Cynthia J. Downs

Selection for increased mass-independent maximal metabolic rate suppresses innate but not adaptive immune function

Both appropriate metabolic rates and sufficient immune function are essential for survival. Consequently, eco-immunologists have hypothesized that animals may experience trade-offs between metabolic rates and immune function. Previous work has focused on how basal metabolic rate (BMR) may trade-off with immune function, but maximal metabolic rate (MMR), the upper limit to aerobic activity, might also trade-off with immune function. We used mice artificially selected for high mass-independent MMR to test for trade-offs with immune function. We assessed (i) innate immune function by quantifying cytokine production in response to injection with lipopolysaccharide and (ii) adaptive immune function by measuring antibody production in response to injection with keyhole limpet haemocyanin. Selection for high mass-independent MMR suppressed innate immune function, but not adaptive immune function. However, analyses at the individual level also indicate a negative correlation between MMR and adaptive immune function. By contrast BMR did not affect immune function. Evolutionarily, natural selection may favour increasing MMR to enhance aerobic performance and endurance, but the benefits of high MMR may be offset by impaired immune function. This result could be important in understanding the selective factors acting on the evolution of metabolic rates.

Testing Hypotheses in Ecoimmunology Using Mixed Models: Disentangling Hierarchical Correlations

Considerable research in ecoimmunology focuses on investigating variation in immune responses and linking this variation to physiological trade-offs, ecological traits, and environmental conditions. Variation in immune responses exists within and among individuals, among populations, and among taxonomic groupings. Understanding how variation and covariation are distributed and how they differ across these levels is necessary for drawing appropriate ecological and evolutionary inferences. Moreover, variation at the among-individual level directly connects to underlying quantitative genetic parameters. In order to fully understand immune responses in evolutionary and ecological contexts and to reveal phylogenetic constraints on evolution, statistical approaches must allow (co)variance to be partitioned among levels of individual, population, and phylogenetic organization (e.g., population, species, genera, and various higher taxa). Herein, we describe how multi-response mixed-effects models can be used to partition variation in immune responses among different hierarchical levels, specifically within-individuals, among-individuals, and among-species. We use simulated data to demonstrate that mixed models allow for proper partitioning of (co)variances. Importantly, these simulations also demonstrate that conventional statistical tools grossly misestimate relevant parameters, which urges caution in relating ecoimmunological hypotheses to existing empirical research. We conclude by discussing the advantages and caveats of a mixed-effects modeling approach.

Do yellow-pine chipmunks prefer to recover their own caches

Abstract Many rodent species scatter hoard seeds within a home range shared with other seed-caching animals. An animal foraging for cached food is likely to encounter the caches of other animals, as well as its own. This study asks the following question: do animals recover primarily their own caches, do they conserve their own caches and search for the caches of other animals, or do they search indiscriminately for cached seeds regardless of who buried them? We tested these ideas using 8 yellow-pine chipmunks inside rodent-proof enclosures. We allowed each subject to cache radioactive seeds, we located the seed caches, and then we paired each rodent cache with an artificial cache in a similar microsite. During search trials, 7 subjects removed mostly their own caches, and 1 subject removed similar numbers of its own caches and paired artificial caches. These results suggest that most yellow-pine chipmunks actively search for food that they have stored, apparently using spatial memory rather than relying on olfaction. However, previous studies have established that chipmunks also are very effective pilferers of caches made by other animals. This apparent discrepancy may result from a difference between enclosures and field sites in the proportion of “own” caches to total caches.

A strong response to selection on mass-independent maximal metabolic rate without a correlated response in basal metabolic rate

Metabolic rates are correlated with many aspects of ecology, but how selection on different aspects of metabolic rates affects their mutual evolution is poorly understood. Using laboratory mice, we artificially selected for high maximal mass-independent metabolic rate (MMR) without direct selection on mass-independent basal metabolic rate (BMR). Then we tested for responses to selection in MMR and correlated responses to selection in BMR. In other lines, we antagonistically selected for mice with a combination of high mass-independent MMR and low mass-independent BMR. All selection protocols and data analyses included body mass as a covariate, so effects of selection on the metabolic rates are mass adjusted (that is, independent of effects of body mass). The selection lasted eight generations. Compared with controls, MMR was significantly higher (11.2%) in lines selected for increased MMR, and BMR was slightly, but not significantly, higher (2.5%). Compared with controls, MMR was significantly higher (5.3%) in antagonistically selected lines, and BMR was slightly, but not significantly, lower (4.2%). Analysis of breeding values revealed no positive genetic trend for elevated BMR in high-MMR lines. A weak positive genetic correlation was detected between MMR and BMR. That weak positive genetic correlation supports the aerobic capacity model for the evolution of endothermy in the sense that it fails to falsify a key model assumption. Overall, the results suggest that at least in these mice there is significant capacity for independent evolution of metabolic traits. Whether that is true in the ancestral animals that evolved endothermy remains an important but unanswered question.

