Parasites and life history variation in a wild mammal
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Date
24/11/2011Author
Hayward, Adam David
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Abstract
The purpose of this thesis is to investigate associations between parasite infection and
host life-history variation in the wild Soay sheep population of the islands of St Kilda,
NW Scotland. Studying host-parasite interactions in wild animal populations is of
interest because of the importance of heterogeneity in resource availability, genetics, and
environmental conditions in determining resistance to parasites, with implications for
human populations and wildlife conservation and management. However, very few
studies are able to investigate these associations in a longitudinal manner, which is
essential in order to understand how infection is associated with life-history variation
across ages and environmental conditions. In this thesis, I investigate associations
between parasite resistance and ageing and the importance of maternal effects on
offspring parasite resistance. I also establish the shape of natural selection on parasite
resistance, and associations between measures of parasite burden and antibody
responses. The principle findings of the analyses presented in this thesis are:
i) Adult sheep of both sexes show a decline in parasite resistance in old age which is
consistent with senescence. Furthermore, the rate of decline in parasite resistance
with age is accelerated in individuals that have experienced more stressful
environmental conditions over their lifespan.
ii) Aspects of maternal phenotype and lamb early life performance are significantly
associated with parasite resistance in lambs. Some of these effects persist into adult
life and may even affect late-life changes in parasite resistance with age.
iii) Analysis of ageing in five female reproductive traits shows that the contributions
of individual senescence, terminal effects, and selective disappearance vary across
traits, and that therefore multiple traits should be studied in order to understand
ageing more fully. Most strikingly, there was no evidence for significant
senescence in the probability of producing twins.
iv) The first estimate of the strength of natural selection on parasite resistance in a
longitudinally-monitored population provided evidence for positive selection on
parasite resistance in lambs but not adults. Selection in lambs also varied across
environmental conditions, being stronger in years of more favourable conditions.
v) Analysis of associations between estimates of parasite burden and antibody
responses showed that an estimate of parasite burden was not correlated with either
a general or parasite-specific antibody response. However, antibody responses
were positively correlated, and there was some evidence for a genetic correlation
between the two in lambs but not adults.