Haematocrit, age and survival in a vertebrate population

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.