The Kingdom of Rarities (21 page)

Our census of the greater one-horned rhinoceros and data gathered from the thirty-five or so radio-collared rhinos we had been intensively monitoring led to a number of conclusions. The first confirmed the observation that Chitwan rhinos shunned the dense mature forest in favor of riverbanks where wild sugarcane swards grew. Much of Chitwan is composed of sal forest, the dominant forest type of the northern part of India and lowland Nepal. Wildlife biologist Bivash Pandav calls it “green desert.” Sal is a member of the family Dipterocarpaceae, the most valued group of timber trees in Asia, comprising about 534 species and reaching its highest diversity in Borneo and Sumatra. Northern India and Nepal are covered by just one species,
Shorea robusta
; it grows tall and stout with deep-furrowed bark and straight, lightly branched trunks and remains green almost year-round. Despite the persistent greenery, few of the plants that grow in this forest, including the tannin-rich sal, produce leaves that herbivores like to eat. Our census confirmed that in much of the park, rhinos are relatively sparse because of the dominance of this forest type.

The rhinos also concentrated their feeding in a small area. One traditional rule in mammalian ecology, especially for herbivores, states that a species' home range is related to its body size. This is a critical insight in the study of rarity. Plant- and seed-eating rodents—a category that includes many narrow-range rarities—can still be abundant numerically. Their home range is often measured in square meters. At the other extreme, elephants and rhinos, according to the rule, should roam widely and live at low densities. Yet radio-collared female rhinos used only 3.5 square kilometers annually and males a slightly larger area, in contrast to elephants, whose home range might be as large as 30 square kilometers. The core part of the rhinos' range was actually less than one square kilometer, remarkable for a giant herbivore. Just as important, the
home ranges of females, including those with calves, overlapped, so many rhinos were packed into a small area. That spring, the density in the riverine grasslands of Chitwan reached thirteen adults per square kilometer—among the highest densities ever recorded for a giant mammal. What, we wondered, accounted for this? And it wasn't just rhinos whose density was unusual, but the park's big carnivores as well. The home ranges of Chitwan's tigers and leopards are smaller than almost anywhere else, with male tigers averaging about 20 square kilometers and females around 6–10 square kilometers. For comparison, in the Russian Far East a male tiger's home range may be as large as 600 square kilometers.

Happily, our long hours of logging data on elephant-back were starting to answer this question, first for rhinos and then for tigers. The twenty-four-hour movements and feeding preference of habituated radio-collared rhinos revealed that not all grasslands were equal for them as feeding spots. The term “elephant grass” encompasses a host of extremely tall grass species, many of which are highly woody and unpalatable to rhinos once the plants grow past the shoot stage. Our results showed that rhinos prefer only about five or six species of these tall grasses. Tall grassland accounts for about 10 percent of Chitwan National Park's vegetation, and 90 percent of that 10 percent is dominated by two species that rhinos eat only as young shoots, one in the genus
Themeda
, the other in
Narenga
. The remaining sliver of grass habitat directly adjacent to the riverbed, covered by a species known in Nepali as
kans
(
Saccharum spontaneum
), or wild sugarcane, is what rhinos really prefer. The wild cane remains green for much of the year and sprouts anew in response to grazing, cutting, fire, or inundation. Year-round, kans accounts for at least half the monthly diet of the rhino; of all the rhinos' potential forage grasses, kans turned out to have the most protein and to be one of the most digestible. So rhinos are not only grassland specialists but also kans connoisseurs. Using their prehensile upper lips to wrap around the stems and their high-crowned molars to chew them up, rhinos are highly proficient mowing machines.

The kans grasslands and some of the other short grasslands adjacent to them or mixed in with them were also the preferred feeding areas for the tiger's prey. The density of deer and wild boars in the kans and riverine forests was among the highest recorded wherever this habitat occurs in Nepal and India.

Floodplains in Asia are among the most productive landscapes on Earth for rhinos, tigers, and other animals as well. Such rich, fertile soil supports, for example, the endangered swamp deer, extinct in Chitwan since 1950 but still present in small numbers in Bardia and abundant in the Suklaphanta reserve. Rare across much of its range in Nepal and India, the deer forms herds that number in the hundreds, until recently in the thousands, in open grasslands in Suklaphanta. The same is true for the endangered hog deer, which is rare elsewhere but quite common in the kans, sometimes traveling in herds of thirty or more. And wild water buffalo, which used to be common in Chitwan but now have been extirpated, were reportedly found only in the riverine grasslands. The foraging patterns of all these large-bodied species illustrate how the highly productive kans, the first elephant grass in the line of succession along riverbanks, is the keystone plant species offering the most critical habitat in this large-mammal ecosystem.

Another critical habitat element for rhinos is water. During the hot, steamy monsoon, rhinos are unable to sweat fast enough to cool off; to compensate, they become almost semiaquatic, spending up to eight hours a day submerged up to their nostrils. When they wallow on their side, their broad bodies form perfect sundecks for rows of amphibians. When the rhinos turn over, the frogs quickly shift to their new perch.

