Do Lions Purr? And Why Are There No Green Mammals?

National Wildlife answers 10 pressing questions about the animal kingdom

  • Doug Stewart
  • Apr 01, 1995

Time again to dig through the mailbag 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, cougars 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. 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.

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 leg is long, featherless and coursing 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 lifespans—dooming many a commercial hive in the process.

Other bug- bugging bugs are exactly what humans are looking for. In the U.S. South, armies of fire ants have an inconvenient tendency to chew through electrical insulation (inside traffic- light poles, for example). Entomologists at the U.S. Department of Agriculture (USDA) are plotting to enlist a South American fly, Pseudoacton, to act as a mini Trojan horse. Female Pseudoacton lay their eggs inside fire ants. The larvae feed on the ants from the inside and finish up by decapitating their hosts.

Another internal parasite being tested by the USDA is a nematode worm that digs through the outer skin of aquatic mosquito larvae and makes itself at home. The worm even goes through several molts there, to its host's increasing detriment.

More subtle are the social parasites. These are animals that mimic the signals and behavior of high- status hosts—queen bees, for example—as a means of enslaving their oblivious underlings. One socially parasitic ant, Teleutomyrmex schneideri, is so adept at subjugating the colonies of another ant species that its own hunting and feeding organs have mostly disappeared along with its worker caste. Explains psychologist Howard Topoff of Hunter College, "The parasitic Teleutomyrmex queen spends much of her life riding on the back of the host queen while being fed by the workers of the host species, an ant called Tetramorium caespitum." And let's not forget hyperparasites: bugs that bug bugs that bug bugs- - and so on. Some of the scenarios involving parasitic wasps suggest a Biblical genealogy ("Euryptoma beset Mesopolobus, which beset Toryus, which beset Syntomaspis, which beset Cynips, which started the trouble by besetting the gall of Cynipidae"). It's a bug- eat-bug world, all right. To quote the poet:

Big fleas have little fleas
Upon their backs to bite 'em,
And little fleas have lesser fleas,
And so, ad infinitum.

How do birds find seeds they've hidden?

Apparently, even with thousands of hiding places to keep track of, birds try to memorize them all. "A black- capped chickadee encounters hundreds of seeds a day, maybe more," says Fernando Nottebohm, a biologist at Rockefeller University. "It stores from one- third to one- half of those seeds, usually singly, and it may do so over an area of 30 acres to a height of about 60 feet." A good sense of smell can't explain the bird's success at recovering what it hides, Nottebohm says, nor can random pecking. "It's very unlikely you'd hit the same spot again unless you remembered it."

So how can a little bird brain have such a fabulous memory? Nottebohm's research offers a clue. In a recent experiment, he measured a dramatic jump in the number of new cells in the black- capped chickadee hippocampus—the part of its brain that seems to be involved in spatial memory. The peak in the recruitment of new cells, which replace older ones that die, comes around October each year, just when the bird's seed- caching is at its most furious. He speculates that the new brain cells are better able to acquire new memories. Captive chickadees don't show the same cell growth. (Nor do humans, alas, regardless of their situation.) Moreover, says Nottebohm, "The hippocampus of birds that hide food is larger than that of birds that don't." He's quick to add that the evidence linking memory to these brain- cell changes is only circumstantial; he plans new experiments to study the connection more definitively.

What animal grows the most over its lifespan?

Probably no creature can match the impressive growth of the ocean mola (also known as the sunfish). A full- size sunfish can stretch 10 feet and weigh 1,200 pounds, which is some 60 million times what it weighed as a hatchling. The egg it emerged from was about the size of this o.

As for live- born animals, the champion is surely the red kangaroo. A 180- pound adult male is born a hairless, translucent, bean- sized object weighing less than a gram—amobile fetus, in effect. It uses its front paws (its back legs are just bumps) to "swim" a short distance through its mother's abdominal fur to her pouch. There it remains clamped onto a nipple for several months while filling out. Newborn kangaroos are so improbably small that nineteenth- century British anatomist Sir Richard Owen concluded they appeared in the pouch as buds that broke off from their mother's teats.

