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

<p class="CxSpFirst">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 (<i>Tachycineta bicolor</i>) 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 (<i>Dermanyssus</i> spp. and <i>Ornithonyssus</i> 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.</p>