Heightened risk of infection does not alter
macronutrient-specific feeding behavior.
Because shifts in behavior and pathology associated with infection could
act as social cues of heightened infection risk to uninfected
conspecifics, we also examined if perceived risk of infection (seeing
sick conspecifics) could alter feeding behavior and macronutrient
selection. Separate lines of evidence indicate that social cues can
influence feeding behavior and alter physiological responses relevant to
immune function (Cornelius et al. 2018, Schaller et al. 2010, Stevenson
et al. 2011, 2012; Love et al. 2021, Gormally and Lopes 2023). Thus, we
predicted that birds exposed to a heightened risk of infection would
have physiological responses relevant to responding to an immune threat
and shift feeding behavior in a way that maximizes immunity. Contrary to
our predictions, we found no evidence for shifts in feeding behavior,
macronutrient intake, gut microbiota, or physiological responses in
birds exposed to a cue of heightened infection risk. It is possible that
the cue of infection was not sufficient to stimulate physiological
changes or alter the feeding behavior of focal birds because we used a
simulated infection (injection with LPS) in this study. The behavioral
effects of LPS typically only last between 2-4 days following injection
(Sköld-Chiriac et al. 2014, Love et al. 2023), thus the cue of infection
elicited by LPS-injection is temporally limited. It is also possible
that we failed to capture the relevant timing of shifts in physiology
following exposure to sick conspecifics. Recent work in Japanese quail
(Coturnix japonica ) found that females exposed to LPS-challenged
males for 3 hours had an upregulation of immune genes in the blood,
suggesting that physiological responses to LPS-challenged individuals
might happen rapidly post-cue (Gormally and Lopes 2023). Additionally,
it might be less costly for organisms to employ behavioral defenses
(such as avoidance behavior) rather than physiological immune defenses
in response to an immune threat in some settings. For example, we
previously documented that zebra finches housed across from
LPS-challenged conspecifics reduce flight activity and increase preening
behavior (Love et al. 2023), and there might be trade-offs in how birds
invest in behavioral versus physiological immune defenses (Zylberberg et
al. 2012). Future work should explore whether an immune challenge or
infection that elicits stronger and longer-lasting behavioral and
physiological signs of disease is capable of influencing feeding
behavior, gut microbiota, and physiological responses in healthy
individuals, as this could have implications for host disease
susceptibility and disease transmission potential.
Although we did not detect an effect of perceived infection risk on any
of the physiological parameters examined in this study, we did observe
changes in testosterone concentrations in male zebra finches over the
course of the experiment. Specifically, we observed a decrease in
testosterone levels in males 3 days after the injection of stimulus
birds, and this decrease occurred in both LPS-focal and saline-focal
males. Physiological stress is known to have inhibitory effects on the
release of testosterone (Da Silva 1999), so it is possible that the
observed decrease in testosterone concentrations in males is related to
handling stress associated with repeated blood sampling. However, we did
not observe a corresponding increase in circulating plasma
corticosterone in birds over the course of the experiment that might be
indicative of acute or chronic stress.
While a heightened risk of infection did not influence feeding behavior
or gut microbiota composition, higher fat consumption was associated
with lower observed richness and lower Shannon diversity across both
treatment groups (LPS-focal, saline-focal). Consistent with this result,
high fat diets in mice result in lower gut microbiota richness and
diversity (Zhang et al. 2012). Similarly, house sparrows experimentally
fed a high-fat urban diet have decreased gut microbiota diversity
(Teyssier et al. 2020). Conversely, protein consumption by focal birds
was unrelated to microbial richness and diversity. Interestingly,
neither fat nor protein consumption predicted richness or Shannon
diversity in the infected birds (LPS-injected, saline-injected). It is
unclear why fat consumption predicted microbial alpha diversity in focal
birds but not injected birds, however larger sample sizes might be
required to confirm this trend and determine the biological
underpinnings of these differences.
The present study extends our understanding of how immune activation can
influence feeding behavior and diet selection in vertebrates. We did not
detect any shifts in feeding behavior, nor any immune or endocrine
changes in response to social cues of infection in the present study.
However, we did detect macronutrient-specific illness-induced anorexia
in LPS-challenged birds, where birds decreased protein but not lipid
intake. Shifts in feeding behavior in sick individuals can affect both
host and parasite fitness and ultimately influence disease severity,
which is inherently related to disease transmission (Hite and Cressler
2019, Povey et al. 2013). Models indicate that sickness-induced
anorexia, like the reduction in caloric intake observed in LPS-birds in
the present study, is capable of enhancing or diminishing disease
severity depending on dietary context (Hite and Cressler 2019),
suggesting that the interactions between infection, resource
availability, and host macronutrient selection can have important
consequences for disease dynamics and deserve further attention.