Population differences in fever and sickness behaviors in a wild passerine: a role for cytokines

Abstract

SUMMARYImmune responses benefit hosts by clearing pathogens, but they also incur physiological costs and tissue damage. While wild animals differ in how they balance these costs and benefits, the physiological mechanisms underlying such differential investment in immunity remain unknown. Uncovering these mechanisms is crucial to determining how and where selection acts to shape immunological defense. Among free-living song sparrows (Melospiza melodia) in western North America, sicknessinduced lethargy and fever are more pronounced in Southern California than in Washington and Alaska. We brought song sparrows from two populations (Southern California and Washington) into captivity to determine whether these differences persist in a common environment and what physiological signals facilitate such differences. As in free-living sparrows, captive California birds exhibited more pronounced fever and lethargy than Washington birds in response to lipopolysaccharide, a nonpathogenic antigen that mimics bacterial infection. After treatment, the two populations showed similar reductions in luteinizing hormone levels, food intake and body mass, although treated birds from California lost more breast muscle tissue than treated birds from Washington. Moreover, California birds displayed higher bioactivity of interleukin-6, a pro-inflammatory cytokine, and marginally higher levels of corticosterone, a steroid hormone involved in stress, metabolism and regulating inflammatory responses. Our results show that immunological differences between these populations cannot be explained by immediate environment alone and may reflect genetic, maternal or early-life effects. Additionally, they suggest that cytokines play a role in shaping immunological variation among wild vertebrates. Supplementary material available online at

Flight performance of western sandpipers Calidris mauri remains uncompromised when mounting an acute phase immune response

“…These results are consistent with the findings of a recent study on homing pigeons, Columba livia, which did not find a difference in plasma haptoglobin concentration or levels of agglutination between birds that raced and those that did not (Matson et al, 2012). The magnitude and the duration of the change in body temperature in response to LPS injection can vary with a number of factors, such as age (Fraifeld and Kaplanski, 1998;Nebel et al, 2013), dose (Maloney and Gray, 1998), population (Adelman et al, 2010b;Adelman et al, 2010a) and season (Owen-Ashley and Wingfield, 2007). Here, we demonstrated a significant effect of flight on body temperature change following LPS challenge.…”

When is it socially acceptable to feel sick?

“…Experiments using immune challenges to examine the flexibility of the immune response under different circumstances are contributing to advancing the understanding of life-history evolution [48], as well as untangling the relative contributions of non-genetic (including, for example, ecological as well as maternal effects) and genetic factors towards the observed differences in behaviour during infection [49,82]. For example, recent studies are helping to shape models of the terminal investment hypothesis [83].…”

Intense flight and endotoxin injection elicit similar effects on leukocyte distributions but dissimilar effects on plasma-based immunological indices in pigeons

“…An alternative explanation is that exercise may not prompt a true systemic inflammatory response, even if muscle cells produce 'inflammation-responsive' IL-6 during exercise (Pedersen and Hoffman-Goetz, 2000). Further investigation of this phenomenon following flight by birds is possible using assays of IL-6 bioactivity (Adelman et al, 2010) or direct measurements of endotoxin in plasma samples.…”