Two Species, One Design
By Fiona Sunquist
If Africa's serval cat and South America's maned wolf look alike and behave similarly, there's a good reason
AS A LIGHT BREEZE shuffles the grass in Tanzania's Serengeti Plains, a serval's spotted shape dissolves and reappears, fading in and out of focus. Small birds scold and dive-bomb the silent form, but the long-legged cat waits, still as a sentinel. Its eyes close as if in a trance, but its enormous, dish-antenna ears slowly turn, listening intently. Minutes pass, then suddenly the ears focus, pinpointing the exact source of a sound.
Hidden beneath the canopy of stems, a grass rat scurries unseen, but definitely heard. The hunter's ears cup the air again. After two cautious steps, the cat springs, floating above the meter-high (3 ft.) grass. Its sinuous body swerves and twists in mid-air, reorienting to each move of the rat. Two forepaws slam into the dirt. The pounce connects. Another meal is secure.
Eight thousand kilometers (5,000 mi.) away, a chestnut-red, stilt-legged maned wolf ambles across a grassy hillside in Brazil. It stops, sniffs the air with a long muzzle and twitches its giant, 18- centimeter (7-in.) ears, directing them at a spot in the grass some 4 meters (12 ft.) away. Head outstretched, like an alien dog on point, it stalks, pausing every few steps to sniff and listen.
A small rodent called a laucha scrambles through the grass. Guided by the sound, the maned wolf pounces, misses, then follows the invisible fleeing quarry with a mad, zig-zag series of stiff-legged jumps. Punching its pointed face deep into the grass, it snaps twice, shakes its head, swallows its prey, then continues the hunt, moving with tireless, rocking motion through the sea of grass, its radar ears prospecting for new opportunities.
Separated geographically by the Atlantic Ocean and genetically by millions of years of evolution, the serval and the maned wolf--one cat, one dog--are hardly close relatives, but both have evolved similar body forms and use the same hunting behaviors. Both make their livings the same way--chasing rodents, lizards and other small creatures that rustle unseen through tall grass. This is their niche, or ecological profession, and a similar lifestyle has resulted in their physical and behavioral resemblances.
Biologists call this duplication of shapes and behaviors in unrelated animals convergent evolution. Just about every imaginable niche in nature, research increasingly confirms, has been filled by creatures that have independently come up with similar biological solutions to the same ecological problems. As the famed English naturalist Charles Darwin put it, "I am inclined to believe that in nearly the same way as two men have sometimes independently hit on the very same invention, so natural selection, working for the good of each being...has sometimes modified in very nearly the same manner, two parts in two organic beings...."
Thus, unrelated creatures all over the world have evolved parallel shapes and structures that help them collect food or move around (flying has been "invented" independently by bats, birds and even dinosaurs, for instance). Others have developed almost identical ways of avoiding predators or communicating. Over eons, each animal arrives independently at the same solution because it's the solution that works most efficiently--like the invention of the wheel or the mousetrap. None of the matches are exact, but all employ the same principles.
Consider the serval and the maned wolf, each non-conformists in their respective families. Among cats, the wiry serval is odd-looking at best, and the maned wolf is not even a wolf but an ancient dog, the sole survivor of a six-million-year-old South American canine lineage. Rather than the slinky, crouching shape of other cats, or the short, foxy outline of most members of the dog family, both seem to have been designed for maximum height. For its size, the serval has the longest legs of any member of the cat family; it stands as tall as a German shepherd but weighs only half as much. A maned wolf is similarly lanky; tall as a Great Dane, it weighs only 23 kilograms (60 lbs.) against the Dane's robust 45 kilograms (120 lbs.).
In the animal kingdom, long legs usually mean speed, but neither the serval nor the maned wolf are particularly fast runners; rather their legs are long so they can hear better. For sound-hunting to work most efficiently in tall grass, ears need an elevated platform. So servals and maned wolves use their extra long necks and limbs to get a better angle on their dinners.
