The New Neotropical Companion

Adaptive Radiation in Bats

The basic bat is a marvel of adaptation. Bats are the

only mammals capable of powered flight (excluding

the unique species that invented airplanes and space

shuttles). Bats appear in the fossil record as far back as

the Eocene, arising approximately 60 million years ago.

Their closest evolutionary relatives are insectivores,

the moles and shrews. The most distinctive adaptation

of bats is the modification of the forelimb (arm) as a

wing, accomplished through elongation and enclosure

of the arm and finger bones within a membrane of skin

(plate 8- 18). Like birds, bats have large hearts, light

body weight, and high metabolism. While most birds

are diurnal, all bats are nocturnal. In terms of species

diversity, birds are much more species rich than bats;

there are approximately 10,000 bird species in the

world, compared with 1,240 bat species. Nonetheless,

bats make up the second- largest order (Chiroptera) of

mammals, surpassed only by the rodents.

Bats in the American tropics are all members of

the suborder Microchiroptera. Most visitors to the

tropics have little idea of the true abundance and

diversity of bats in the forests and surrounding habitats.

Microchiropterans traditionally capture insect prey on

the wing and avoid obstacles by using echolocation. They

emit loud, high- pitched vocalizations (mostly inaudible

to humans) that bounce off objects of approximately the

same size as the wavelength of the emitted sound, thus

providing the bat with an effective system of sonic radar

for locating small nearby objects, such as flying insects.

Most microchiropterans display very prominent pinnae,

or external ears, that aid in receiving the echolocation

signals, including those from inanimate objects such as

tree branches. Microchiropteran bats usually have small

eyes and presumably poor eyesight, relying instead on

their incredible echolocation skills (plate 8- 19).

The nocturnal nature of bats makes them a challenge to

study. Louise Emmons, in her book Neotropical Rainforest

Mammals: A Field Guide (2nd ed., 1997) states that there

are generally more species of bats in a Neotropical rain

forest than all other mammals combined, and that bats

make up 39% of all mammal species in the Neotropics.

Researcher Bruce Miller has done years of work on the

bat community in Belize using a computerized sound-

detection system to detect vocalizations of free- flying

bats, as well as direct capture of bats to document species

present. Miller has been able to census bat diversity by

recording the animals’ vocalizations on sonograms and

has documented 84 bat species in Belize, a country about

Plate 8- 17. The Large Ground Finch (Geospiza magnirostris)

has a massive seed- crushing bill. It is one of the 14

Darwin’s finches, which have substantially diverged in bill

characteristics. Photo by John Kricher.

Plate 8- 18. The Neotropical Pygmy Fruit- eating Bat

(Dermanura phaeotis) provides an example of bat external

anatomy. Note the large ears and leaf- like nose, adaptations

for the bat’s sonar system. The membranous wings are

supported by the forearm (prominently visible) and the finger

bones. A mammal, the bat is otherwise covered with hair.

Photo by Bruce and Carolyn Miller.

Plate 8- 19. The well- named Wrinkle- faced Bat (Centurio senex)

displays large pinnae (external ears) and wrinkled facial skin,

adaptations to enhance high- frequency sound detection.

Note the small eyes. Photo by Bruce and Carolyn Miller.

chapter 8 evolutionary cornucopia 121