Immunosenescence in the wild? A longitudinal study in a long‐lived seabird

1. Longitudinal studies of various vertebrate populations have
demonstrated senescent declines in reproductive performance and survival
probability to be almost ubiquitous. Longitudinal studies of potential
underlying proximate mechanisms, however, are still scarce.  2.
Due to its critical function in the maintenance of health and viability,
the immune system is among the potential (mediators of) proximate
mechanisms that could underlie senescence.  3. Here, we studied
three innate immune parameters - hemagglutination titre, haemolysis titre
and haptoglobin concentration - in a population of common terns (Sterna
hirundo) known to undergo actuarial senescence. We repeatedly sampled
birds of known sex and age across 11 years and used random regression
models to (i) quantify how immune parameters vary among individuals, and
(ii) describe within-individual age-specific changes in, and potential
trade-offs between, immune parameters. 4. Our models revealed no
differences between males and females in hemagglutination titre and
haptoglobin concentration, and very low among-individual variation in
these parameters in general. Within individuals, hemagglutination titre
increased with age, while haptoglobin concentration did not change. We
found no indication for selective (dis)appearance in relation to
hemagglutination titre or haptoglobin concentration, nor for the existence
of a trade-off between them. Haemolysis was absent in the majority (76%)
of samples. 5. Common terns do not exhibit clear senescence in
hemagglutination titre and haptoglobin concentration and show very little
among-individual variation in these parameters in general. This may be
explained by canalisation of the immune parameters or by the colonial
breeding behaviour of our study species, but more longitudinal studies are
needed to facilitate investigation of links between species’
characteristics and immunosenescence in wild animals.