Imperiled species reproduces in a quiet refuge
Many zoos and wildlife parks have replaced cages with more natural enclosures, but they may be paying less attention than they should to sound, says Suzi Wiseman, a graduate student in environmental geography at Texas State University-San Marcos.
At last week’s meeting of the Acoustical Society of America, Wiseman reported on sound and the white rhinoceros at a Texas game park. She says she chose the site because it is one the few parks that can actually breed white rhinos in captivity, indicating that they are healthier and/or happier than the average captive rhino.
One reason, she says, may be the rural location of their home, the Fossil Rim Wildlife Center. But how, exactly, does that good ol’ rural life help?
One possibility is sound, says Wiseman, who has just finished preliminary exams for a Ph.D. that she expects shortly.
White rhinos, natives of tropical and subtropical savannahs and grasslands, “Have the most sensitive hearing of all animals, according to animal trackers,” Wiseman says.
You can take the rhino out of the wild, but can you take the wild out of the rhino?
Concerned that rhinos, which have huge territories and can run 35 mph, spend too much time standing around at zoos looking bored, Wiseman several years ago started “enrichment practices,” playing audio recordings and watching for a response. “In the Texas summer,” she says, “I played the sound of a storm rolling in, and they jumped up and started looking around and sniffing the air. I played a bird call, and they started poking their heads into the bushes, looking for birds. I played frog calls, and they went over to the moat and looked at the water.”
But when she played zoo sounds, like a child crying, “I got a fairly disinterested response.”
When we commented that these experiments seemed pretty straightforward, Wiseman responded, “I was astonished that nobody had thought to do this.”
In her report last week, Wiseman described an exhaustive program to measure the full panoply of sound, stretching beyond the optimal human range of 20 to 20,000 hertz to:
Infrasound: vibrations slower than a person with perfect hearing can detect
Ultrasound: vibrations above the 20,000 limit to human hearing
Geosound: vibrations in the Earth made by wind, traffic and footfalls
Daddy sang bass
We asked how low a rhino could hear, and Wiseman admitted, “It’s controversial. Rhinos have been recorded in the literature vocalizing down to 4 or 5 hertz … but there are indications that it’s a great deal lower than that. Some say infrasound is just a byproduct of their large size, but if an animal vocalizes at a particular frequency, and has most of the energy in their call at low frequency, I assume they can hear that frequency. The hard part is proving it.”
Infrasonic noise, she points out, “tends to be pretty typical in urban areas,” due to traffic and other concomitants of development.
In the future, Wiseman wants to record soundscapes at zoos and other captive environments and compare animal health in different environments. “Where they are healthier, and what is the sound like?”
Typically, acoustic studies look at volume and pitch. But Wiseman probed more deeply, saying, “I think this is the first study that used broad-based measurements to look at the whole soundscape.”
She looked, for example, at entropy — a ten-dollar term for variations in sound frequency and energy. “We know humans respond poorly to a fluctuating sound, and we know animals respond to a sudden noise out of nowhere.” Sirens vary quickly in pitch and intensity and are designed to grab our attention – and create an emotional response. “The point is you get stressed out if the sound moves,” Wiseman says.
A sound idea?
To gauge response to the sound, Wiseman suggests measuring stress hormones in urine. “We might be able to play back a positive sound track to rhinos and see if the stress hormones go down.”
If the research continues to support the importance of sound to captive-animal health, zoos will be able to adapt, Wiseman says. 50 years ago, she notes, zoo relied on cages, but “now all of them have improved. I hope soundscapes might be another area for improvement.”
Among her suggestions:
Using embankments or fences to block noise and rubber mats to insulate against severe ground vibration
Building a waterfall to create white noise that masks stressful noise
Silencing back-up alarms on zoo vehicles (“I’m sure a lot of animals don’t like that”)
Switching off equipment when possible
Zoos “are doing a wonderful job with the facilities they have,” Wiseman says, “but if they are surrounded by freeways….”
Knowledge is power, Wiseman contends. “Are there particular frequency ranges associated with particular problems? Which species are sensitive to these frequencies? Maybe we can move them to another location with less noise.”
– David J. Tenenbaum