In human populations, threatened wildlife species and livestock, some individuals get infected with larger parasite burdens than others, and some get sicker than others when infected. Infections and sickness should reduce survival and reproductive success and so natural selection should remove genes associated with weak immune responses, leaving only resistant, healthy individuals. But this isn’t the case- individuals vary in their immune responses, parasite loads, and health when infected. How has natural selection led to this variation?
I use data collected from the remarkable long-term study of a wild population of Soay sheep living on the St Kilda archipelago, NW Scotland to attempt to answer this question.
My PhD work with Loeske Kruuk investigated causes of variation between individuals in parasite resistance (including age and maternal effects) and how parasite burden affected fitness in lambs and adults in the sometimes harsh and sometimes benign winter conditions experienced by the sheep.
More recently, I have worked with Andrea Graham (Princeton) and Dan Nussey (Edinburgh) to determine how natural selection acts upon defence against infection in the Soay sheep. Our most recent work focused on tolerance of infection: the ability of a host to maintain health or fitness in the face of increasing parasite burden. We showed that (A) Soay sheep vary in their tolerance of infection: some sheep show a faster decline in body weight with increasing parasite load than others, with (B) the least tolerant sheep showing a greater loss of body weight across the range of infection that we saw. We also found that tolerance was under selection in the population, because (C) individuals which lost less weight under infection had, on average, more offspring across their lives, and indeed (D) tolerance was under selection of comparable strength to body weight.
This work has increased my interest in the drivers of variation in immune responses, which I’m studying using data collected from experiments on domesticated sheep, performed by collaborators Tom McNeilly and Al Nisbet (Moredun Research Institute).
Currently, I’m interested in determining how pregnancy and lactation affect immune responses. This represents an evolutionary trade-off between reproduction and immune function, because energy used for producing offspring can’t be used for fighting infections, and vice versa, and because reproductive hormones interact intimately with the immune system. I plan to collect data on Soay sheep maternal and lamb immune responses to determine how investment in reproduction is associated with immune function, and what the consequences of this are for mother and lamb.