Experimental increase in predation risk causes a cascading stress response in free-ranging snowshoe hares

Extensive research confirms that environmental stressors like predation risk can profoundly affect animal condition and physiology. However, there is a lack of experimental research assessing the suite of physiological responses to risk that may arise under realistic field conditions, leaving a fragmented picture of risk-related physiological change and potential downstream consequences on individuals. We increased predation risk in free-ranging snowshoe hares (Lepus americanus) during two consecutive summers by simulating natural chases using a model predator and monitored hares intensively via radio-telemetry and physiological assays, including measures designed to assess changes in stress physiology and overall condition. Compared to controls, risk-augmented hares had 25.8% higher free plasma cortisol, 15.9% lower cortisol-binding capacity, a greater neutrophil:lymphocyte skew, and a 10.4% increase in glucose. Despite these changes, intra-annual changes in two distinct condition indices, were unaffected by risk exposure. We infer risk-augmented hares compensated for changes in their stress physiology through either compensatory foraging and/or metabolic changes, which allowed them to have comparable condition to controls. Although differences between controls and risk-augmented hares were consistent each year, both groups had heightened stress measures during the second summer, likely reflecting an increase in natural stressors (i.e., predators) in the environment. We show that increased predation risk in free-ranging animals can profoundly alter stress physiology and that compensatory responses may contribute to limiting effects of such changes on condition. Ultimately, our results also highlight the importance of biologically relevant experimental risk manipulations in the wild as a means of assessing physiological responses to natural stressors.

From process to pattern: how fluctuating predation risk impacts the stress axis of snowshoe hares during the 10-year cycle

Predation is a central organizing process affecting populations and communities. Traditionally, ecologists have focused on the direct effects of predation--the killing of prey. However, predators also have significant sublethal effects on prey populations. We investigated how fluctuating predation risk affected the stress physiology of a cyclic population of snowshoe hares (Lepus americanus) in the Yukon, finding that they are extremely sensitive to the fluctuating risk of predation. In years of high predator numbers, hares had greater plasma cortisol levels at capture, greater fecal cortisol metabolite levels, a greater plasma cortisol response to a hormone challenge, a greater ability to mobilize energy and poorer body condition. These indices of stress had the same pattern within years, during the winter and over the breeding season when the hare:lynx ratio was lowest and the food availability the worst. Previously we have shown that predator-induced maternal stress lowers reproduction and compromises offspring's stress axis. We propose that predator-induced changes in hare stress physiology affect their demography through negative impacts on reproduction and that the low phase of cyclic populations may be the result of predator-induced maternal stress reducing the fitness of progeny. The hare population cycle has far reaching ramifications on predators, alternate prey, and vegetation. Thus, predation is the predominant organizing process for much of the North American boreal forest community, with its indirect signature--stress in hares--producing a pattern of hormonal changes that provides a sensitive reflection of fluctuating predator pressure that may have long-term demographic consequences.

Genetic basis of incidence and period length of circadian rhythm for locomotor activity in populations of a seed beetle

Circadian rhythms are ubiquitous in a wide variety of organisms, although their genetic variation has been analyzed in only a few species. We found genetic differences in the circadian rhythm of adult locomotor activity among strains of the adzuki bean beetle, Callosobruchus chinensis, which differed in origin and have been maintained in isolation. All beetles in some strains clearly had free-running rhythms in constant darkness whereas most beetles in other strains were arrhythmic. The period of free-running rhythm varied from approximately 19 to 23 h between the strains. F(1) males from reciprocal crosses among strains with different periods of circadian rhythms had circadian periods that were intermediate between their parental strains. Segregation of the circadian rhythm appeared in the F(2) generation. These findings are consistent with the hypothesis that variation in the period length of circadian rhythm is explained by a major autosomal gene with additive effects and no dominance. This hypothesis was supported by the joint scaling test for the free-running period in the F(1) and F(2) generations. We discuss possible causes for genetic variation in circadian rhythm in the C. chinensis strains in terms of random factors and selection.

The sensitive hare: sublethal effects of predator stress on reproduction in snowshoe hares

