Abstract
Understanding trade-offs in wild populations is difficult, but important
if we are to understand the evolution of life histories and the impact
of ecological variables upon them. Markers that reflect physiological
state and predict future survival would be of considerable benefit to
unravelling such trade-offs and could provide insight into individual
variation in senescence. However, currently used markers often yield
inconsistent results. One underutilised measure is haematocrit, the
proportional of blood comprising of erythrocytes, which relates to the
blood’s oxygen-carrying capacity and viscosity, and to individual
endurance. Haematocrit has been shown to decline with age in
cross-sectional studies (which may be confounded by selective
appearance/disappearance). However, few studies have tested whether
haematocrit declines within-individuals or whether low haematocrit
impacts survival in wild taxa. Using longitudinal data from the
Seychelles warbler (Acrocephalus sechellensis), we demonstrated that
haematocrit increases with age in young individuals (<1.5
years) but decreases with age in older individuals (1.5–13 years). In
breeders, haematocrit was higher in males than females and varied
relative to breeding stage. High haematocrit was associated with lower
survival in young individuals, but not older individuals. Thus, while we
did not find support for haematocrit as a marker of senescence, high
haematocrit is indicative of poor condition in younger individuals.
Possible explanations are that these individuals were experiencing
dehydration and/or high endurance demands prior to capture, which
warrants further investigation. Our study demonstrates that haematocrit
can be an informative metric for life-history studies investigating
trade-offs between survival, longevity and reproduction.