Biology Reference
In-Depth Information
caterpillars and adult moths varies locally and geographically; the orange patch on the
caterpillars may cover from 20 to 90% of the body, while the female moth's hindwing
varies from pale orange to bright red. Studies in Finland by Carita Lindstedt, Johanna
Mappes and colleagues have shown that more orange caterpillars and redder adult
moths are more likely to be rejected by predatory birds, even though their levels of
toxins are no greater than those of less bright individuals (Lindstedt
et al
., 2008). Bright
colours are, therefore, more effective as warning signals. Why, then, aren't all the
caterpillars and moths brightly coloured? Experiments reveal that the aposematic
colours are both costly to wear and costly to make.
Costly to wear
: The size of the caterpillar's orange patch is heritable, so laboratory
selection lines were used to produce caterpillars with large and small orange patches. At
low temperatures, caterpillars with smaller orange patches (hence more black) grew
faster because they could absorb heat more effectively and, hence, forage for longer each
day. Caterpillars reared at low temperatures developed smaller and darker orange
patches, suggesting that the costs of thermoregulation have selected for phenotypic
plasticity in aposematic colouration (Lindstedt
et al
., 2009).
Costly to make
: In another experiment, caterpillars were reared on plants with either
a low or a high concentration of glycosides. Both groups of caterpillars and subsequent
adult moths had equal levels of toxins in their bodies, so excess toxins were disposed of
effectively. However, detoxification was costly because moths reared on the high
concentrations produced fewer offspring. Furthermore, although the diet did not affect
caterpillar colouration, female moths reared on high toxin concentrations developed
less bright hindwings. This suggests that resources devoted to getting rid of toxins left
fewer resources for pigment production (Lindstedt
et al
., 2010).
… both costly to
wear, and costly
to make
Conspicuousness versus crypsis
As a defence against predators it may pay to be cryptic, but this may conflict with
the advantage of being conspicuous for other activities such as territory defence or
mate attraction. As an example of this trade-off, in many species of birds the males
are brightly coloured in the breeding season but moult into duller female-like
plumage after breeding.
John Endler's (1980, 1983) work on the coloration of guppies (
Poecilia reticulata
)
provides an illuminating experimental study of this trade-off. Endler studied several
isolated populations of these little fish in the streams of Trinidad and Venezuela. Males are
more colourful than females. Three types of colour can be distinguished. (a)
Pigment colours
(carotenoids — red, orange and yellow), which are obtained from the diet. If fish are fed
carotenoid-free food then these colours fade within a few weeks (Kodric-Brown, 1989).
(b)
Structural colours
(iridescent blue and bronze) produced by reflection of light from
scales. (c)
Black spots
(melanin), which are partly under nervous control and can increase
or decrease in size. Laboratory experiments showed that brighter colours brought a mating
advantage. Females were particularly attracted to the orange spots (Houde, 1988).
To test whether there was counter-selection against bright colours due to predation,
Endler sampled streams with different predator communities. He found that males
living in streams with greater predation pressure were duller in colour, having both
fewer colour spots per fish and also smaller spots (Fig. 4.17a). Not only did predation
intensity influence guppy coloration but the type of predator was also important. In
In guppies,
brighter males
have a mating
advantage …
… but suffer
increased
predation
