Investigating the first stages of coevolution between the Pacific koel and its newest host,the red wattlebird
Date
2017
Authors
Abernathy, Virginia
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Abstract
Avian obligate brood parasites lay their eggs in the nests of
other bird species and never provide their own parental care.
This behaviour is a model example of coevolution and while
multiple studies and reviews have discussed the different types
of adaptations and counter-adaptations hosts and brood parasites
evolve, there have only been a handful of empirical studies
focused on how quickly coevolution can occur in a host-brood
parasite system. Additionally, little is known about the early
stages of brood parasite and host coevolutionary interactions.
Understanding the rates of coevolution between brood parasites
and their hosts is an important step in uncovering aspects about
the process of speciation, determining which traits represent
true genetic change and can aid in conservation decisions of
endangered potential hosts, especially as brood parasites expand
their breeding ranges with environmental changes.
I investigated these issues by capitalising on the recent
exploitation of the Red Wattlebird (Anthochaera carunculata) by
the brood-parasitic Pacific Koel (Eudynamys orientalis). I
conducted a literature review on factors that influence the rate
of coevolution between avian obligate brood parasites and their
hosts and performed observational and experimental studies at two
sites where wattlebirds have experienced different durations of
parasitism: Sydney (parasitism for 38-86 years) and Canberra
(parasitism for 8-33 years). I determined that host switching can
pose challenges for both the host and brood parasite, as
parasitised wattlebird nests fledged significantly fewer young
than unparasitised wattlebird nests, but fledged similar numbers
of wattlebird and koel young. The koel’s later breeding season
relative to the wattlebird’s and the koel’s poor timing of
egg laying may have contributed to the low success of koel eggs.
Mobbing experiments demonstrated that naïve hosts can learn to
recognise a brood parasite within 33 years or less, but the speed
at which this defence spreads throughout the population may be
constrained by low parasitism rates. Egg rejection experiments
indicated that more than 38 years is required for egg ejection to
evolve in wattlebirds, as they only ejected model eggs at an
extremely low rate in Sydney and Canberra, while two older hosts
showed high levels of ejection at both sites. Lastly, I found
evidence that the koel likely evolved mimicry of the eggs of one
of its old hosts, which allowed it to exploit several other host
species, including the wattlebird, due to all of its hosts having
similar egg morphology.
My results support the many theoretical models which agree that
at least 30 or more years is required for egg ejection alleles to
spread throughout a population. This process may be slowed
because wattlebird eggs appear so similar to koel eggs, making it
more difficult for wattlebirds to recognise parasitic eggs.
However, I discovered that naïve hosts without specific
anti-parasite traits can still utilise generalised defences, such
as mobbing, in order to reduce the impact of brood parasitism,
and that host switching can also be difficult for the brood
parasite, as it may not be well-adapted to the new host’s
breeding season or behavioural habits.
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Anthochaera carunculata, Brood parasitism, Coevolution, Eudynamys orientalis, Pacific Koel, Red Wattlebird
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