Ageing in a variable habitat: environmental stress affects senescence in parasite resistance in St Kilda Soay sheep.
Despite widespread empirical evidence for a general deterioration in the majority of traits with advancing age, it is unclear whether the progress of senescence is chronologically determined, or whether factors such as environmental conditions experienced over the lifespan are more important. We explored the relative importance of 'chronological' and 'environmental' measures of age to changes in parasite resistance across the lifespan of free-living Soay sheep. Our results show that individuals experience an increase in parasite burden, as indicated by gastrointestinal helminth faecal egg count (FEC) with chronological age. However, chronological age fails to fully explain changes in FEC because a measure of environmental age, cumulative environmental stress, predicts an additional increase in FEC once chronological age has been accounted for. Additionally, we show that in females age-specific changes are dependent upon the environmental conditions experienced across individuals' life histories: increases in FEC with age were greatest among individuals that had experienced the highest degree of stress. Our results illustrate that chronological age alone may not always correspond to biological age, particularly in variable environments. In these circumstances, measures of age that capture the cumulative stresses experienced by an individual may be useful for understanding the process of senescence.
Hayward
Adam D
AD
Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, UK. adam.hayward@ed.ac.uk
Wilson
Alastair J
AJ
Pilkington
Jill G
JG
Pemberton
Josephine M
JM
Kruuk
Loeske E B
LE
D000375:Aging / Q000276:immunology*; D000818:Animals; D017753:Ecosystem; D005243:Feces / Q000469:parasitology; D005260:Female; D008297:Male; D008954:Models, Biological; D010270:Parasite Egg Count; D010273:Parasitic Diseases, Animal / Q000276:immunology*; D012606:Scotland; D012756:Sheep; D012757:Sheep Diseases / Q000276:immunology* / Q000469:parasitology; D013312:Stress, Physiological / Q000276:immunology*