Lecture 13 - brood parasitism Flashcards
why is parasitism expected to be selected for in evolution?
because parental care is costly so putting this cost on another species is useful
what are intraspecific brood parasites and a couple of examples?
they lay eggs in clutches of conspecifics
- european starling and burying beetle
how are cliff swallows cryptic with their brood parasitism?
they lay their eggs in their own nest then it up and move into the nest of another
the masked weavers breed in large colonies with a third of their eggs laid parasitically - how have they evolved defence against this parasitism?
- dramatic egg divergence
- females also have the ability to recognise their own eggs
describe parasitism in american coots
- 41% of pairs parasitised
- almost half of those eggs are rejected by the host
- process of recognition has been studied by colour ranking the eggs
- the hosts were rejecting eggs that were a different colour rank but accepting parasitism eggs that were similar in colour
- explains why it wasnt 100% rejection rates
what are interspecific brood parasites?
Lay eggs in clutches of one or
more host species
- occurs in alot of taxa
how many bird species are obligate interspecific brood parasites?
100 - 1%
describe interspecific parasitism in cuckoo catfish?
- parasitise the cichlid which is a mouth brooding fish - when the cichlids are spawning the catfish deposits their own eggs within the cichlid clutch - cichlid takes all the eggs including catfish ones into mouth - catfish end up then eating all the cichlid offspring when it grows in the mouth - very bad for the cichlids
why do we expect brood parasitism to be a co-evolutionary arms race?
because it is a conflict of interest between a host and a parasite
5 facts about the natural history of cuckoos and hosts
- 10 main host species in Europe
- female lays 15-20 eggs per season
- unusual egg-laying behaviour- most birds lay in the early hours of the morning but they lay in the afternoon
- Females specialise on one host species and usually lays a mimetic egg - they remove one of the host egg to replace with their own
- egg is very small compared to body size
there are different types of cuckoos for the different mimetic eggs they lay what are these known as?
each kind known as a ‘gens’ - e.g. pipit cuckoos or wagtail cuckoos
why do we still only have a single cuckoo species, why havent each gens lineage diversified?
because males show no specialisation - males maintain gene flow between the different gentes of cuckoo females
why does parasitism result in zero reproductive success for hosts?
because the cuckoo chick hatches quicker than the host ones and it knocks the other chicks out the nest
describe co-evolution
brood parasitism results in adaptations of host defences which results in counter-adaptations by the brood parasites
Davie and Brooke did an experiment to see if cuckoos evolved in response to hosts by placing model eggs in reed warbler nests - how did they test to see why the cuckoo waits until the host starts laying?
- Placed model egg in nest before and after start of laying
results:
before host lays: 100% rejected
after host lays: 0% rejected
how did davie and brook test why cuckoos lay in the afternoon?
Placed model egg in nest in am and pm
Results:
morning: 50% rejected
afternoon: 0% rejected
how did davie and brook test why cuckoos lay so quickly?
Placed stuffed cuckoo by nest for 5 mins
results:
model + stuffed cuckoo: 45% rejected
model egg only: 0% rejected
how did davie and brook test why cuckoos lay a small egg?
Placed large model egg (expected size) in nest
results:
large model egg (10g): 40% rejected
usual model egg (3.4g): 0% rejected
how did davie and brook test why cuckoos lay a mimetic egg?
Placed model egg of different gens in reed warbler nests pied wagtail type: 81% rejected redstart type: 70% rejected meadow pipit type: 44% rejected reed warbler type: 0% rejected
what is the main conclusion from davie and brooks experiment?
cuckoos have evolved in response to host defences
explain the experimental evidence for have hosts evolved in response to cuckoos?
- They placed non-mimetic model eggs in nests of 13 suitable UK hosts - some showed extremely good rejection ability and other not
- then placed non-mimetic model eggs in nests of 9 unsuitable hosts - all 9 species didnt reject the eggs
conclusion from egg rejection by hosts experiment?
the difference in behaviour of suitable and unsuitable hosts suggests that rejection has evolved only in parasitised species - some are better than others but it hasnt evolved at all in unparasitised species
how can sympatry and allopatry test for co-evolution?
In Iceland, there are hosts but no cuckoos - when you compare rejection rates the UK has much higher rates than Iceland due to the host species never being exposed to the cuckoos before
what can you conclude from the sympatry/allopatry experiments?
can conclude that it is parasitism that is selected for egg rejection and in the absence of parasitism rejection ability is much lower and many cases, zero
give an example of observational evidence of parasite evolution
- cuckoo finch parasitises the tawny-flanked prinia
- the Prinia has more diverse egg polymorphism than any other bird
- hosts use several aspects of parasitic egg appearance to reject foreign eggs
- clutches collected since 1970s
- over that period of time both the host eggs and cuckoo finch eggs have become more diverse than before suggesting an evolutionary arms race
- co-evolutionary process = parasites have tracked host eggs
why do mimicry and defences vary?
all we are able to do is look at a ‘snapshot’ of a continuing evolutionary arms race with no particular end
- different hosts and parasites will be at different points along that continuum
describe the sequence of events for parasitism to evolve?
(i) before parasitism - no rejection e.g. unsuitable hosts / hosts in allopatry
(ii) parasitism - selection favours rejection depends on parasitism rate
- the number of generations for rejection gene to spread depends on percentage of nests parasitised
iii) evolution of mimicry by parasite - specialisation results in gentes
(iv) host defences ‘win’ - no parasitism, so host defences weaken… no rejection…
describe how the brown- headed cowbird currently gets away with not mimicking eggs of nests and how this is now changing?
because it is a generalist with 216 hosts so parasitism of particular species isnt that often so they take longer to evolve rejection ability - however >50% of some hosts are parasitise putting an extreme selection pressure on some species that have now evolved rejection
what is the ghost of the parasitism past?
some species are now never parasitised by cuckoos but yet still show rejection ability
why may rejection be costly?
- recognition errors (eject own eggs)
- ejection costs (damage own eggs)
what are other possible defences instead of egg rejection
nest defence as seen in the common grackle