1. Prey responses to high predation risk can be morphological or behavioural and ultimately come at the cost of survival, growth, body condition, or reproduction. These sub-lethal predator effects have been shown to be mediated by physiological stress. We tested the hypothesis that elevated glucocorticoid concentrations directly cause a decline in reproduction in individual free-ranging female snowshoe hares, Lepus americanus. We measured the cortisol concentration from each dam (using a faecal analysis enzyme immunoassay) and her reproductive output (litter size, offspring birth mass, offspring right hind foot (RHF) length) 30 h after birth. 2. In a natural monitoring study, we monitored hares during the first and second litter from the population peak (2006) to the second year of the decline (2008). We found that faecal cortisol metabolite (FCM) concentration in dams decreased 52% from the first to the second litter. From the first to the second litter, litter size increased 122%, offspring body mass increased 130%, and offspring RHF length increased 112%. Dam FCM concentrations were inversely related to litter size (r(2) = 0.19), to offspring birth mass (r(2) = 0.32), and to offspring RHF length (r(2) = 0.64). 3. In an experimental manipulation, we assigned wild-caught, pregnant hares to a control and a stressed group and held them in pens. Hares in the stressed group were exposed to a dog 1-2 min every other day before parturition to simulate high predation risk. At parturition, unsuccessful-stressed dams (those that failed to give birth to live young) and stressed dams had 837% and 214%, respectively, higher FCM concentrations than control dams. Of those females that gave birth, litter size was similar between control and stressed dams. However, offspring from stressed dams were 37% lighter and 16% smaller than offspring from control dams. Increasing FCM concentration in dams caused the decline of offspring body mass (r(2) = 0.57) and RHF (r(2) = 0.52). 4. This is the first study in a free-ranging population of mammals to show that elevated, predator-induced, glucocorticoid concentrations in individual dams caused a decline in their reproductive output measured both by number and quality of offspring. Thus, we provide evidence that any stressor, not just predation, which increases glucocorticoid concentrations will result in a decrease in reproductive output.

Heavy metal exposure, reproductive activity, and demographic patterns in white-footed mice (Peromyscus leucopus) inhabiting a contaminated floodplain wetland

We examined the concentrations of selected metals and selenium (Se) in the tissues of white-footed mice (Peromyscus leucopus) collected at a constructed wetland originally created as a retention basin for sediments dredged from Lake DePue, Illinois. These sediments were contaminated with high concentrations of cadmium (Cd), zinc (Zn), and other elements as a result of nearby smelting operations. White-footed mice inhabiting the former retention basin experienced greater exposure to Cd, Pb, and Se than those from nearby reference sites. Concentrations of Cu and Zn in livers of mice from the contaminated wetland and adjacent floodplain reference site were greater than in mice from the more-distant reference sites. Judging by concentrations in their kidneys, white-footed mice inhabiting the floodplain adjacent to the contaminated wetland had greater exposure to Cd than those from the two more-distant reference sites. Concentrations of Hg in tissues of mice did not vary appreciably among sites. Concentrations of Cd and Se in the tissues of some white-footed mice from the contaminated wetland exceeded critical concentrations observed in experimental studies of laboratory mice and rats; with few exceptions tissue Pb concentrations were below published effects levels. However, we did not detect changes in abundance, demographics, or reproductive activity that might suggest population-level effects of contaminant exposure. Mean weight of embryos expressed as a function of crown-rump length did not differ among locations sampled, and no gross lesions indicative of exposure to heavy metals were observed. Kidney and liver weight, corrected for body weight, were nominally, though not significantly, lowest in both male and female mice from areas of increased Cd and Pb exposure. Metals dredged from Lake DePue were still bioavailable 25 years after deposition. However, small mammal populations are resilient to environmental stressors and we did not detect differences in population parameters suggesting that the population of white-footed mice inhabiting the contaminated wetland was at risk from increased exposure to these contaminants.

Reproductive changes in a cyclic population of snowshoe hares

Reproductive output was estimated for a cyclic population of snowshoe hares (Lepus americanus) in the Kluane Lake region of the southwest Yukon Territory. Data collected by five researchers were collated over 8 years (1989–1996). Pregnant hares were captured and held in cages until they gave birth so that reproductive characteristics could be measured. Pregnancy rate, litter size, and neonate size fluctuated significantly throughout the cycle, changes occurring about 2 years before corresponding changes in density. Pregnancy rates were nearly 100% early in the breeding season, but declined up to 20% in the last gestation periods of the year. The number of litters produced in a breeding season varied between two (decline phase) and four (low, early increase phase). Litter size varied among years as well as among litters within a year, larger litters being born later in the breeding season. The body mass and size of newborn hares varied by 5–33% among years. The combined changes in pregnancy rate and litter size resulted in a cyclic change in total reproductive output ranging from a low of 6.9 young per female during the decline phase to a maximum of 18.9 during the second year of the low and early increase phases.

Differential predation by coyotes on snowshoe hares

Differential predation on particular sex or age classes of a population can arise as a result of predator preferences or prey attributes. I examined the impacts of age, size, and body mass of snowshoe hares, Lepus americanus, on their susceptibility to predation by coyotes, Canis latrans. I observed coyote predation on naïve radio-collared hares during a fortuitous natural experiment: a coyote entered a predator exclosure fence in which hares of all ages had no previous experience with terrestrial predators, thus separating age from experience with this predator. I contrasted this manipulation with populations in which hares grew up in the presence of coyotes. Prey naiveté per se did not influence coyote predation, but older hares appeared to be more susceptible to coyote predation than younger ones. There were no obvious effects of body mass or size on coyote predation during the winter.