Read Online The Meaning of Human Existence by Edward O. Wilson - Free Book Online
Authors: Edward O. Wilson
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size and structure of the molecule, as well as the amount released in each message, and finally the sensitivity of smell to it in the receiving organism.
Consider a female moth summoning males of her species in the night air. The nearest available male may be a kilometer away—the equivalent of about fifty miles when converted from moth body length to human body length. As a consequence the sex pheromone must be powerful, and so it has proved in real cases studied by pheromone researchers. A male Indian meal moth, for example, is stirred to action by as few as 1.3 million molecules per cubic centimeter. You might think that is a lot of pheromone, but it is actually a vanishingly small amount when compared with, say, a gram of ammonia (NH 3 ), which contains 10 23 molecules (one hundred billion trillion) molecules. The pheromone molecule needs to be not only powerful to attract the right kind of male, but also of some rare structure or other, making it highly unlikely to attract a male of the wrong species—or worse, a moth-eating predator. So precise are some sex attractants of moths that those of closely related species differ only by a single atom, or possession or location of a double bond, or even just an isomer.
The male moth of species with such a high level of exclusivity faces a severe problem in finding a mate. The ghostlike active space he must enter and follow starts at a pinpoint on the female’s body. It proceeds as a roughly ellipsoid (spindle-shaped) entity until it finally dribbles out to a second pinpoint, then disappears. In most cases the male cannot find the target female by simply moving from a weak concentration of odor along a gradient of ever-increasing concentration, as we do when sniffing out the source of a hidden kitchen smell. It uses another, but at least equally effective method. Upon encountering the pheromone plume the male flies upwind until he reaches the calling female. If he loses the active space, which can happen easily as a breeze shifts and warps the odor stream, he zigzags from side to side through the air until he enters the active space again.
The same magnitude of olfactory power this requires is commonplace throughout the living world. Male rattlesnakes find willing females by following pheromone tracks. Both sexes, their tongues flicking in and out to smell the ground, close in on a chipmunk with no less the precision of a hunter tracking a mallard duck with the barrel of his gun.
The same degree of olfactory skill exists anywhere in the animal kingdom whenever there is a need to make fine discriminations. Among mammals, includinghuman beings, mothers can distinguish the odor of their own infants from those of others. Ants can separate nestmates from aliens in tenths of a second with sweeps of their two antennae over the bodies of approaching workers.
The design of the active space has evolved to communicate many kinds of information in addition to sex and recognition. Guard ants inform nestmates of the approach of enemies by releasing alarm substances. These chemicals are simple in structure compared to sex and trail pheromones. They are released in large quantities, and their active spaces travel far and fast. There is no need for privacy. On the contrary, there is good reason for friend and foe alike to smell them—and the sooner the better. The purpose is to stir alertness and action, and among as many nestmates as possible. Pumped-up fighters rush into the field upon detecting an alarm pheromone, while at the same time nurses carry the young deeper into the nest.
A remarkable pheromone-and-allomone combination is used as a “propaganda substance” by an American species of slave-maker ant. Slavery is widespread in ants of the north temperate zone. It starts when colonies of the slave-making species conduct raids on other ant species. Their workers are shiftless at home, seldom engaging in any domestic chore. However, like indolent Spartan warriorsof ancient
Consider a female moth summoning males of her species in the night air. The nearest available male may be a kilometer away—the equivalent of about fifty miles when converted from moth body length to human body length. As a consequence the sex pheromone must be powerful, and so it has proved in real cases studied by pheromone researchers. A male Indian meal moth, for example, is stirred to action by as few as 1.3 million molecules per cubic centimeter. You might think that is a lot of pheromone, but it is actually a vanishingly small amount when compared with, say, a gram of ammonia (NH 3 ), which contains 10 23 molecules (one hundred billion trillion) molecules. The pheromone molecule needs to be not only powerful to attract the right kind of male, but also of some rare structure or other, making it highly unlikely to attract a male of the wrong species—or worse, a moth-eating predator. So precise are some sex attractants of moths that those of closely related species differ only by a single atom, or possession or location of a double bond, or even just an isomer.
The male moth of species with such a high level of exclusivity faces a severe problem in finding a mate. The ghostlike active space he must enter and follow starts at a pinpoint on the female’s body. It proceeds as a roughly ellipsoid (spindle-shaped) entity until it finally dribbles out to a second pinpoint, then disappears. In most cases the male cannot find the target female by simply moving from a weak concentration of odor along a gradient of ever-increasing concentration, as we do when sniffing out the source of a hidden kitchen smell. It uses another, but at least equally effective method. Upon encountering the pheromone plume the male flies upwind until he reaches the calling female. If he loses the active space, which can happen easily as a breeze shifts and warps the odor stream, he zigzags from side to side through the air until he enters the active space again.
The same magnitude of olfactory power this requires is commonplace throughout the living world. Male rattlesnakes find willing females by following pheromone tracks. Both sexes, their tongues flicking in and out to smell the ground, close in on a chipmunk with no less the precision of a hunter tracking a mallard duck with the barrel of his gun.
The same degree of olfactory skill exists anywhere in the animal kingdom whenever there is a need to make fine discriminations. Among mammals, includinghuman beings, mothers can distinguish the odor of their own infants from those of others. Ants can separate nestmates from aliens in tenths of a second with sweeps of their two antennae over the bodies of approaching workers.
The design of the active space has evolved to communicate many kinds of information in addition to sex and recognition. Guard ants inform nestmates of the approach of enemies by releasing alarm substances. These chemicals are simple in structure compared to sex and trail pheromones. They are released in large quantities, and their active spaces travel far and fast. There is no need for privacy. On the contrary, there is good reason for friend and foe alike to smell them—and the sooner the better. The purpose is to stir alertness and action, and among as many nestmates as possible. Pumped-up fighters rush into the field upon detecting an alarm pheromone, while at the same time nurses carry the young deeper into the nest.
A remarkable pheromone-and-allomone combination is used as a “propaganda substance” by an American species of slave-maker ant. Slavery is widespread in ants of the north temperate zone. It starts when colonies of the slave-making species conduct raids on other ant species. Their workers are shiftless at home, seldom engaging in any domestic chore. However, like indolent Spartan warriorsof ancient
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