Investment in Constitutive Immune Function by North American Elk Experimentally Maintained at Two Different Population Densities

Natural selection favors individuals that respond with effective and appropriate immune responses to macro or microparasites. Animals living in populations close to ecological carrying capacity experience increased intraspecific competition, and as a result are often in poor nutritional condition. Nutritional condition, in turn, affects the amount of endogenous resources that are available for investment in immune function. Our objective was to understand the relationship between immune function and density dependence mediated by trade-offs between immune function, nutritional condition, and reproduction. To determine how immune function relates to density-dependent processes, we quantified bacteria killing ability, hemolytic-complement activity, and nutritional condition of North American elk (Cervus elaphus) from populations maintained at experimentally high- and low-population densities. When compared with elk from the low-density population, those from the high-density population had higher bacteria killing ability and hemolytic-complement activity despite their lower nutritional condition. Similarly, when compared with adults, yearlings had higher bacteria killing ability, higher hemolytic-complement activity, and lower nutritional condition. Pregnancy status and lactational status did not change either measure of constitutive immunity. Density-dependent processes affected both nutritional condition and investment in constitutive immune function. Although the mechanism for how density affects immunity is ambiguous, we hypothesize two possibilities: (i) individuals in higher population densities and in poorer nutritional condition invested more into constitutive immune defenses, or (ii) had higher parasite loads causing higher induced immune responses. Those explanations are not mutually exclusive, and might be synergistic, but overall our results provide stronger support for the hypothesis that animals in poorer nutritional condition invest more in constitutive immune defenses then animals in better nutritional condition. This intriguing hypothesis should be investigated further within the larger framework of the cost and benefit structure of immune responses.

HIGH RELATIVE HUMIDITY INCREASES PILFERING SUCCESS OF YELLOW PINE CHIPMUNKS

Abstract Scatter-hoarding animals store food items to be used later when food is scarce. However, other individuals can pilfer food stores because caches are not usually defended. We tested how associative learning contributes to foraging success of pilferers searching for scatter-hoarded food. We conducted a field-based, seed-removal experiment to test 2 hypotheses. First, yellow pine chipmunks (Tamias amoenus) will learn to associate buried food with recurring objects faster than they will learn to associate food with singular (distinctive) objects. Second, they will learn to associate buried food with man-made objects faster than they will learn to associate food with natural objects. Rodents clearly learned to associate objects with buried food regardless of distinctiveness or origin. The observed pattern of seed removal suggested that high relative humidity (RH) events (storm systems) increased seed odor, facilitating olfaction by rodents, and increasing the rate of seed removal. We tested this hypothesis in a laboratory experiment using 8 wild-caught yellow pine chipmunks and 5 levels of RH (17%, ∼27%, ∼50%, ∼75%, or ∼95%). Foraging success at 17–75% RH was not different from random, but at 95% RH seed recovery was significantly higher than random. High RH facilitates discovery of buried seeds, and with higher foraging success, associative learning of cache markers may be easier. Thus, cache pilfering may be facilitated by high humidity.

Immune response to a Trichinella spiralis infection in house mice from lines selectively bred for high voluntary wheel running.

SUMMARY Four lines of mice bred for high voluntary wheel running (HR lines) have high baseline circulating corticosterone levels and increased daily energy expenditure as compared with four non-selected control (C) lines. High corticosterone may suppress immune function and competing energy demands may limit ability to mount an immune response. We hypothesized that HR mice have a reduced immune response and therefore a decreased ability to fight an infection by Trichinella spiralis, an ecologically relevant nematode common in mammals. Infections have an acute, intestinal phase while the nematode is migrating, reproducing and traveling throughout the bloodstream, followed by a chronic phase with larvae encysted in muscles. Adult males (generation 55 of the selection experiment) were sham-infected or infected by oral gavage with ~300 J1 T. spiralis larvae. During the chronic phase of infection, mice were given wheel access for 6 days, followed by 2 days of maximum aerobic performance trials. Two weeks post-infection, infected HR had significantly lower circulating immunoglobulin E levels compared with infected C mice. However, we found no statistical difference between infected HR and C mice in numbers of encysted larvae within the diaphragm. As expected, both voluntary running and maximum aerobic performance were significantly higher in HR mice and lower in infected mice, with no line type-by-infection interactions. Results complement those of previous studies suggesting decreased locomotor abilities during the chronic phase of T. spiralis infection. However, despite reduced antibody production, breeding for high voluntary wheel exercise does not appear to have a substantial negative impact on general humoral function.

