom of the Mediterranean Sea, where the soft body can find safety by anchoring itself in a hole or a crevice, and the lobed proboscis can be extruded out, three feet or more, to grope for passing morsels of food. But the proboscis of
Bonellia
collects more than nourishment; it also collects mates.

When a tiny sexless larva of the same species comes into contact with this proboscis, the larva attaches itself there on the tube and (apparently in response to chemical signals) begins the process of turning into a dwarf
Bonellia
male. Eventually the mature male, still no larger than a caraway seed, will make his way up the proboscis and into the female's gut, claiming a permanent home within the uterus. There he will live off her as a parasite, a feckless but useful gigolo, conveniently on hand to fertilize her eggs.

In finding its female host, the
Bonellia
larva has found also its own sexual identity, its own selfhood. If the same larva had
not
blundered upon a female proboscis, it would (in most cases, though there are exceptions) eventually have settled down in a rocky cleft and grown into a large bulb-and-tube female itself. The presence or absence of a female proboscis is the crucial environmental fact, in the life of each young
Bonellia,
that settles the matter of sex.

This is ESD at its most vivid, and back as early as 1920
Bonellia
had already become quietly famous among zoologists as the leading exemplum of the phenomenon. But environmental sex determination seemed then just an oddity, an aberration, the kind of garish and mildly repugnant trick that one would expect from an obscure group of marine invertebrates like the spoon worms. Today we know better. ESD has been discovered also among orchids, nematode worms, crustaceans, lizards, at least one species
of fish, four or five species of turtle, and the American alligator.

In the case of the alligator, an elaborate set of experiments and field observations has recently proved that sex determination for this species involves little or no genetic component. Instead, the sex ratio in a litter of hatchling alligators seems to be completely dependent upon the temperature at which those eggs were incubated. An alligator nest maintained at eighty-six degrees F. or cooler will produce nothing but females. The same batch of eggs, if kept at ninety-three degrees or warmer, will hatch out as all males. Alligator eggs have an incubation period of about sixty-five days, but sex determination seems to occur during just the second and third weeks. At temperatures between the range of eighty-six and ninety-three degrees, the nest will yield a mix of males and females.

Still, the interesting issue is
why.
Why has the alligator come to depend upon thermal signals, rather than genetic coding, to set the sexual identities of its offspring? Why has
Bonellia
evolved a system using social contact (or the lack of it) for the same purpose?

And from that pair of questions derives another, even more puzzling: What could
Bonellia
and the American alligator have in common?