The secrets of the serval's sound-hunting techniques were uncovered by a Dutch researcher, Aadje Geertsema, who now lives in Tanzania Armed with little more than a notebook, binoculars and patience, Geertsema habituated several of these slender cats to her presence, allowing her to watch them hunt. "Servals are so tuned in to noises that they often stop and sit for 10 minutes or more, just turning their heads. Their eyes are closed, they don't bother to look, they're just listening," she says. "Unless they are extremely hungry, servals rarely bother to hunt in windy weather--strong winds seem to interfere with their ability to pinpoint prey."
Maned wolves differ only slightly. "They point their large ears at the grass and stand stock still, probably focusing on high-frequency squeaks and the sound of leaves and grass moving," says James Dietz, a behavioral ecologist at the University of Maryland. But Dietz, who studied maned wolves in Brazil, found that the wolf has added a small refinement to the serval's technique. "Once it has a general fix on the prey's position, the wolf lightly taps the ground with one forefoot. The noise startles the rodent into flight, then the wolf lunges, stabbing with its forelegs to pin the prey to the ground."
Other specialized ways to make a living lead to similar convergences. Eating ants, for instance. Despite an abundance of ants and termites, few animals specialize on feeding on this diminutive prey. Those that do look strikingly similar. Australia, South America, Africa and Asia all boast ant-eating specialists. The echidna, giant anteater, giant armadillo and pangolin are among them.
The reason for so few ant predators is that ants and termites make less than ideal prey. Termites have had more than 50 million years to develop defenses to avoid being eaten. A breach in the colony walls brings thousands of soldiers rushing to defend their home. Each species has a different way of responding. Some bite with giant pincers, others spray noxious chemicals. Moreover, ants are generally low-quality food to begin with, says anteater authority Kent Redford, now science program director for The Nature Conservancy's Latin American Division. "There is a lot of space between individual ants and termites, and when you put down a sticky tongue, you get a lot of other indigestible stuff like dirt and twigs," Redford says.
To offset these challenges, the various anteaters have all come up with similar adaptations. Long tongues and lots of sticky spit seem to be the best way to collect such minute morsels, and teeth are of no use to an animal that processes as much dirt at it does food. Consequently, a very long tongue, reduced teeth (or none at all), giant salivary glands and a huge, hook-like claw on each front foot are standard equipment for the professional anteater. These tools have given us some of the world's weirdest-looking creatures--all similar to each other.
In Nigeria and Cameroon, the giant pangolin, a 23 kilogram (60 lb.) artichoke look-alike, cruises savannas and forests searching for ant and termite nests. Using heavy-duty digging claws on its front feet to hack its way into cement-hard mounds, it probes the holes with a 38 centimeter-long (15-in.) tongue. Copious quantities of honeylike saliva glue the scurrying ants to its tongue and horny ridges on the roof of the mouth then scrape off the ants, which are swallowed whole. Pangolins have no teeth so food is "chewed" in a modified stomach. A feeding pangolin closes both nostrils and eyes, using specially thick eyelids to protect against ant bites. The scaly ant terminator can suck down 200,000 ants a day--quite a feat, considering it can feed for only about 60 seconds at any single mound before being chased away by ant soldiers.
On the other side of the world, in South America, a shaggy, Shetland pony-sized giant anteater stumps across the grasslands on an ant-seeking mission identical to that of the pangolin. Sniffing constantly, it locates an anthill or termite mound, then carefully uses the tip of one of its huge claws to make a small cut at the base of the mound. Pushing its slender snout into the opening, it moves its tongue in and out rapidly, as often as 160 times a minute. Speed is of the essence. According to Redford, giant anteaters "specialize in surprise attacks and usually feed for only 10 to 30 seconds at each location." Just as with the pangolin, their eat-and-run tactics are a response either to the arrival of ant and termite soldiers or the rapid departure of other ants worth eating.
