Introduction
Nutrition is critical to the immune system and can influence how hosts
respond to infection (Cunningham-Rundles et al. 2005, Ponton et al.
2011, Amar et al. 2007). Among ecological studies, the effect of the
availability of resources on immune processes and disease outcomes has
received significant attention (Becker et al. 2018, Strandin et al.
2018, Moyers et al. 2018), but the quality of those resources and their
nutritional make-up (e.g., macronutrient content) are also important
(Povey et al. 2013, Cotter et al. 2011).
Macronutrients like lipids, carbohydrates, and protein vary in their
biological availability and can influence processes ranging from
cellular function to whole organism performance (Warne 2014). Diet
macronutrient content can influence physiological processes important in
responding to pathogens, including hormonal and immune responses. For
example, kittiwake chicks fed a low-lipid diet had higher baseline and
stress-induced concentrations of the hormone corticosterone (Kitaysky et
al. 2001), which plays a role in responding to energetic demands and is
known to both stimulate and suppress immune responses in different
contexts (Roberts et al. 2007, Da Silva 1999). Dietary nutrients can
also impact other hormones known to influence immune function, such as
testosterone (Roberts et al. 2007, Da Silva 1999). For example, humans
consuming higher levels of dietary fat have higher baseline testosterone
concentrations, while subjects consuming higher levels of dietary
protein have lower testosterone levels (Volek et. 1997).
Diet can also directly affect immunity without the mediation of
hormones. In captive white ibis, birds that were fed anthropogenic
dietary items like white bread had reduced bacterial killing ability,
but corticosterone levels and the other immune parameters investigated
were unaffected by diet treatment (Cummings et al. 2020). Other work has
found that restriction of dietary protein can limit immune activity (Lee
et al. 2006, Povey et al. 2009) and high lipid diets can increase
mortality rates during infection (Adamo 2008, Adamo et al. 2010). The
macronutrient composition of diets can also differentially affect
different components of the immune system. For example, in insects, the
optimal macronutrient composition of diets varies for different
immunological parameters (Cotter et al. 2011). Thus, optimal diet
selection may vary based on the type of immune threat that organisms are
experiencing.
Despite the apparent need for nutritional resources to mount an immune
response, some organisms respond to infection with sickness-induced
anorexia (Adamo et al. 2007, Adelman and Martin 2009, Povey et al.
2013). This reduction in food intake following an immune challenge is
thought to reduce the risk of ingesting additional infectious agents or
may function to starve pathogens and parasites of key nutrients
(Kyriazakis et al. 1998, Adamo et al. 2007, Adelman and Martin 2009).
Further, caloric restriction during illness can improve host health and
recovery (Cheng et al. 2017; Wang et al. 2016).
In addition to influencing the quantity of food that animals consume,
infection can also alter diet selection. In caterpillars challenged with
a viral infection, infected individuals select diets with a higher
protein to carbohydrate ratio when compared with control individuals,
and infected individuals placed on a high protein diet are more likely
to survive infection (Povey et al. 2013). Shifts in diet preference
during infection that optimize recovery and survival are referred to as
“self-medication” behaviors and have been documented in several taxa
(Huffman and Seifu 1989, Hutchings et al. 2003, Povey et al. 2013).
Thus, animals can shift feeding behaviors and selectively feed on foods
with desirable macronutrient composition in response to an immune
threat.
