Biology Reference
In-Depth Information
On a steamy July evening in Chitwan, we had just sat down to dinner when we heard tre-
mendous 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 under-
stand for both rarity theory and conservation, especially when rare species reach low popula-
tion 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 genet-
ic 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
