Elephants know best time for a trunk call
Namibian elephants prefer to broadcast their very low-frequency calls at exactly the times of day when the air is best for carrying sound a long way, research has found.
In a three-week study that incorporated a range of meteorological equipment and an array of eight microphones, Garstang and team found 42% of all elephant calls were made during the stable air period three hours after sunset.
The next most popular calling time during the two hours after sunrise, also a time when the acoustics of the atmosphere are the best for carrying calls great distances.
"This project was an attempt to demonstrate in the field predictions we had made with mathematical, acoustical models," says Garstang.
Of about 1300 calls recorded during the study, 94% fell within those two time periods, he says.
Garstang's interest extends to other animal calls as well, he says, but elephants were the logical animal to study, since they produce the loudest calls that, because they are made at infrasound frequencies, should theoretically reach the furthest.
The "stable air" that Garstang referred to is air that is layered, or stratified, by temperature with an inversion layer, like the air that traps smog closer to the ground in many cities.
The stratified air and the inversion help to bend sound back toward the ground, therefore propagating it further along, says Garstang.
Extremely stratified air, such as can exist in the Arctic, has been known to carry human conversations at audible levels for over 3 kilometres, according to the University of Colorado researchers at the National Snow and Ice Data Center.
The sun warming the ground each day in Namibia, however, causes heat to rise (often called "thermals") and break up the layering, along with the best opportunity to call to distant elephants.
To verify this theory, Garstang's team set up elaborate atmospheric measurement equipment and flew weather balloons throughout the study to keep constant track of the wind speeds and temperatures at various heights in the atmosphere above the study area.
Elephants need to call great distances in order to reproduce, says Garstang. Female elephants have only a brief period of fertility and need to get the best mates in a timely manner.
The only way they can do this is to put out an oestrus call, he says, and listen for what sorts of answers she receives.
It appears that elephants do know that morning and evening are best for calling, without any knowledge of why it's best, says Garstang. Elephants have simply learned what times work best and stuck to them.