From The Handicap Principle:
A Missing Piece of Darwin's Puzzle
(pp. 85-88)
by
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Editor's note: In The Handicap Principle, Amotz and Avishag Zahavi present an important new theory, with wide-ranging implications, to explain certain wildlife phenomena - elaborate displays, odd physical characteristics, and extravagant behavioral patterns - that have puzzled researchers for years. These features (such as the elephant seal's cumbersome snout, or the inefficient bounding of a fleeing gazelle) handicap an animal by hindering its performance, or by making it easier for an adversary to attack. Why evolve a handicap? The Zahavis argue convincingly that it is a signal of an animal's health and prowess, showing that the animal is strong enough to survive and thrive despite the cost involved in overcoming an impediment. Such demonstrations are highly useful both in deterring rivals and predators, and in attracting mates. Here, some examples of what might be described as animal attitude.
Body Parts that Emphasize the Direction of Gaze
An aspect of vision that signals often show is the direction a bird or animal is looking in. The California quail has a long black crest that curls forward. We were puzzled by it: the quail we were familiar with, the European quail, spends practically all its life on the ground in dense vegetation. What could such a bird show off with a crest of feathers? The mystery was solved when we saw the California quail in its natural habitat. Unlike the European quail, the California quail gives its loud call while standing on such high observation posts as the tops of rocks or bushes. Its black crest is visible from afar and shows the direction of its gaze. When the crest appears as a straight line extending upward from its head, the quail is gazing directly at an observer; when the crest appears arched, the quail is looking to the side. But by the same means, an observer can tell where the quail is not looking. The shape of its crest allows the quail to display its confidence; plumes on guardsmen's helms and Roman centurions' helmets may have evolved to fulfill the same function.
In Israel, there is only one lark that has a crest; the crested lark is also the only lark in Israel that sings standing on a hillock, a post, a fence, or a bush, as well as in flight. The crest shows clearly what direction it is facing; in flight, a crest is not seen and is not an effective showing-off signal. In humans and in some monkeys, noses show the direction of an individual's gaze. The nose is more developed in males than in females, and more in adults than in the young - an indication of its use for showing off. Small tufts of feathers on both sides of the head, as in owls and other birds, and short horns, such as the bony protrusions on the heads of giraffes, also let an observer see from afar which way the animal's head is turned.
The arched horns of gazelles, antelopes, and other such animals also show off the direction of their gaze; we found that out at the Shushlui Nature Reserve in South Africa. Till then we had always toured African reserves by car. At the Shushlui, a student doing research offered Amotz a chance to sightsee on foot. Unprotected and unarmed among lions, rhinoceroses, and buffaloes, Amotz couldn't help but listen to every sound and keep a constant watch on his surroundings. The animals were not used to the sight of humans walking; antelopes barked at the sight. Because of the distance, neither their eyes nor even the markings on their faces could be seen, but their horns always looked even and distinct to Amotz and his guide and followed them around, proof that the antelopes were looking at them.
The real adventure of the tour came in an encounter with a square-lipped rhinoceros. Square-lipped rhinos have two horns: the large frontal horn serves as a weapon; Amotz had a hard time trying to figure out the function of the second smaller horn behind it - until the rhino cow charged.
The guide had assured Amotz that square-lipped rhinos are usually peaceful, and that even if they charge, one can usually deter them at the last minute, or escape behind a tree. They came within a hundred yards of a rhino cow and her young calf. The guide, who probably wanted to show off, suggested getting nearer. Amotz had no choice but to come along; being left on his own was an even more frightening prospect than the rhinos.
The rhinoceros cow snorted, stamped, and charged. Amotz and the guide clapped their hands to deter her, and indeed at about twenty yards she swerved aside. The instant she turned, the small horn could be seen: until then it had been hidden behind the large, threatening frontal horn. It was only then that it became clear that the function of the rhino's smaller horn is to show what direction its owner is facing.
The small horn lets one tell whether a rhinoceros is confident or hesitating. A rhino with a well-developed back horn cannot hide its intentions; any hesitation, any change of direction or glance sideways is unmistakable. Such horns function this way only in an open landscape, where they can be seen from afar. The horns of Asiatic rhinos, which live in dense vegetation, do not show the direction of the rhino's gaze in a similar manner, and indeed their back horns have degenerated into small horny bumps.
