Body size and circulating levels of different molecules from tree swallows (Tachycineta bicolor) exposed to different doses of lipopolysaccharide (LPS)

The purpose of mounting an immune response is to destroy pathogens, but
this response comes at a physiological cost, including the generation of
oxidative damage. However, many studies on the effects of immune
challenges employ a single high dose, meaning that the consequences of
more mild immune challenges are poorly resolved. We tested whether the
degree of immunological challenge in tree swallows (Tachycineta bicolor)
affects oxidative physiology and body mass, and whether these metrics
correlate with parasitic nest mite load. We injected 14-day-old nestlings
with either 0, 0.01, 0.1, or 1 mg lipopolysaccharide (LPS) per kg body
mass, then collected a blood sample 24-h later to quantify multiple
physiological metrics, including oxidative damage (i.e., d-ROMs),
circulating amounts of triglyceride and glycerol, and levels of the acute
phase protein haptoglobin. After fledging, we identified and counted
parasitic nest mites (Dermanyssus spp. and Ornithonyssus spp.). We found
that only nestlings injected with 1 mg LPS/kg body mass, which is a common
dosage in ecoimmunological studies, lost more body mass than individuals
from other treatment groups. However, every dose of LPS resulted in a
commensurate increase in oxidative damage. Parasitic mite abundance had no
effect on oxidative damage across treatments. Amount of oxidative damage
correlated with haptoglobin levels, suggesting compensatory mechanisms to
limit self-damage during an immune response. We conclude that while only
the highest-intensity immune challenges resulted in costs related to body
mass, even low-intensity immune challenges result in detectable increases
of oxidative damage.