In the classic John Ford western, The Man Who Shot
Liberty Valance, there is a traditional western shootout between an eastern lawyer, Rance Stoddard (James Stewart) and the vicious outlaw, Liberty Valance
(Lee Marvin). Valance’s first shot injures Stoddard and just as Valance is
about to finish him off, Stoddard manages to squeeze off a shot and Valance
falls, dead. Unknown to the characters and the audience, at that point in the
film, Valance was actually killed by Tom Doniphon (John Wayne), who shot a
fraction of a second after Stoddard. Had the situation been real, a witness
would have heard a single shot and thought that it was Stoddard’s gun that
killed Valance.
This sonic illusion, called the precedence effect
illusion (PEI),
occurs when two identical sounds, separated in space, are
emitted close together in time. A witness perceives them as coming from the
direction of the first emitted sound. This effect enables us to locate the true
source of a sound and not be fooled by sonic reflections that bounce off other
objects in the environment. Without the PEI we would be unable to identify the
true source of a sound.
Animals such as frogs, birds, crickets, or katydids, use
auditory signals (calls) to establish territories and attract mates. In most
frogs neighboring males alternate their calls, giving an approaching female a
clear choice between competing males. But a few frog species are known to call
almost synchronously, potentially creating a PEI. It works like this: male A
starts to call. A few thousandths of a second later, nearby male B calls. A
female in the vicinity will perceive a single call coming in the direction of
frog A, due to the PEI, and hop or swim toward male A.
Why did frog B not wait a bit before starting to call? If
he delayed calling, he would stand out and have a better chance of attracting a
nearby female. If mating preference was the only factor at work here then
synchronous calling should never have evolved.
In Central America there are bats that use their keen
sense of hearing to prey on calling male frogs. In addition, there are midges,
mosquito-like flies, that hear with their antennae. They can locate a calling
frog and, like mosquitoes, take a blood meal from it. In the act of feeding these
midges can also transmit diseases to the frogs. Could such predators and parasites
make synchronous calling advantageous? If they are subject to the PEI in the
same way as humans they would be attracted to the frog that began calling first.
Suddenly, synchronous calling doesn’t seem like such a bad choice when there are
predators about.
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| Pug-nosed Tree Frog (Wikimedia by Dario/CC0) |
In Central America, found from Costa Rica through Panama,
the Pug-nosed Tree Frog, Smilisca sila, is a synchronous caller and also
preyed upon by the bats and midges. A research group, working at the
Smithsonian Tropical Research Institute in Panama, recorded the calls of
individual Pug-nosed frogs, creating a sound library. From this library they could
use software to create an artificial chorus of frogs, broad cast from a pair of
speakers separated by three to four feet. In this way they could randomly
assign the calls from the sound library to each speaker and randomly determine
which caller would be the leader (the first to initiate calling) and which the
follower. The follower lagged the leader by 23 thousandths of a second, a value
based on actual Pug-nosed choruses.
Captured bats were tested in a flight cage with a video
recorder. Bats were judged attracted to a speaker is they approached it to a
distance of less than 20 centimeters. To determine the attraction of midges
traps were placed around the speakers and the number of midges collected at the
leader and follower speakers were counted.
About 70% of the bat attacks were made on the leader
speaker, 30% on the follower speaker. The response of midges was not as
distinct, ~60% were trapped in the leader speaker, ~40% in the follower speaker. The
difference, in both cases, was statistically significant.
So it appears that in Pug-nosed Tree Frogs it’s a matter
of who croaks first!
Reference:
Legett, H.D., Hemingway, C.T., and Bernal, X.E. (2019).
Prey Exploits the Auditory Illusions of Eavesdropping Predators. The American
Naturalist doi: 10.1086/707719.