APS209 Animal Behaviour
Brood Parasites
Aims
1. To present the concept of a
coevolutionary arms race between brood parasites and their hosts.
2. To describe the biology of cuckoos
and experimental tests of the function of behaviour
3. To examine whether the host-parasite
arms race is ongoing or in evolutionary equilibrium.
Objectives
1. To understand the
principles underlying coevolutionary arms races
2. To understand how
adaptations and counter-adaptations may be tested in field experiments
3. To understand the
importance of studying mechanisms in the study of behaviour
Brood parasites
Brood parasites effectively parasitise
the parental care of their hosts. Such parasitism may occur among members of
the same species (intra-specific brood parasitism, which is a fairly widespread
behaviour among birds) or different species (inter-specific brood parasitism).
About 1% of all bird species are obligate brood parasites.
Coevolutionary ‘arms race’
Parasites and their hosts (and equally,
predators and prey) are expected to be in a coevolutionary arms race whereby
host defences select for adaptations in the parasite, which in turn result in
counter-adaptations from the hosts, and so on. Such arms races may reach a
stable evolutionary equilibrium of adaptation and counter-adaptation, or may
remain evolutionarily dynamic with continuous adaptive change occurring, of
which we can observe only a snapshot.
Common cuckoos Cuculus canorus and their hosts
The common cuckoo has about 10 regular
host species in the UK. Various features of female egg-laying behaviour have
been shown, by experiments, to be adaptive responses to host egg-rejection
defences. Among these adaptations is the production of eggs that mimic the host
eggs. Thus each female will specialise on a particular host species, with those
specialising on a particular host being referred to as a ‘gens’ (plural
‘gentes’), as in ‘reed warbler gens’, ‘meadow pipit gens’, etc.
Hosts have also evolved in response to
parasitism, exhibiting a range of egg rejection abilities. Suitable hosts are
more discriminating than unsuitable hosts. Furthermore, in places where cuckoos
and hosts are sympatric, hosts are more discriminatory than when hosts are not
parasitized.
Continuing arms race or evolutionary
equilibrium?
Variation among species in egg
rejection ability suggest that hosts and their parasites are in a continuing
arms race. However, it may not always pay to be a rejector if the costs of
parasitism are low, or if the costs of rejection are high. Alternatively, the ability
of a host to discriminate may depend on the rejection mechanism.
Reading: see Chapter 12 in
John Alcock’s Animal Behavior (2009: pp. 379-419); also Nick Davies’ book:
Cuckoos, cowbirds and other cheats (2000; T & AD Poyser, London).
Davies NB & Brooke
ML (1988) Cuckoo versus reed warblers: adaptations and counter-adaptations.
Animal Behaviour 36: 262-284.
Davies NB &
Brooke ML (1988) An experimental study of coevolution between the cuckoo, Cuculus canorus, and its hosts I Host
egg discrimination. J. Animal Ecology 58: 207-224.
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