Do lions purr? And why are there no green mammals?

National Wildlife, August-Sept, 1995 by Doug Stewart

Once again, National Wildlife answers 10 pressing questions about the animal kingdom

Time again to dig through the mail-bag for questions about wildlife that have lately vexed, intrigued or otherwise baffled our readers. So, without further ado, here are the answers to our editors' top 10 choices of your queries.

Do lions purr?

Contented lions might if they could, but they can't. Only the smaller cats - not just house cats, but also bobcats, ocelots, lynxes and others - have what it takes to purr. The relevant apparatus is a tightly connected linkage of delicate bones running from the back of the feline tongue up to the base of the skull. When in a purring frame of mind, a cat vibrates its larynx, which in turn sets the twiglike hyoid bones to resonating. No one is sure why cats evolved this ability, but one possibility is that a mother's purr helps camouflage the mewling of her vulnerable nursing kittens, a sound that might otherwise alert and attract predators. All purring cats can make the distinctive sound continuously, both breathing in and breathing out.

In big cats - lions, tigers, leopards, jaguars - a length of tough cartilage runs up the hyoid bones to the skull. This feature prevents purring but also gives the larynx enough flexibility to produce a full-throated roar - 114 decibels' worth in the case of one lion tested. The sound can be loud enough to be near a human's pain threshold. Purring ability, rather than size or behavior, is one of two chief distinctions between the two main genera of cat, Felis and Panthera. (The other difference is that the eyes of the former have pupils that narrow to vertical slits.) These genera are sometimes called "the purring cats" and "the roaring cats," respectively, although among the latter only the lion roars habitually. Other large cats are more apt to snarl, yowl, hiss, spit, grunt or cough.

One big cat that purrs but can't roar is the cheetah. Biologists place it in a genus all its own (Acinonyx), simply because it can't retract its claws completely. Also unique to the cheetah is a high-pitched chirp, said to resemble a canary's. "When I first heard it," Theodore Roosevelt once wrote, "I was sure that it was uttered by some bird, and I looked about quite a time before finding it was the call of a cheetah."

Why are there no green mammals?

No one knows for sure. Mammals are overwhelmingly earth-colored - mousy, you could say. A few sort-of-green mammals do exist: Tree sloths turn grayish-green when algae grows on their fur. Australia's ringtail opossums have bands of black and yellow on their hair that can look a grizzled olive drab. You could argue that a diatom-encrusted whale is green. But nonmammal tree frogs, praying mantises and parakeets are all luminous, unapologetic greens. Green vegetation fills the natural world, and many of its denizens use green as camouflage. Why not mammals?

The short answer is that mammals are hairy. Mammalian hair has only two kinds of pigment: one that produces black or brown hair and one that produces yellow or reddish-orange hair. Mixing those two pigments is never going to yield a bright, contestable green.

Mammalogist Maria Rutzmoser of Harvard's Museum of Comparative Zoology suggests a more complex explanation: that small mammals - the ones needing protective coloration the most - typically live on the ground, scurrying in leaf litter. "Dead leaves aren't green," she points out. "They're brown."

Finally, most predators of mammals are other mammals, and mammals usually have poor color vision; ergo, green wouldn't help.

Still, evolution has given us wonders ranging from the hawk's retina, to the mathematician's brain, to the lion's roar. Given enough time, natural selection could surely produce green fur.

Why do flamingoes stand on one leg?

Most likely, to stay warm. Whether sleeping through a 12-hour equatorial night or loafing for a bit after breakfast, flamingoes - along with storks, ibises, herons and other long-legged wading birds - typically draw one leg in, pull their heads down, tuck their bills under a wing and fluff out their feathers to conserve heat.

Though thin as a reed, a flamingo's long, featherless leg courses with blood vessels - a perfect radiator. To stay flight-ready, however, the birds must keep warm around the clock; on cool nights, they can't afford to leave two radiators on. Long-legged birds aren't alone in this habit: Perching birds like canaries and zebra finches assume the same pose, just not as noticeably.

A locking mechanism above a flamingo's foot keeps its leg from collapsing as the bird drowses, and the same exquisite sense of balance that lets a wading bird hold its head absolutely level while stalking through a marsh during the day prevents it from toppling over at night.

Do bugs bug bugs?

Do they ever! "There are very few insects that don't get parasites," says mite expert Bruce Smith of Ithaca College. "In fact, there are very few organisms of any kind that don't." Smith reports that close to a thousand mites can hitch a ride on a single dragonfly. There are mites that live aboard no-see-ums, the tiny biting flies that are themselves small enough to fly through window screens. Another mite, Acarapis woodi, makes its home in the breathing passages of honeybees. This opportunistic behavior doesn't suffocate the bees, but it does shorten their life spans - dooming many a commercial hive in the process.