Two groups of birds--New World vultures, and African and Indian vultures, both of which scavenge dead animals--also illustrate convergence over a feeding niche. The bodies and behaviors of these birds look so similar that scientists had long lumped them in the same family. In both groups, for instance, bald skin on the head and neck keeps neck feathers from getting nasty and also helps in heat regulation, while large broad wings are perfect for soaring while searching for meals. But recent genetic testing using DNA has revealed that New World vultures share a common ancestry with storks, while Old World vultures are descended from the same ancestor as are eagles. "Vultures are an absolute textbook example of convergent evolution," says David Houston of Glasgow University in Scotland.
Despite their virtually identical lifestyles, convergence in this case is not quite exact. "African vultures live in open areas, rely on eyesight to find their meals and cruise at high altitudes so they can get a really good view of a large area of grassland below them," Houston says. "They can spot a dead animal from 3,000 feet [900 meters] up." But American turkey vultures have had to come up with another sense in order to find carcasses because their food often is obscured by trees. "American turkey vultures hunt by flying just above the level of the tree canopy, sniffing the air all the time. When they get a whiff of rotting meat, they circle to pinpoint the strongest smell, then hop down into the canopy and follow their beaks to the carcass." This self-appointed clean-up brigade is so effective, according to Houston, that in South America "vultures consume more meat than all other mammalian predators put together."
While some animals have evolved similar ways of feeding, others have arrived independently with the same strategy to avoid becoming food. Hedgehogs, tenrecs, spiny anteaters and porcupines have spines to defend themselves. Others, like aardwolves, cats and peccaries fluff up their fur to make themselves look larger and more intimidating.
Predator pressure is one of the main reasons why the world's tropical forests are home to a variety of small, anonymous-looking animals with hunched over backsides. A wide assortment of rodents and ungulates have come to look and behave so alike that even an expert would be hard pressed to tell them apart. At nearly 2 kilograms (5 lbs.), the agouti (a rodent) and the Asiatic mouse deer (a member of a primitive family of its own) are the smallest of the group, while muntjac (small deer) and duiker (antelopes) can tip the scales at 15 kilograms (40 lbs.).
Whatever their size, these look-alikes avoid being eaten by behaving in very similar ways. Quiet, cryptic and solitary, they escape predators by running fast and low through tangled vegetation in small parcels of forest they know intimately. All are built on the same compact body plan--low wedge-shaped heads without antlers for pushing through dense brush, and high rounded hindquarters with lots of muscle for speedy takeoffs. All of them live by browsing on scattered high-energy foods such as flowers and nuts, and they give birth to single, precocious infants that are born ready to run and need little care from the mother.
Similarly, the wide, turbid rivers of Africa and South America are full of fish that independently have evolved parallel solutions to communicating in conditions where you can't see a fin in front of your face. Some of those involve electricity. Tucked into a hole in the bank of a South American river, a male knifefish broadcasts a very low-frequency electric signal to identify himself. That night, as a group of females swims past his hiding place, he changes his tone and produces a sort of electric courtship song.
All around, the electric signals of other fish ping, rasp and beep in the darkness. Under murky conditions, generating your own power seems to be the way to go. Electric signals are fast, they can go around stumps or rocks, and they penetrate silt and darkness. Some fish even can produce enough power to stun prey or shock would-be predators, but most use the electricity to sense objects and send and receive messages. In West Africa, a meter-long (3 ft.) elephant fish sends electric signals into the silty river water in a threat display designed to intimidate other males.
Bizarre and limitlessly inventive though they may seem, these animal solutions to life have to work within a major constraint. Natural selection can operate only on what it has, and in the case of animals with backbones, that means a basic body plan with four limbs, two eyes, a nose, mouth, tongue and so on. Tongues and legs can become longer, ears larger, but busy people are unlikely to evolve a third arm, no matter how much they need it, because three arms are not in a vertebrate's basic body plan.
No matter what your mother told you, she could never have developed another pair of eyes in the back of her head or have become "all ears," because the basic blueprint only calls for two of each. But on the other hand, vertebrate wings are just modified front legs, so given a lot of time and a good enough reason to do so, pigs just might fly.
International Wildlife
September/October 1996