Lecture 13 - brood parasitism

Question 1

why is parasitism expected to be selected for in evolution?

Answer 1

because parental care is costly so putting this cost on another species is useful

Question 2

what are intraspecific brood parasites and a couple of examples?

Answer 2

they lay eggs in clutches of conspecifics

- european starling and burying beetle

Question 3

how are cliff swallows cryptic with their brood parasitism?

Answer 3

they lay their eggs in their own nest then it up and move into the nest of another

Question 4

the masked weavers breed in large colonies with a third of their eggs laid parasitically - how have they evolved defence against this parasitism?

Answer 4

• dramatic egg divergence

- females also have the ability to recognise their own eggs

Question 5

describe parasitism in american coots

Answer 5

• 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

Question 6

what are interspecific brood parasites?

Answer 6

Lay eggs in clutches of one or
more host species
- occurs in alot of taxa

Question 7

how many bird species are obligate interspecific brood parasites?

Answer 7

100 - 1%

Question 8

describe interspecific parasitism in cuckoo catfish?

Answer 8

• 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

Question 9

why do we expect brood parasitism to be a co-evolutionary arms race?

Answer 9

because it is a conflict of interest between a host and a parasite

Question 10

5 facts about the natural history of cuckoos and hosts

Answer 10

• 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

Question 11

there are different types of cuckoos for the different mimetic eggs they lay what are these known as?

Answer 11

each kind known as a ‘gens’ - e.g. pipit cuckoos or wagtail cuckoos

Question 12

why do we still only have a single cuckoo species, why havent each gens lineage diversified?

Answer 12

because males show no specialisation - males maintain gene flow between the different gentes of cuckoo females

Question 13

why does parasitism result in zero reproductive success for hosts?

Answer 13

because the cuckoo chick hatches quicker than the host ones and it knocks the other chicks out the nest

Question 14

describe co-evolution

Answer 14

brood parasitism results in adaptations of host defences which results in counter-adaptations by the brood parasites

Question 15

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?

Answer 15

• Placed model egg in nest before and after start of laying
  results:
  before host lays: 100% rejected
  after host lays: 0% rejected

Question 16

how did davie and brook test why cuckoos lay in the afternoon?

Answer 16

Placed model egg in nest in am and pm
Results:
morning: 50% rejected
afternoon: 0% rejected

Question 17

how did davie and brook test why cuckoos lay so quickly?

Answer 17

Placed stuffed cuckoo by nest for 5 mins
results:
model + stuffed cuckoo: 45% rejected
model egg only: 0% rejected

Question 18

how did davie and brook test why cuckoos lay a small egg?

Answer 18

Placed large model egg (expected size) in nest
results:
large model egg (10g): 40% rejected
usual model egg (3.4g): 0% rejected

Question 19

how did davie and brook test why cuckoos lay a mimetic egg?

Answer 19

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

Question 20

what is the main conclusion from davie and brooks experiment?

Answer 20

cuckoos have evolved in response to host defences

Question 21

explain the experimental evidence for have hosts evolved in response to cuckoos?

Answer 21

• 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

Question 22

conclusion from egg rejection by hosts experiment?

Answer 22

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

Question 23

how can sympatry and allopatry test for co-evolution?

Answer 23

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

Question 24

what can you conclude from the sympatry/allopatry experiments?

Answer 24

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

Question 25

give an example of observational evidence of parasite evolution

Answer 25

• 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

Question 26

why do mimicry and defences vary?

Answer 26

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

Question 27

describe the sequence of events for parasitism to evolve?

Answer 27

(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…

Question 28

describe how the brown- headed cowbird currently gets away with not mimicking eggs of nests and how this is now changing?

Answer 28

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

Question 29

what is the ghost of the parasitism past?

Answer 29

some species are now never parasitised by cuckoos but yet still show rejection ability

Question 30

why may rejection be costly?

Answer 30

• recognition errors (eject own eggs)

- ejection costs (damage own eggs)

Question 31

what are other possible defences instead of egg rejection

Answer 31

nest defence as seen in the common grackle