Biology Reference
In-Depth Information
models; in populations where a particular model is more common, a greater proportion
of the palatable species mimics this model (Sheppard, 1959).
If predation on models increases as mimics become relatively more common, why
don't models evolve to escape the mimics? A likely explanation is that whereas there is
selection on mimics for improved mimicry, any mutant model which is different in
colour will suffer increased predation because it will be rare and not recognized as
noxious (Nur, 1970). Therefore, Batesian mimicry may be the outcome of an arms race
that the mimic has won (Ruxton
et al
., 2004).
Trade-offs in prey defences
Prey defences have costs as well as benefits. Firstly, resources are limited, so there are
allocation costs
. Increased investment in defence means fewer resources for growth and
reproduction. Secondly, alternative benefits may be forfeited, so there are
opportunity
costs
. Improved crypsis on an oak tree may limit habitat choice because of increased
conspicuousness on other backgrounds, and may restrict other activities, for example
signalling to rivals or mates. An example of each is discussed, while recognizing that
these two kinds of cost are often closely related.
Two costs
Costs of aposematism
Theoretical studies suggest that investment in aposematic signals should vary depending
on predation pressure (Endler & Mappes, 2004) and trade-offs between investing in
repellent defences versus colourful signals (Speed & Ruxton, 2007; Blount
et al
., 2009).
This topic deserves more empirical studies and we focus on one example.
The wood tiger moth,
Parasemia plantaginis
, is aposematic at both the larval and adult
stage. The caterpillars are hairy and black with an orange patch. The adult female has
cryptic forewings and orange-red hindwings (Fig. 4.16). Both are moderately distasteful
to insectivorous birds and the source of the toxins (iridoid glycosides) is the food plant
of the caterpillars (ribwort plantain,
Plantago lanceolata
). The colouration of both
Wood tiger moths
have warning
colours …
(a)
(b)
Fig. 4.16
Variation in aposematic colouration among individual wood tiger moths,
Parasemia plantaginis
.
(a) The caterpillars vary in the size of the orange patch. (b) The hindwings of female moths vary from bright red
to pale orange. Photos © Eira Ihalainen.