Shifts in diet caused by environmental availability and selective
feeding can affect the immune system through shifts in the gut
microbiome (Zheng et al. 2020). The gut microbiome regulates multiple
aspects of host health, including metabolism and the development of the
host immune system (Hird 2017, Kau et al. 2011, Zheng et al. 2020). Gut
microbial communities can also influence how hosts respond to disease,
as disrupting microbial communities with antibiotics can alter immunity
and increase susceptibility to bacterial and parasitic infections in
humans and wildlife (Buffie et al. 2012, Zheng et al. 2020, Knutie et
al. 2017). Host diet plays an important role in shaping the composition
and diversity of gut microbial communities (Pan and Yu 2014, Singh et
al. 2017, Bodawatta et al. 2021) and diet-induced shifts in gut
microbiota can alter host immune responses (Zheng et al. 2020). For
example, in humans and mice, a high-fat westernized diet alters gut
microbial communities and increases inflammation (Agus et al. 2016,
Statovci et al. 2017). Shifts in diet can also cause changes in the gut
microbiome that increase host resistance to parasites. In nestling
bluebirds, food supplementation with mealworms increased gut bacterial
diversity and the abundance of Clostridium spp., which was
associated with higher nestling antibody responses and lower numbers of
nest parasites (Knutie 2020). These studies provide evidence that diet
can alter host responses to parasites through shifts in the gut
microbiome. Thus, organisms may be able to optimize responses to
infection through shifts in feeding behavior that alter gut microbial
composition. Despite the clear implications for host health and wildlife
disease dynamics, few studies have investigated how dietary
macronutrients, gut microbiota, physiology, and feeding behavior
interact to shape host responses related to infection.
Infection-induced shifts in feeding and activity can also be detected by
conspecifics and viewing a sick neighbor can result in healthy
individuals altering their feeding behavior in a way that optimizes or
primes the immune system to fight off infection (Castella et al. 2008;
Povey et al. 2013). Further, recent research in social organisms
indicates that behavioral and physiological changes can occur in
response to public information (Cornelius et al. 2018; Cornelius 2022,
Schaller, et al. 2010, Stevenson et al. 2011, 2012; Love et al. 2021).
In red crossbills, social information from conspecifics that is
indicative of low food-abundance reduces the expression of
glucocorticoid and mineralocorticoid receptors in the brain (Cornelius
et al. 2018). Additionally, red crossbills that observed food-restricted
neighbors before becoming food-restricted themselves ate more food,
conserved more body mass, and were in better condition than birds
without this predictive social information (Cornelius 2022). Since
nutritional state is known to greatly impact disease outcomes (Murray et
al. 1998, Chandra 1996, Lochmiller & Deerenberg 2000), prophylactic
behaviors involving macronutrient selection in response to cues of
disease could help prepare organisms for an impending immune threat.
The goals of this study were to 1) enhance our understanding of how
dietary macronutrients affect host physiology and the gut microbiome, 2)
characterize how feeding behavior and macronutrient selection are
influenced by immune threats that are direct (i.e., immune activation)
or perceived (e.g., observing a sick conspecific), and 3) provide
insight into whether shifts in feeding behavior alter the gut microbiome
and physiological processes relevant to disease susceptibility and
transmission. We explored these relationships in zebra finches
(Taeniopygia guttata ) through two separate experiments
investigating 1) how diet macronutrient content affects immunity,
hormonal responses, and the gut microbiome, and 2) how perceived and
actual immune threats shape feeding behavior (caloric intake and diet
macronutrient selection). Additionally, because social information from
conspecifics regarding food (Cornelius et al. 2018; Cornelius 2022) and
disease risk (Schaller, et al. 2010, Stevenson et al. 2011, 2012; Love
et al. 2021) can alter physiology, we also investigated whether
perceived risk of infection (seeing immune-challenged conspecifics)
alters physiological responses pertinent to immune function and feeding
behavior, specifically, complement activity, and corticosterone and
testosterone blood plasma concentrations. We predicted that the
different diets would induce changes in the gut microbiome and
physiology. We also predicted that birds given an immune challenge and
birds with a social cue of heightened infection risk would increase
protein consumption, as studies in insects indicate that high protein
diets are associated with increased immune capabilities and high lipid
diets are associated with increased mortality during infection (Povey et
al. 2013, Adamo 2008, Adamo et al. 2010). Additionally, we predicted
that perceived risk of infection (seeing sick conspecifics) would alter
physiological responses, and that this effect might be mediated through
shifts in feeding behavior and subsequent changes in the gut microbiome.
Identifying and understanding the factors that contribute to variation
in avian responses to infection is of broad interest and integral to
improving our understanding of avian epidemiology, especially since
birds are hosts for diseases relevant to wildlife, domestic animals, and
human health (Reed et al. 2003).