How do hibernating animals know when to wake up?

For true hibernators -- deep winter sleepers that go cold and almost stop breathing—warm weather alone isn't necessarily enough. Biologist Alan French of the State University of New York at Binghamton has studied three hibernating species that share the same alpine meadows in the high Sierras: yellow- bellied marmots (or western woodchucks), Belding ground squirrels and jumping mice.

"Marmots, male and female, will come up right through the snowpack," French says. "They're using an internal calendar. There's no apparent environmental trigger." Weighing nearly 10 pounds, the big males have cushions of fat to live on if they emerge from their burrows too early for browsing.

Male squirrels, one- tenth the size of big marmots, generally do the same: Once their biological alarm clock goes off, they dig out. The females, however, are more cautious. Being skinnier than males, says French, they're more vulnerable to bad weather. They'll rouse themselves periodically as spring approaches and check the temperature of the soil plugging up their burrow. If the plug's warm enough, they dig out; if not, they go back to sleep. By keeping the plug frozen, French has managed to keep captive females hibernating 12 months in a row.

Jumping mice, a tenth again smaller, play it safest of all. They wait for the temperature down at their nests to tell them when to get up and stay up. "These guys can't afford to gamble," French says. "They stand to die too easily in an early spring snowstorm."

Do animals see color?

That depends on the animal and whether seeing in color helped its ancestor's survive. Animals didn't evolve color vision so their surroundings would look prettier. Rather, color vision is chiefly a tool for picking out objects (food, predators, potential mates) from their backgrounds, especially when the objects aren't moving.

Many flying insects see not only color but also ultraviolet light, which butterfly wings and flower petals often reflect—"bee purple," it's sometimes called. The brilliant colors of flowers are actually evidence of insects' color vision: Flowering plants evolved color to lure pollinators. Plants pollinated by moths and bats, by contrast, tend to have white flowers, which show up best at night.

Fish need color vision to make up for a lack of contrast underwater, though the range of color they can see may be narrow indeed. Deep- sea fish see innumerable shades of blue since so much of their world is blue; red doesn't penetrate the depths, so they've never developed the ability to see it. Up near the surface, where water doesn't filter out so much of the color spectrum, fish not only see yellows and reds but also wear them to attract mates or warn intruders. Small wonder that the rainbow- colored fish so beloved of snorkeling tourists are shallow- water reef dwellers.

Tourists aside, mammals are generally colorblind. (That's right. Bulls have no idea the matador's cape is red.) As mammals evolved, they seem to have lost the ability to see colors that longer- established creatures—birds, reptiles and winged insects, in particular—see vividly. "For two- thirds of our history, we mammals were mostly small, nocturnal, rat- sized things," says biologist Dan Blackburn of Trinity College in Hartford. "To compete ecologically in the Mesozoic Era, mammals had to do something that reptiles weren't doing, such as being active at night." Since colors are virtually impossible to see in the dark, mammals came to rely more on their noses than their eyes. Our cats and dogs can hold their nocturnal ancestors responsible for their poor color vision and for their great sense of smell).

Primates, being diurnal, seem to have re- evolved color vision. "One thing primates do very well is live in trees," says Blackburn. "Color vision gives you extra information about your environment when you're jumping from branch to branch." That humans see colors in the green- yellow region most acutely is a throwback to our days struggling to survive in the verdant wild. It also helps explain why readers of L.L. Bean catalogs can evidently discriminate among two dozen shades of green—including spruce, teal, sage, hunter, jade, loden, aqua, emerald, olive, forest, bluegrass and seabreeze.

For primates, color vision is also useful for meal selection, Blackburn says. "If you're feeding on fruits and vegetables, as opposed to dark little insects, you need to be able to tell the ripeness of things and whether certain colored berries will make you sick. An insectivore like a shrew will just eat everything in its path—earthworms, grubs, whatever."

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