Sunday, November 19, 2006

Why are Malaysian leopards black rather than spotted gold?

Malaysian Mystery Leopards
By Fiona Sunquist

Dec/Jan 2007, vol. 45 no. 1

A new study reveals that the leopards of peninsular Malaysia are black rather than the typical spotted gold, but the question remains: Why?

“Another black leopard,” biologist Kae Kawanishi thought as she leafed through pages of color prints just back from the processing lab. Stacks of yellow-and-white albums filled with images covered the surface of her tiny desk at her home in Kuala Lumpur, Malaysia, while hundreds more albums filled with neatly labeled photos from previous months’ surveys lined a nearby bookshelf.

Kawanishi was in the last month of her three-year study of tigers in peninsular Malaysia’s Taman Negara National Park. Although the epic camera-trapping study—in which she and her research team set up cameras in the rain forest so that animals would trigger trip wires and photograph themselves—was designed primarily to estimate tiger density, Kawanishi’s work also gave the world its first glimpse of the full array of wildlife in a Southeast Asian lowland rain forest. Kawanishi and her team, bankrolled largely by the Save the Tiger Fund through the University of Florida’s Malaysia Tiger Project, racked up thousands of photos of Malaysian wildlife. Day and night, tigers, tapirs, elephants and porcupines strolled past the 150 or so cameras set along game trails. Dozens of little known species, including clouded leopard, dhole, golden cat and the goatlike serow, tripped the infrared sensors and photographed themselves. Among the most unexpected finds was Taman Negara’s leopards, which triggered the cameras more than 100 times. The surprising thing about them: In every one of the photos the big cats were black.

Looking for an explanation as to why this usually rare color phase was prevalent, Kawanishi checked with the forest people that helped her team with the camera-trapping study. “About 400 Orang Asli [original people] live in the park,” she explains. “They are hunter-gatherers. They know every animal in the forest. When we asked them about the various cats, they recognized clouded leopards, tigers and black leopards, but no one recognized a large spotted cat. To the Orang Asli all leopards are harimau kumbang, or black leopards.” The fact that the Orang Asli never saw spotted leopards at least proved that the cameras were not lying.

Leopards—large, usually golden cats with black spots—range throughout much of Africa, avoiding only desert areas, and across the Middle East into Asia, including Java and Sri Lanka and extending as far north as Siberia. They feed on creatures varying in size from rodents to hoofed animals in excess of 200 pounds. Competing with lions or tigers in parts of their habitat, they often drag their prey up into trees, where larger cats and scavengers cannot get at it. An estimated 500,000 leopards survive worldwide, although some of the 22 recognized subspecies are endangered. They range in weight from 60 to more than 200 pounds.

Black leopards, also known as black panthers, are rare in the wild. Almost never seen in Africa, they show up only occasionally in southern India. Historical reports and hunters’ stories from a century ago record that as many as half of the leopards in the Malay Peninsula may have been black, but as Kawanishi has just discovered, black leopards now seem the norm in that part of the world. The reason might be as simple as camouflage or as complex as disease resistance.

Just as albino individuals -— all white with unpigmented, red eyes -— crop up occasionally in many species, so too do melanistic, or all-black, individuals. The color comes from a pigment called melanin, the same pigment responsible for suntans. Both albinism and melanism are influenced by genes. In domestic cats and leopards, a single recessive gene controls dark coat color, so both black and spotted leopard cubs can occur in the same litter, and black cubs can be born to spotted parents.

Black leopards have a reputation for being more aggressive than spotted leopards, and early biologists thought they were a separate species. Not so. “Black leopards are simply leopards with dark coats,” says John Seidensticker, senior scientist at the National Zoo in Washington, D.C. “If you look closely, you can see the faint outline of spots in the black leopard’s dark fur.”

Black leopards are black because they have a mutation in the gene responsible for coat color. But there is more to color than meets the eye. Genetic mutations that affect coat color can have multiple physical effects, influencing two or three seemingly unrelated physical characteristics. Lack of pigment cells is implicated, for example, in the deafness typical of white cats with blue eyes. A clue to the link between pigment genetics and deafness lies in the fact that white cats with one blue and one yellow eye are often deaf on the side with the blue eye. Studies of lab mice suggest that pigment cells are involved in maintaining fluid in ear canals. Without the fluid, the canals collapse, causing deafness as the auditory nerves degenerate.

Similarly, the characteristic dark ears, nose color pattern and crossed eyes of Siamese cats are all caused by the same gene. The gene that causes defective “frizzle-trait,” or curly, feathers in chickens also decreases their egg laying and causes changes in their hearts and kidneys. Recent research on mice has shown that a mutation in the gene that produces yellow fur results in mice that are chronic overeaters, while another mutation that produces white-spotted fur causes anemia and sterility. Scientists are using mouse models to study the processes that cause these conditions in humans.

In the natural world, rare genetic variations that occur in pigment genes can help animals to adapt better to their habitats. These color differences often provide a selective advantage through camouflage. For example, a recent study of rock pocket mice in the deserts of Arizona and New Mexico by Michael Nachman and his coworkers at the University of Arizona found that pocket mice tend to match the ambient color of their habitat—usually a light sandy color. However, in areas where the mice live on dark, rocky lava flows, the rodents wear dark, almost black fur. This camouflage presumably provides more protection from great horned owls, a significant predator of these mice.

The adaptive benefits of black fur for leopards are more difficult to interpret. Researchers Eduardo Eizirik, Stephen O’Brien and their colleagues at the National Cancer Institute’s Laboratory of Genomic Diversity in Maryland have mapped, cloned and sequenced the genes responsible for black coats in cats. “Melanism is particularly common in the cat family,” Eizirik says. “This is important because it means that dark fur, or something connected with it, has a survival benefit.” One theory is that animals living in dark, humid forests generally have darker fur for camouflage. African leopards spend most of their lives in open habitats with dappled sunlight, where spots are the best disguise. In Malaysian forests, black coats may be better camouflage than spots. “The most likely benefit of melanism is camouflage for hunting,” Eizirik says.

However, the black leopard story may prove more complicated than mere camouflage. O’Brien, chief of the Laboratory of Genomic Diversity, suggests that there are many other selective pressures besides being able to hide. “Another explanation is that about 70 percent of selective pressures associated with the biological environment involve microbes and diseases,” O’Brien says. Recent studies have shown that coat-color genes also affect the immune system. “The types of receptors used for coat colors are also used by viruses to enter cells,” O’Brien says. “It is plausible that some of these color mutations are adaptive—relics of historic epidemics.”

The fossil record indicates that leopards have roamed in peninsular Malaysia for more than 150,000 years. The first to arrive there must have been spotted, like their relatives in Africa and Asia. Over time, spotted coats gave way to black, as Kae Kawanishi’s snapshots show. It might be that the black coats arose as an immune system response to an ancient epidemic. Or it might be, as Kawanishi suggests, “In the dimly lit rain forests of Malaysia, black leopards are less visible than spotted leopards—they are better concealed from prey and from the tigers they share habitat with.” In the end, the reason all Malaysian leopards are black remains, like the cats themselves, a mystery.

Fiona Sunquist is a long-time contributor to National Wildlife. issueID=112&articleID=1413

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