When male wolves feel amorous, they are famously known to howl. When male cheetahs find themselves in the same spot, so to speak, they resort to a more modest bark, a staccato yip that scientists say does much more than simply catch a receptive female's ear.
The so-called “stutter-bark,” according to new research, seems literally to turn the female on, triggering a cascade of reproductive hormones that results in the release of eggs for fertilization.
“We think this is the first known case of mammalian ovulation by acoustic inducement,” said Matthew Anderson, a researcher in the San Diego Zoo's behavioral biology division who recently documented the discovery with Fred Bercovitch, associate director of the zoo's Conservation Research Department.
The cheetah research is part of the zoo's larger experiment in “sensory ecology,” an effort to better understand how animals use sound (and other signals) to communicate.
Wild elephants, for example, are known to transmit messages via audible calls and also by long-distance ground vibrations picked up by other elephants through their feet.
The okapi, an African ungulate that resembles a zebra-cow-giraffe hybrid, sometimes communicates using low-frequency calls inaudible to humans and, more importantly, to predators.
Recently, scientists at the zoo's Conservation Research Department and the Hubbs-SeaWorld Research Institute announced a first-ever study of polar bear hearing sensitivity. They hope the knowledge gained will help wildlife managers better protect wild bears confronted by an increasingly crowded and noisy environment.
The findings concerning cheetahs' vocal cues are currently under peer review for scientific publication.
Cheetahs have long been considered atypical members of the cat family, perhaps not true “big cats” at all. For example, cheetahs are the sole members of their genus, Acinonyx, which in Greek means “no-move-claw,” a reference to the fact that unlike almost all other cats, cheetahs cannot completely retract their claws.
Those claws help cheetahs run faster than any other land animal; yet cheetahs cannot climb trees. Nor can they roar like other big cats. Instead, they purr, growl, chirp, yowl and bark.
Not that males bark or females reproduce very often. Unlike lions and tigers, female cheetahs do not have regular estrus cycles – periods of increased physiological and sexual arousal. Rather, they experience long stretches of non-estrus, according to Anderson.
“In nature, cheetahs mate very intermittently. No one knows exactly why. It may be because females spend most of their lives alone, roaming very large home ranges. It wouldn't pay for them to get pregnant all of the time.”
It's apparently up to the male cheetah to inform the female when the time is right. Bercovitch said scientists had long suspected a link between the male's stutter-bark and female fertility, but no one had closely or systematically studied it.
He and Anderson did so by observing the Wild Animal Park's cheetah population (one of the few places where captive cheetahs have been successfully bred), recording episodes of stutter-barking, then analyzing fecal samples for changes in female reproductive hormone levels.
“You can't measure all of the hormones this way,” said Bercovitch. “It's more of an indirect assumption of ovulation.”
Nonetheless, Anderson and Bercovitch say they have noted a definite correlation between stutter-barking and successful mating.
Male cheetahs don't stutter-bark often, maybe a few times a year. The barking begins one day with just a few instances, but becomes more frequent and louder in subsequent days.
Unlike other cheetah vocalizations, the stutter-bark seems to convey a single message, a specific call with specific intent. “It's very distinct,” said Anderson, “rhythmic and staccato. Not at all like a howl. More like the barking of a tiny dog, except that it's lower and carries a long way.”