Page 50 - 2005 DT 12 Issues

Can you hear me now? called microbats. They are typically porpoises, killer whales and sperm
small compared to the remaining species whales—eventually developed organs
that make up the megabat group. With to produce high frequency chirps and
wilight signals the time when a couple of exceptions, only microbats inner ear structure to hear the returning
many animals begin their day’s use sonar. The distance between pressure sound. Both adaptations are necessary
Twork. Pit vipers, with their
heat-sensors, and scorpions that can pulses in a bat’s ultrasonic chirps and for echolocation. Ancestors of today’s
echoes determines how small a target dolphins could probably echolocate by
detect delicate vibrations in the sand, the predator can detect. Modern bats use about 18 million mya.
emerge from the warm rocks to silently their sonar not only for hunting, but as a Mammals have the ability to hear
hunt; owls peer into the darkness, their general navigation tool for getting high frequencies because of the charac-
large eyes locking onto in and around their roosts. teristic arrangement of those tiny bones
an unwary rodent in the Some bats use sonar to com- in their ears and the structure of the
brush; bats stream into the municate with each other or identify cochlea, the inner ear. Young adult hu-
sky, darting and swooping after members of their own mans can hear sounds with frequencies
“invisible” insects. Each of these species, and can even between 20 hertz, or cycles per second,
nocturnal hunters has evolved differentiate the sex of and 20 kilohertz (kHz). Bat frequen-
different adaptations that help another bat (which comes cies run as high as 120 kHz. Bottlenose
them survive. Only the bats, in handy for scoring dolphin calls range as high as 150 kHz.
however, are able to soar among a date). A German These abilities provide a formidable
their prey with impunity, lock- study of five species predator advantage.
ing in on their targets with the of the Myotis genus The battle is not all one-sided, how-
deadly accuracy of sonar. discovered that each ever. The hunted have developed talents
Echolocation in mammals sub-species emits a and tactics of their own to even the odds.
is at least 50 million years old. different multiple, Some fish, like the American shad, can
Its use has evolved independently or harmonic, of the hear frequencies as high as 180 kHz
by a diverse group of animals over same fundamental fre- (most other fish can hear no higher than
time. Because bats are small quency. The bats’ ears 3 kHz). As the sound intensifies, they
creatures, their delicate bones are extremely sensitive form a tight school, then scatter in panic
don’t often survive fossilization, to hearing their own harmonic fre- when a dolphin closes in, even leaping
therefore, their evolutionary history quency, rendering prey of different sizes out of the water.
is sketchy. However, based on DNA evi- or distances invisible to other species Certain moths can detect a bat’s
dence and estimated long-term genetic when echolocating. chirps 40 meters away, about 4 times
mutation rates, the common ancestor Unlike bats, fossils of aquatic the distance at which a bat can detect
of modern bats probably lived about 64 mammals that lived during a 30-mil- the moths. Random evasion techniques
million years ago (mya). lion-year span of time record the evo- used by the moths make their behavior
The oldest bat fossils recovered so lution of features that have allowed more unpredictable, raising the odds of
far date to about 54 mya, shortly before modern members of the whale family survival in the biological arms race.
a warmer climate spurred an explosion to echolocate. The earliest whale ances- A few birds use sonar, too, but
of plant and insect species. But accord- tors, the pakicetids, lived about 50 mya they can’t produce or hear the high
ing to Mark S. Springer, an evolutionary and were land mammals that foraged in frequencies required to track small
biologist at the University of California, rivers and streams. Although over time targets such as insects. Except for the
Riverside, these fossils aren’t dramati- their lower jawbone became a sound oilbird of South America, all birds that
cally different from living bats. Without receptor that allowed them to hear well
soft tissue structures to analyze, it’s underwater, they had no ability to gener-
not certain that these early bats could ate high frequency sounds. That came
echolocate, but fossilized stomachs full later when early whale nostrils moved
of insects suggests that they could. up from the nose to the top of the head.
About 800-or-so of the more than One of two major lineages of the fam- echolocate were thought to be swiftlets
1,100 species of bats fall into the group ily tree, toothed whales—which include of the Aerodramus genus, which can
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