Within-lifetime trade-offs but evolutionary freedom for hormonal and immunological traits: evidence from mice bred for high voluntary exercise.

SUMMARY Chronic increases in circulating corticosterone (CORT) generally suppress immune function, but it is not known whether evolved increases necessarily have similar adverse effects. Moreover, the evolution of immune function might be constrained by the sharing of signaling molecules, such as CORT, across numerous physiological systems. Laboratory house mice (Mus domesticus Linnaeus) from four replicate lines selectively bred for high voluntary wheel running (HR lines) generally had baseline circulating CORT approximately twofold higher than in four non-selected control (C) lines. To test whether elevated baseline CORT suppresses the inflammatory response in HR mice, we injected females with lipopolysaccharide (LPS). All mice injected with LPS exhibited classic signs of an inflammatory response, including sickness behavior, loss of body mass, reduced locomotor activity (i.e. voluntary wheel running), enlarged spleens and livers, elevated hematocrit and elevated inflammatory cytokines. However, as compared with C mice, the inflammatory response was not suppressed in HR mice. Our results, and those of a previous study, suggest that selective breeding for high voluntary exercise has not altered immune function. They also suggest that the effects of evolved differences in baseline CORT levels may differ greatly from effects of environmental factors (often viewed as ‘stressors’) that alter baseline CORT during an individual’s lifetime. In particular, evolved increases in circulating levels of ‘stress hormones’ are not necessarily associated with detrimental suppression of the inflammatory response, presumably as a result of correlated evolution of other physiological systems (counter-measures). Our results have important implications for the interpretation of elevated stress hormones and of immune indicators in natural populations.

Ectoparasite performance when feeding on reproducing mammalian females: an unexpected decrease when on pregnant hosts

Reproduction is an energy-demanding activity in mammalian females, with increased energy requirements during pregnancy and, especially, during lactation. To better understand the interactions between parasitism and host reproduction, we investigated feeding and reproductive performance of fleas (Xenopsylla ramesis) parasitizing non-reproducing, pregnant or lactating gerbilline rodents (Meriones crassus). Based on energetic considerations, we predicted that feeding and reproductive performance of fleas would be lowest on non-breeding females, moderate on pregnant females and highest on lactating females. We estimated feeding performance of the fleas via absolute and mass-specific bloodmeal size and reproductive performance via egg production and latency to peak oviposition. Host reproductive status had no effect on either absolute or mass-specific bloodmeal size or the day of peak oviposition, but significantly affected the daily number of eggs produced by a female flea. Surprisingly, and contrary to our predictions, egg production of fleas fed on pregnant rodents was significantly lower than that of fleas on non-reproducing and lactating rodents, while no difference in egg production between fleas feeding on non-reproducing and lactating hosts was found. Our results suggest that differences in parasite reproduction when feeding on hosts of different reproductive status are not associated with the different energy requirements of the hosts at non-breeding, pregnancy and lactation but rather with variation in hormonal and/or immune status during these periods.

Host reproductive status and reproductive performance of a parasite: offspring quality and trade-offs in a flea parasitic on a rodent.

We investigated offspring quality in fleas (Xenopsylla ramesis) feeding on non-reproducing, pregnant or lactating rodents (Meriones crassus) and asked whether (a) quality of flea offspring differs dependent on host reproductive status; (b) fleas trade off offspring quantity for quality; and (c) quality variables are inter-correlated. Emergence success was highest when parents exploited pregnant hosts, while development time was longest when parents exploited lactating hosts. Male offspring from fleas fed on non-reproductive and pregnant hosts were larger than those from lactating hosts whereas female offspring from fleas fed on pregnant hosts were larger than those from both lactating and non-reproductive hosts. Male offspring survived under starvation the longest when their parents exploited lactating hosts and the shortest when their parents exploited pregnant hosts. Female offspring of parents that exploited lactating hosts survived under starvation longer than those that exploited non-reproductive and pregnant hosts. Emergence success and development time decreased as mean number of eggs laid by mothers increased. Fleas that were larger and took longer to develop lived significantly longer under starvation. These results indicate the presence of a trade-off between offspring quantity and quality in fleas exploiting female Sundevalls jird in varying reproductive condition but this trade-off depended on the quality trait considered.