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).