The Pause That Enmeshes
by Michael Greenfield
with Roger Martin
In
most species of noisemaking animals, females prefer the sexual calls
of some males over others. Usually, the preference is based on the energy
of the call. Females often prefer loud calls or calls delivered at a
high rate. But females also may exhibit preferences that have nothing
to do with loudness or frequency.
For example, among many species of frogs and acoustic
insects, females seem to prefer the male who makes the first of two
or more calls that occur closely in time. In psychophysics, this is
known as a "precedence effect," and variations of it also occur among
animals that signal each other visually, rather than acoustically, such
as fireflies and fiddler crabs.
A precedence effect influences human hearing, too --
although, as far as we know, not in the context of mate choice! That
is, humans pay less attention to a second sound in a series when the
second comes too close on the heels of the first.
It's currently unclear why animals would have evolved
to give preferential notice to the first of two signals they receive
from potential mates or other sources.
Precedence effects in hearing have had an effect on
the evolution of calling behavior in animals. That is, evolution has
favored the survival of males that do not call immediately after the
onset of a neighbor's call. The males that wait increase their production
of leading calls and presumably get more attention from females.
Now in situations where many animals are crowded together,
there's a danger: A male's calling rate might drop sharply. Therefore,
we hypothesize that the males learn to attend selectively to those around
them -- just as humans, say, learn to focus on one person or a small
group of individuals at a cocktail party and tune out others.
Túngara frogs are among the more common amphibians in
Central America and northern South America. Female túngara frogs display
a strong preference for leading male calls, and males are inhibited
from calling following a neighbor. However, playback experiments conducted
in the acoustic laboratories at the Smithsonian Tropical Research Institute
in Panama demonstrated that males apply this inhibitory mechanism in
relation only to a subset of nearby signaling neighbors.
This research will be published in a future issue of Ethology
under the title "Frogs Have Rules: Selective Attention Algorithms Regulate
Chorusing in Physalaemus Pustulosus (Leptodactylidae)."
Selective attention is regulated by a combination of
several rules. If those rules were contained in an instruction manual
passed out to túngara frogs at birth, here's how they'd read:
In determining when to make your courting call
- Pay attention to (allow yourself to be inhibited
by) the loudest (usually nearest) neighbor and any additional ones
whose calls are only 6 to 8 decibels weaker than the loudest.
- Pay attention only to two neighbors when the
calls are all weak or the second one is much weaker than the first.
- Pay attention to three neighbors when the calls
are loud or all neighbors are approximately equal in loudness.
These rules suggest that frog chorusing is a complex
social feature that ultimately evolved in the context of sexual competition.
Getting there first, or avoiding being drowned out by coming in second
a hair after whoever does get there first, seems to be important to
a male túngara frog in search of a mate.