Body Parts that Handicap Fighting
The special snout or proboscis of the elephant seal hangs down and covers its face. The snout prevents mature males from seeing objects right in front of their mouths, and they cannot feed or bite rivals without swinging it aside. The elephant seal's snout, like a cock's comb, can be grabbed and torn by a rival. The larger the snout, the better proof it is of the male's status: only a large, strong, and experienced male can afford to fight burdened with such a snout.
The puffin and a few other members of the Alcidae family nest in burrows; these birds develop a special horny layer around their beaks during breeding season. Richard Wagner, who studied the razorbill - a member of this family - found that the resident of the burrow protects it from within by presenting its beak to a rival in such a way that the intruder can grasp it.[1] The depressions in the rough, horny sheath of the beak let rivals get a good grip on it. If the beaks were smooth, as they are outside breeding season, rivals could not get a good hold on the beak of a puffin inside its burrow. Only a bird sure of its strength can afford to let its rival grab it effectively. The size of the beak's horny sheath, the number of grooves in
it, and the decorations that show it off to best advantage all increase with age. Members of the Alcidae family who do not nest in burrows do not develop such sheaths.
The beard of the male ibex puzzled us for many years. Clearly it is a signal: adult males have them, but not females or young. But what does the beard show off? It is not heavy; it may change somewhat the appearance of its owner's head but is dwarfed in this respect by the adult male's gigantic horns. The solution came from a totally unexpected direction. Giora Ilani, who studied leopards at the Em Gedi Nature Reserve, found that ibexes are an important part of the leopard's diet. A leopard can overcome even a large ibex: it grabs the ibex by its face, covers the ibex's nose and mouth with its own mouth, and suffocates it.[2] Anything, such as a beard, that makes it easier for a leopard to grab hold of the ibex's face makes it more difficult for the ibex to escape. An ibex that grows a beard is showing off its contempt for leopards and its confidence in its ability to escape them.[3] Another such body part is the loose skin on the lower jaws of adult male moose; wolves go for this dangling skin when they attack moose.
The heavy branched antlers of deer and the heavy arched or curling horns of goats and ibexes are not efficient fighting weapons. In fact, Darling [4] notes that the lunge of an antlerless deer is far more dangerous than that of an antlered deer. Yet it is clear that the larger and more branched a deer's antlers, the more attractive they are to females and the more of a deterrent to other males.[5]
From time to time one finds deer without antlers (hummel) or with straight antlers (pronghorns). Darling remarks that in battle, straight, sharp antlers can wreak a great deal of harm. But such deer disappear from natural populations: the straight antlers may be better weapons but are less effective when used to threaten and to display one's strength. It may well be, then, that heavy, cumbersome horns and antlers evolved not simply as weapons, but rather as handicaps that show off the strength of the animals that carry them. Females seem to be looking for males that are stronger overall, not for males who can kill better.
Amotz Zahavi is professor of zoology at the Institute for Nature Conservation Research, Tel Aviv University. From 1969-1988, Avishag Zahavi was professor of plant physiology at the Volcani Center for Agricultural Research.
Endlinks
Bullies of the Bird World - discusses Amotz Zahavi's work on the cuckoo and how it coerces other birds into hatching and raising its offspring. From the August/September 1997 issue of National Wildlife magazine.
How Females Choose Their Mates - from Scientific American, this article discusses mate choice in several species and relates this decision to the Handicap Principle.
World Wildlife Fund - the international organization dedicated to conserving wildlife and wild places. The extensive site offers news on environmental issues, fact sheets on endangered species, field reports, country profiles, an Expert Database and the Green Web Directory.
CSIRO Wildlife and Ecology Research Programs - research projects conducted by the Australian Commonwealth Scientific and Industrial Research Organization Wildlife and Ecology Division, to study, manage, and conserve Australia's wildlife, plant and land resources. Projects include National Rangelands, Tropical Forests, Tropical Savannas, Vertebrate Fauna and Pests, Land Use and Biological Diversity, and Resource Futures.
wURLd Wild Web - an extensive listing of animal-related resources: animal welfare, wildlife protection, conservation, and research organizations. Listings cover mammals, marine mammals, birds, reptiles and amphibians, fish, insects, and other invertebrates. Also offers resources for young people.
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