Biology Reference
In-Depth Information
The parasite has also evolved remarkable variation in its eggs
(Fig. 4.21). Wouldn't it be marvellous if individual cuckoo
finches could target those host individuals for whom their egg
would be a good match? But they don't, perhaps because such
selectivity would be too difficult to achieve. Instead, the
parasite lays eggs at random and so suffers high rates of
rejection because its egg is often a poor match for the host
eggs (Spottiswoode & Stevens, 2010). Thus, host egg signature
variation can be an effective defence.
The egg arms race: a coevolutionary
sequence
These experiments enable us to reconstruct the likely sequence
of the cuckoo-host arms race at the egg stage (Davies & Brooke,
1989a, 1989b).
Fig. 4.21
Tawny-flanked prinia eggs
(outer circle) and cuckoo finch eggs (inner
circle). The diversity of egg 'signatures' in
the host leads to a signature-forgery arms
race between host and cuckoo and
remarkable diversity in egg colours and
markings within a species. From
Spottiswoode and Stevens (2012). Photo by
Claire Spottiswoode.
(i)
At the start, before small birds are parasitized, they
show little, if any, rejection of foreign eggs (small birds
with no history of cuckoo parasitism, because they are
unsuitable as hosts, do not reject).
(ii)
In response to parasitism, hosts evolve egg rejection
(hosts do reject) and more distinctive individual egg
signatures (hosts have more egg variation).
(iii)
In response to host rejection, cuckoos evolve egg mimicry
(egg mimicry in the different host races reflects the degree
of host discrimination).
(iv)
If cuckoo egg mimicry is sufficiently good, and if parasitism levels are not too high,
then it may be best for hosts to accept most cuckoo eggs to avoid the costs of
mistakenly rejecting their own eggs from unparasitized clutches (Box 4.2).
Once cuckoos
evolve egg
mimicry, host egg
rejection is costly
Egg rejection versus chick rejection as a host defence
Hosts occasionally abandon common cuckoo chicks (Grim
et al
., 2003) but why don't
they always do so? The cuckoo chick is much larger than their own chicks and its gape
is a different colour. Size and colour are the cues used to reject odd eggs; why not pick up
on these same cues at the chick stage too?
Arnon Lotem (1993) suggested an ingenious solution to this puzzle. At the egg
stage, experiments have shown that hosts imprint on their eggs the first time they
breed and then they reject eggs that differ from this learnt set (Rothstein, 1982; Lotem
et
al., 1995). Such imprinting is not a foolproof defence. Firstly, if hosts are unlucky
and are parasitized in their first clutch then they may misimprint on the foreign egg
too and so regard it as one of their own. Secondly, once the parasite has evolved egg
mimicry then hosts may make recognition errors and reject one of their own eggs
Why accept the
cuckoo chick? A
cognitive
challenge?
