Biology Reference
In-Depth Information
prey type they are probably less likely to attack by mistake if it is conspicuous (Guilford,
1986). Mistakes occur because a predator often attacks an item before it is certain that
it is a prey. This may be advantageous when prey are mobile or when there is intense
competition for food. Given that predators may make hasty decisions, it may pay
unpalatable prey to be conspicuous to reduce recognition errors.
Studies of the desert locust,
Schistocerca gregaria
, provide a beautiful example of the
relative advantages of crypsis and bright colouration. Under low population density,
juvenile locusts occur in a 'solitarious' phase; they are green and cryptic, move about
slowly, avoid one another and avoid feeding on plants with defensive chemicals, such as
alkaloids. However, under high population density (when crypsis is no longer possible),
the juveniles develop into a 'gregarious phase'; they become brightly patterned black
and yellow, are attracted to one another and change to a more active foraging style,
including toxic plants in their diet which makes them unpalatable to predators (Despland
and Simpson, 2005). Experiments show that predators learn faster to avoid unpalatable
locusts when they are in this conspicuous black and yellow colouration rather than a
cryptic green, so the change to the aposematic form at high density is adaptive in
reducing predation (Sword
et al
., 2000).
Cryptic and
conspicuous
locusts
The evolution of warning colouration
At the start of the chapter, two general questions for prey defences were posed. Clearly,
bright colours are advantageous (question 1). But how did they evolve (question 2)?
One possibility is that conspicuous colours evolved first, followed by distastefulness. For
example, some brightly coloured birds like kingfishers are distasteful (Cott, 1940). Their
colours may have been favoured for better mate attraction or territory defence and
then, because they also increased conspicuousness to predators, this then favoured the
evolution of distastefulness. The other possibility is that distastefulness came first. This
may apply to those insects, such as caterpillars of the monarch butterfly,
Danaus
plexippus,
which feed on plants containing toxins and incorporate the toxins in their
bodies as a defence against predation. It is plausible that here unpalatability evolved first
followed by conspicuousness. In this case, then, bright coloration evolves specifically as
a warning device.
This last scenario poses an interesting problem. Imagine a population of unpalatable
but cryptic larvae. A mutation arises in an adult which causes its larvae to be more
conspicuous. These larvae would then surely be more obvious to predators and so
more likely to perish. Although the predator may, as a result of its experience with the
nasty taste, decide never to touch the brighter form again, because it is a rare mutant
the predator is unlikely to encounter another one. Thus, the mutation goes extinct
during the sampling and never has the chance to spread. How, then, can warning
colours ever evolve?
R.A. Fisher (1930) was the first to propose a solution. He realized that distasteful,
brightly coloured insects were often clumped in family groups (Table 4.2 gives an
example). In this situation, because of the grouping, the predator does encounter others
with the bright coloration, namely the siblings of the individual which perished in the
sampling. Thus, their lives are saved and some copies of the gene for conspicuous
Conspicuousness
and
distastefulness:
which evolved
first?
Fisher's
hypothesis:
warning colours
may have evolved
through their
effects on survival
of relatives in the
same group
