females. The other is female choice of males that exhibit
characteristics signaling high fitness. Characteristics of
bright plumage in birds may represent generations of
female choice.
In the tropics sexual selection in birds is evident in
numerous bird families but perhaps the most dramatic
examples are among the cotingas (Cotingidae) and
manakins (Pipridae), both of which depend heavily on
diets of fruit.
An Example: The Guianan Cock- of- the- rock
The Neotropical Guianan Cock- of- the- rock (Rupicola
rupicola) is one of many marvels that draw people,
in particular those with a fondness for birds, to the
Neotropics. This large, nearly chicken- size cotinga
provides an outstanding example of sexual selection.
As the name suggests, the Guianan Cock- of- the- rock
is found in northeastern South America (Venezuela,
Guyana, Suriname), extending well into the Brazilian
rain forest. Pepper Trail, following a well- known series
of studies on cotingas by David Snow, documented the
elaborate courtship of this species in a classic series of
studies.
Males are chunky and have brilliant golden- orange
plumage, with black on the wings and the short tail
(plate 10- 18). In flight they resemble winged, Day- Glo
orange footballs. Beaks, legs, eyes, and even the very
skin is bright, vivid orange. The male’s already striking
plumage is further enhanced by delicate, elongate
orange wing plumes and a thick, crescent- like fan of
feathers extending from the base of the bill to the back
of the neck. Females are dull brown, with neither the
wing plumes nor the head fan.
Males gather in the rain forest understory in
confined courtship areas called concentrated leks. Each
male clears an area of ground in which to display and
defends perches in the vicinity of its display site. The
lek becomes crowded, occupied by several dozen males,
spaced as closely as 1.5 m (5 ft) apart. When a female
approaches a lek, each male displays, first by landing on
the ground and posturing to her. Each displaying cock
strokes its wing plumes and turns its head fan sideways,
presenting its profile to the female while staring at her
with its intense orange eyes. The object of each cock’s
display is obviously to mate, presumably by suitably
impressing the female. Females do not appear to be
easily impressed. A hen will typically visit a lek several
times before engaging in copulation. These visits,
called mating bouts, always excite the males to display.
Ultimately only one male on a lek will get to mate with
a visiting female, who may return to mate with him a
second time before laying eggs. No extended pair- bond
is formed, only a brief coupling. The cock returns to
the lek, continuing to court passing hens, while the
newly fertilized hen attends to nest building, egg laying,
incubation, and raising the young.
Darwin reasoned that in some species female choice
was the dominant factor in selecting male appearances.
Put very simply, males are colorful (or musical or noisy
or perform complex dances) because females have
tended through generations to mate mainly with males
having these features. Since plumage color is heritable
(as are behavioral rituals), gaudy coloration was selected
for and continually enhanced. Recent work in sexual
selection suggests that females learn much about the
evolutionary fitness of males by signals communicated
both by plumage condition and male courtship behavior.
The appearance and behavior of males has been shown
to be an honest signal of good health, correlating with
lack of parasites, strong immune system, agility, and
coordination. In other words, females are not being
frivolous in driving male evolution toward more
elaborate, gaudy plumage and exotic behavior but
are looking intently for reliable signals, expressed in
plumage and behavior, of male fitness.
The other facet of sexual selection recognized by
Darwin is that males must compete among themselves
for access to females. Male– male competition takesPlate 10- 18. Male Guianan Cock- of- the- rock. Photo by Andrew
Whittaker.164 chapter 10 tropical intimacy: mutualism and coevolution