Ectoparasite presence and brood size manipulation interact to accelerate
telomere shortening in nestling jackdaws
Abstract
Early-life conditions impact fitness, but whether the combined effect of
extrinsic stressors is additive or synergistic is not well known. This
is a major knowledge gap, because exposure to multiple stressors is
likely to be frequent. Telomere dynamics may be instrumental when
testing whether combined stressor effects are additive or synergistic,
because many factors affect telomere shortening, and telomere shortening
predicts survival. We evaluated the effects of manipulated brood size
and natural infestation by the carnid fly Carnus hemapterus on nestling
growth and telomere shortening of wild jackdaws (Corvus monedula).
Telomere length, measured in blood using TRF, shortened on average by
264 bp. In enlarged broods, nestlings’ telomeres shortened more when
parasitized, while in reduced broods there was no effect of infestation
on telomere shortening. We conclude that there is a synergistic effect
of number of siblings and Carnus infestation on telomere shortening
rate: blood-sucking parasites may negatively impact telomeres by
increasing cell proliferation and/or physiological stress, and coping
with infestation may be less successful in enlarged broods with
increased sibling competition and lower per capita feeding rate. Larger
nestlings had shorter telomeres independent of age, brood manipulation
or infestation. Growth was independent of infestation but in enlarged
broods, nestlings were lighter at fledging. Our findings indicate that
(i) evaluating consequences of early-life environmental conditions in
isolation may not yield a full picture due to synergistic effects, and
(ii) effects of environmental conditions may be cryptic, e.g. on
telomeres, with fitness consequences expressed beyond the temporal
framework of the study.