Teamwork: When Animal Enemies Unite

Studying species from snakes to birds to spiders, biologists are uncovering clues to how mutually beneficial relationships evolve between predator and prey

04-01-2008 // David Brian Butvill

Mr. T, a tubby and fidgety fellow with shifty eyes and a forehead peppered with warts, cautiously enters a local hole-in- the-wall. The air inside is heavy, but the dimly lit walls are draped in exquisite silk. In a far corner, an immense figure with a hunched back and beady eyes crouches into fight position.

That's when Mr. T--a narrow-mouthed toad--begins violently snapping his sticky tongue around the giant tarantula's burrow, ensnaring fleeing ants. After the feast, he settles down for a rest beneath the spider's hairy legs--an exceedingly brave move. "Tarantulas eat toads," says Eastern Michigan University spider ecologist Cara Shillington.

These two species seem to have an agreement, however. "They make good roommates," Shillington says. "Tarantula burrows are typically humid," providing suitable shelter for toads, she explains. Tarantulas also offer security because they can kill most toad predators. Narrow-moutheds, meanwhile, rid tarantula nests of ants and other pests that eat spider eggs. It's an example of one of the most unusual types of reciprocally beneficial associations between species: when predator and prey unite.

Mutually beneficial associations, known as mutualisms, abound in nature. Any gardener knows, for example, that where there are sap-sucking aphids, there are also ants to tend them. The aphids share their sweets with the ants, which vigorously protect their sugar daddies from wasps, spiders and other predators.

But why, and how, would enemies such as toads and tarantulas team up? Fred Gehlbach, a biologist from Baylor University in Texas, has found clues in the U.S. Southwest. Screech owls in this region fortify abandoned woodpecker chambers and other empty cavities with rodents, songbirds and other prey. They eat what they can, and any scraps that remain become their nests. By the time females lay eggs, the matted, rotting mess is packed with maggots and other parasites.

At some point, however, especially during wet years, an owl will return from the night's hunt with a juicy, wormlike blind snake, a skilled subterranean insect hunter that secretes a slippery coat of slime as protection against nips and bites. Because this mucous makes the snake hard to handle, the ensuing parent-chick handoff almost always results in a fumble, and the snake quickly burrows into the nest.

The owl's young may lose a nutritious meal, but they gain a much greater good in the long-run. "Live-in blind snakes eat the [pests] that compete with nestlings for stored owl prey," says Gehlbach. As a result, he has found that more owlets fledge, and they're healthier, than those reared in snake-free nests. Living amid a large supply of food, the snake will stay put until the owl family departs. Then it slithers down the tree and returns to its earthen home. "The relationship," Gehlbach says, "could be an example of an early-stage [mutualism]." Indeed, as clearly accidental as the association started out, pest-controlling blind snakes are common guests of at least four owl species, he adds.

Less direct interactions may also turn into strong mutualisms under the right conditions. Consider cottonmouth snakes and fish-eating birds such as pelicans and herons on islands off the northwestern coast of Florida. The birds are skilled hunters but sloppy eaters that transform their habitat into giant sushi buffets. Though cottonmouths elsewhere tend to eat anything they can swallow--including small birds, eggs and chicks--snakes on these islands rely almost entirely on the shorebirds' seafood delivery service. "The snakes feed rarely, if ever, on colonial-nesting birds or their eggs," says University of Florida zoologist Harvey Lillywhite, who discovered the phenomenon. Thanks to the abundance of easy food, cottonmouth populations on the islands have boomed, which, in turn, benefits the birds. "The snakes deter other nest predators such as raccoons, arboreal snakes and rats," says Lillywhite.

An equally unlikely partnership forms in freshwater lakes throughout the eastern United States, where the common drive to nurture and protect young, coupled with an overlapping spawning season, unites tiny minnows called golden shiners and their predators. Many shiners don't bother to build their own nests, spawning instead in the pits of larger fish, such as bowfins, that would normally eat them. The beefy bowfins take care of nest defense, while the shiners serve as bowfin midwives, darting among the mix of eggs to keep them well oxygenated and clean of bacteria. Outside the spawning season, the two species return to their cat-and-mouse ways.

While it's difficult to imagine that first time a shiner finned over to a bowfin family, "there was probably variation in the propensity of predators to eat shiners, and in the boldness of shiners to approach predators," says Isabelle Côté, an ecologist from Simon Fraser University in Canada. "Get the two correct variants together, and voilà," a mutualism is born. Some 35 minnow species and their predators form similar associations.

Once such win-win relationships develop, they can get reinforced in unexpected ways. Côté recently found that some cleaner fish in the Caribbean treat their own predators better than other clients, attending to them more quickly and initiating more of the cleanings. While this might sound like suicide, "it appears to advertise that the cleaner is a cleaner--about to perform a beneficial service--and so shouldn't be eaten," says Côté. "Some clients remember the treatment they've received from individual cleaners, and either return if treated well or switch cleaners if treated poorly." Either behavior, or both in combination, strengthens the relationship between species.

Indeed, a closer look at any mutualism will likely reveal a host of factors at play. Take the narrow-mouthed toad and its tarantula host. When spider ecologist Shillington offered tarantulas similarly sized toads and frogs of different species, spiders consistently avoided narrow-moutheds. "The toads secrete toxins they pick up from their ant diet," she says. And Shillington suggests that pest-killing toads may provide their innkeepers an extra benefit. She recalls a report of a lifeless snake lying partway out of a burrow. The tarantula inside was eating it, when out hopped a toad. "The snake probably followed it into the burrow," says Shillington. Now that's teamwork.

Costa Rica-based David Brian Butvill writes about science and nature for a variety of publications.

Bacteria and The Beast

Bacteria have joined forces with everything from algae to great apes in a way that benefits both parties. "Without microbes, most other organisms wouldn't have even evolved," says Michigan State University microbiologist Todd Ciche. All animals and plants house bacteria that process food or fight disease. In return, the microbes get a relatively stable environment in which to live. Recently, Ciche discovered a nematode that can't even procreate without a bacterial pal. The soil-dwelling worm waits for an insect to pass by, then slices into it--and vomits. "It regurgitates symbiotic bacteria in the insect blood," says Ciche. "The infection produces the conditions required by the nematode to grow and reproduce." It's a two-way road: The infection--a bacterial reproductive blitz--won't happen if the microbes invade the insect on their own. "The bacteria are unable to thrive unless the nematode introduces them into the insect," says Ciche. Eventually, hundreds of thousands of new nematodes, all packed with the very bacteria that gave them life, emerge from the insect cadaver.

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Brainy Business?

In Brazil and the African country of Mauritania, two of the most clever animals on Earth have become fishing buddies. Fishermen in these areas place nets in the shallow waters near their villages, then beat the sea's surface with a stick. The water-slapping is a sort of dinner bell for dolphins, which respond by chasing schools of fish into the nets. What seafood doesn't get tangled, is cornered, making for easy pickings for the dolphins. By working together, both hunters increase their catch.

Bird-brains have also developed ways to work with humans. A woodpecker in Africa called the honeyguide feeds on beeswax, meaty insect grubs and honey. Trouble is, pecking open a beehive takes too much effort. So the bird leads honey-gathering badgers, baboons--or humans--to bee nests to do the work. The Boran peoples of Kenya are so adept at translating "little birdie" language that they can estimate direction and distance to hives from the bird's calls, flight patterns and perching heights. After arriving at a nest, the humans and honeyguide share the sweets. --David Brian Butvill

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