Evolution, 4th Edition

252 CHAPTER 10

length. The clipped feathers were glued onto the tails of males in the second group,

increasing their tail length by 50 percent. The third and fourth groups were con-

trols: in one there was no manipulation, and in the other the tails were clipped

and then glued back on. All the males were then released, and their mating suc-

cess was measured by the number of females that nested on their territories. The

results were striking. Males with lengthened tails attracted more than twice as

many females as males with shortened tails and the control males, showing that

long tails are favored by sexual selection on males (FIGURE 10.5). This experiment

confirms Darwin’s logic: exaggerated secondary sexual traits are favored by sexual

selection because they increase male mating success.

The second part of Darwin’s hypothesis is that exaggerated secondary sexual

traits decrease survival. While the hypothesis hasn’t been tested with widowbirds,

it has been with many other species. One is the túngara frog (Physalaemus pustu-

losus) [36]. Males spend the night crowded together in ponds and puddles, where

they attract females with calls that are reminiscent of some 1980s video game.

Some males make a simple call known as a whine. Others make more complex

calls by adding one or more “chucks” to the end of the whine. Experiments using

speakers that play calls with different numbers of chucks show that females are

attracted to calls with more chucks. This preference results because the chuck’s

acoustic frequencies match those to which

the female’s inner ear is most sensitive.

This presents a puzzle: why don’t

males always make calls with chucks? The

answer is that a male’s call attracts more

than just females. Many bats are nocturnal

predators that can use echolocation (much

like sonar) to find their prey. But some bats

simply listen for sounds made by their

prey. The fringe-lipped bat (Tra c h o p s c ir-

rhosus) preys on male túngara frogs that it

finds using their calls. Experiments show

that chucks decrease the survival of males

because they make the male easier for the

bat to localize (FIGURE 10.6). When bats

are presented with two speakers, one

playing a frog call with chucks and the

other with just a simple whine, they more

often attack the speaker playing the call

with chucks.

Secondary sexual traits experience a

tug-of-war between their effects on two

fitness components: survival and mating

success (FIGURE 10.7). These traits evolve

to a compromise that maximizes a male’s

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_10.05.ai Date 12-15-2016

Mean number of nests per male

Shortened Controls

2

1

0

Elongated

FIGURE 10.5 An experimental test of Darwin’s hypothesis that secondary sexual traits

increase mating success. Male long-tailed widowbirds (Euplectes progne) have extremely

long tail feathers. Tails were artificially shortened in one group and lengthened in another.

There were two control treatments: no manipulation was done in one group of males, and

tails were cut and then reattached in the other group. Results from the two controls were

not significantly different, so they are shown together here. Mating success was measured

by the number of females nesting on each male’s territory. Males with lengthened tails

had the greatest success, as predicted by the hypothesis. (After [1].)

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_10.06.ai Date 12-20-2016

(B)

(A)

0

5

0

5

Frequency (kHz)

0 500

Time (msec)

Whine

Whine Chucks

FIGURE 10.6 Calls of the male túngara frog (Physalaemus pustulosus) attract females

for mating, but they also attract predators. (A) A male is attacked by a fringe-lipped

bat (Trachops cirrhosus) that has located the frog by his calls. (B) This sonogram of a

male’s call is a plot of frequency against time. Some males make calls that consist only

of a “whine” component, while others add one or more “chuck” components. The

chuck is attractive to females, but also more conspicuous to the bats. (B from [36].)

10_EVOL4E_CH10.indd 252 3/22/17 2:25 PM