Rhinos concentrate close to rivers, near the ribbon of wild cane, and in areas pockmarked by wallows. Chitwan offers all of these features. With rhinoceroses so locally abundant there, I had to keep reminding myself just how globally rare these creatures truly are. On a typical hot spring morning in 1987 in the Pipariya grassland, for example, rhinos, especially mothers and calves, seemed to be everywhere.
Within the span of two hours, in an area no bigger than a shopping-mall parking lot, we counted thirty-five individuals, the highest concentration I had ever seen. A morning stroll across the grassland would demonstrate the truth of local abundance to any ecologist who doubted that such a thing was possible, even for a rhino.

On a steamy July evening in Chitwan, we had just sat down to dinner when we heard tremendous crashing sounds in the forest next to camp. Galloping across the compound at full tilt were two male rhinos, one chasing the other. The bellow of the chaser sounded like the earth ripping apart. Ignoring us completely, the males thundered past the dining area and straight through a barbed-wire fence, snapping the strands as if they were party ribbon.

This vignette of competition among males sparked a new research focus for us: the link between dominance and breeding success in males. Male rhinos don't use their horns when they fight; instead they use daggerlike tusks housed in the lower jaw. Remarkably, we found, it was the size and condition of the males' tusks that determined who had sex and who was vanquished. When we recaptured losers to replace their radio collars damaged in the duel, the vanquished males always had shorter incisors or broken teeth, as compared with the intact, larger incisors of the males that usurped them.

This relationship between dominance among males and breeding success is vital to understand for both rarity theory and conservation, especially when rare species reach low population levels. It is of even greater concern where the species is polygamous and one long-lived male can often monopolize breeding for many years. This could in turn lead to inbreeding, which would reduce the genetic vigor of the population and lead to the kind of downward spiral that rare species must avoid. This mechanism is the presumed cause of severe declines among isolated small populations of cheetahs and some mountain sheep.

According to the data we collected, however, these rhinos were in no danger of genetic decline. By radio-collaring the seven bulls that had been dominant in our intensive study area during the five-year field project, we found that six different males had rotated through the top position. One reigned for as long as a year and a half, but the tenure of two other males lasted less than two months. By gauging the age of each calf they produced, we gained a proxy for how many rhinos each male had likely sired while dominant. In some cases, the answer was zero because females cycled into estrus, or breeding stage, only every sixteen months or so.

The rapid turnover among breeding males implied a high degree of genetic mixing, in our view. To test this theory, we took blood samples for future analysis from many of the rhinos we sedated. Ecologist Gary McCracken led this part of our effort and, during a visit to his lab in 1988, revealed some startling news. “You won't believe this. Your rhino samples have among the highest levels of genetic variation ever recorded for mammals!” The results ran contrary to a commonly accepted aspect of rarity, that rare species typically have little genetic variability, especially those on the brink of extinction.

How had the rhinos managed to accumulate so much genetic variability in the first place? And how had they been able to retain it when their numbers crashed after 1950? The answer to the first question was simple. As would most ancient mammals, rhinos over the course of millions of years had accumulated a lot of mutations—and thus genetic variability—during their evolution. When rhinos were common across their range, they were highly mobile and able to spread their genetic material among their populations and maintain a large breeding stock. In so doing, the species built up a large genetic reservoir. This is also likely true of other large mammals where individuals have moved long distances from their birth areas and there has been a lot of genetic exchange among populations.

The answer to the second question also seemed straightforward. We knew that the collapse of the rhino population was relatively
recent. With more than 1,000 rhinos likely in the Chitwan area before 1940 or so, and a crash to 60 to 80 individuals by the early 1960s, only a dozen or so rhino generations had elapsed before the population started rapidly expanding again in the 1960s. McCracken explained: “Populations lose variability by rhino generation, not year by year—a rhino doesn't breed every year, and few rhino generations had elapsed since 1950. So little loss occurred.” Here, then, was an example of a large-mammal population that had survived a bout of near extinction and still harbored high levels of genetic variability.

When a large species becomes especially rare or extinct, there may be many consequences for the ecosystem of which it has been a part. Large mammals that dominate an ecosystem and then disappear or shrink to a level at which they become “functionally extinct” no longer perform their long-standing ecological roles, whatever those might be. In chapter 1, I described the experience that triggered my interest in rarity—understanding the role of greater one-horned rhinos in the dispersal of seeds of the tree
Trewia nudiflora
and the creation of
Trewia
woodlands in the grasslands. Lecturers in plant and animal ecology in the United States and Europe often mention the extinct terrestrial giants only in passing. Bison, mastodons, giant ground sloths, North American rhinos, woolly rhinos, and their allies no longer play their former parts as landscape engineers, so they are typically ignored except as historical curiosities or examples among the early extinctions. But through their browsing, trampling, grazing, wallowing, and manuring, giant herbivores have over time played a major role as nature's architects, shaping the evolution of plant traits.

In most of the world today, no living laboratories remain in which to test theories about the ecological roles of big mammals. In areas of Chitwan, however, giant herbivores lived at such high densities that it was possible to study how plants and giant mammals might have interacted since the Miocene epoch, over 20 million years ago.
All one needed was an elephant to maneuver through the grass and follow the rhinos, a bit of curiosity, and some basic gardening skills.