The metabolic performance predicts home range size of bank voles: a support for the behavioral-bioenergetics theory

Individual heterogeneity in ixodid tick infestation and prevalence of Borrelia burgdorferi sensu lato in a northern community of small mammalian hosts

Heterogeneous aggregation of parasites between individual hosts is common and regarded as an important factor in understanding transmission dynamics of vector-borne diseases. Lyme disease is vectored by generalist tick species, yet we have a limited understanding of how individual heterogeneities within small mammal host populations affect the aggregation of ticks and likelihood of infection. Male hosts often have higher parasite and infection levels than females, but whether this is linked to sexual body size dimorphism remains uncertain. Here, we analysed how host species, sex, and body mass influenced Ixodes ricinus tick infestations and the infection prevalence of Borrelia burgdorferi sensu lato (s.l.) in three species of small mammals involved in the enzootic transmission cycle of Lyme disease in Norway from 2018 to 2022. Larval and nymphal ticks were found on 98% and 34% of all individual hosts, respectively. In bank voles and wood mice, both larval and nymphal tick infestation and infection probability increased with body mass, and it increased more with mass for males than for females. Tick infestation in the common shrew increased with body mass and was higher in males, while pathogen infection was higher in females. Sex-biases in infestation did not correspond with level of sexual body mass dimorphism across species. This study contributes to our understanding of how individual heterogeneity among small mammalian hosts influences I. ricinus tick aggregation and prevalence of B. burgdorferi s.l. at northern latitudes.

Modeling heterothermic fitness landscapes in a marsupial hibernator using changes in body composition

Hibernation is an adaptive strategy that allows animals to enter a hypometabolic state, conserving energy and enhancing their fitness by surviving harsh environmental conditions. However, addressing the adaptive value of hibernation, at the individual level and in natural populations, has been challenging. Here, we applied a non-invasive technique, body composition analysis by quantitative magnetic resonance (qMR), to calculate energy savings by hibernation in a population of hibernating marsupials (Dromiciops gliroides). Using outdoor enclosures installed in a temperate rainforest, and measuring qMR periodically, we determined the amount of fat and lean mass consumed during a whole hibernation cycle. With this information, we estimated the daily energy expenditure of hibernation (DEEH) at the individual level and related to previous fat accumulation. Using model selection approaches and phenotypic selection analysis, we calculated linear (directional, β), quadratic (stabilizing or disruptive, γ) and correlational (ρ) coefficients for DEEH and fat accumulation. We found significant, negative directional selection for DEEH (βDEEH = - 0.58 ± 0.09), a positive value for fat accumulation (βFAT = 0.34 ± 0.07), and positive correlational selection between both traits (ρDEEH × FAT = 0.24 ± 0.07). Then, individuals maximizing previous fat accumulation and minimizing DEEH were promoted by selection, which is visualized by a bi-variate selection surface estimated by generalized additive models. At the comparative level, results fall within the isometric allometry known for hibernation metabolic rate in mammals. Thus, by a combination of a non-invasive technique for body composition analysis and semi-natural enclosures, we were characterized the heterothermic fitness landscape in a semi-natural population of hibernators.

Physiology can predict animal activity, exploration, and dispersal

Physiology can underlie movement, including short-term activity, exploration of unfamiliar environments, and larger scale dispersal, and thereby influence species distributions in an environmentally sensitive manner. We conducted meta-analyses of the literature to establish, firstly, whether physiological traits underlie activity, exploration, and dispersal by individuals (88 studies), and secondly whether physiological characteristics differed between range core and edges of distributions (43 studies). We show that locomotor performance and metabolism influenced individual movement with varying levels of confidence. Range edges differed from cores in traits that may be associated with dispersal success, including metabolism, locomotor performance, corticosterone levels, and immunity, and differences increased with increasing time since separation. Physiological effects were particularly pronounced in birds and amphibians, but taxon-specific differences may reflect biased sampling in the literature, which also focussed primarily on North America, Europe, and Australia. Hence, physiology can influence movement, but undersampling and bias currently limits general conclusions.

Energetic dissociation of individual and species ranges

The use of energy is universal to all life forms and all levels of biological organization, potentially linking processes operating at variable scales. Individual and species ranges might be energetically constrained, yet divergent metabolic limitations at both scales can disassociate these individual and species traits. We analysed comparative energetic and range data to unravel the mechanistic basis of the dissociation between individual and species range sizes observed among mammalian species. Our results demonstrate that basal, or maintenance, metabolism negatively correlates with individual ranges, but, at the same time, it positively correlates with species ranges. High aerobic capacity, i.e. maximum metabolic rate, positively correlates with individual ranges, but it is weakly related to species range size. These antagonistic energetic constraints on both ranges could lead to a disassociation between individual and species traits and to a low covariation between home and species range sizes. We show that important organismal functions, such as basal and maximum metabolic rates, have the potential to unravel mechanisms operating at different levels of biological organization and to expose links between energy-dependent processes at different scales.

Chronic Background Radiation Correlates With Sperm Swimming Endurance in Bank Voles From Chernobyl

Sperm quantity and quality are key features explaining intra- and interspecific variation in male reproductive success. Spermatogenesis is sensitive to ionizing radiation and laboratory studies investigating acute effects of ionizing radiation have indeed found negative effects of radiation on sperm quantity and quality. In nature, levels of natural background radiation vary dramatically, and chronic effects of low-level background radiation exposure on spermatogenesis are poorly understood. The Chernobyl region offers a unique research opportunity for investigating effects of chronic low-level ionizing radiation on reproductive properties of wild organisms. We captured male bank voles (Myodes glareolus) from 24 locations in the Chernobyl exclusion zone in 2011 and 2015 and collected information on sperm morphology and kinetics. The dataset is limited in size and there overall was a relatively weak correlation between background radiation and sperm quality. Still, some correlations are worth discussing. First, mid-piece segments of spermatozoa tended to be smaller in bank vole males from areas with elevated background radiation levels. Second, we demonstrated a significant positive relationship between background radiation dose rates and the proportion of static spermatozoa among males within and among study locations after 10 as well as 60 min of incubation. Our results provide novel evidence of damaging effects of low dose ionizing radiation on sperm performance in wild rodent populations, and highlight that this topic requires further study across the natural gradients of background radiation that exist in nature.

Individual quality and phenology mediate the effect of radioactive contamination on body temperature in Chernobyl barn swallows

Anthropogenic stressors, such as radioactive contaminants released from the Chernobyl and Fukushima Daiichi accidents, deteriorate ecological and evolutionary processes, as evidence for damaging effects of radioactive contamination on wildlife is accumulating. Yet little is known about physiological traits of animals inhabiting contaminated areas, and how those are affected by individual quality and phenology. We investigated variation in body temperature of wild barn swallows, Hirundo rustica, exposed to radioactive contamination from the Chernobyl accident in Ukraine and Belarus. We tested whether exposure to variable levels of radioactive contamination modified core body temperature of birds, and whether individual and phenological characteristics modulated radiosensitivity of body temperature. We showed that barn swallow body temperature varied with exposure to environmental radioactive contamination and that individual characteristics and phenology affected radioactive exposure. Increased radiosensitivity and up-regulation of body temperature were detected in birds of low body condition, high risk of capture, and in animals captured late during the day but early during the season. These results highlight the complex ways that the body temperature of a wild bird is impacted by exposure to increased radioactive contamination in natural habitats. By impacting body temperature, increased radioactive contamination may compromise energetic balance, jeopardize responsiveness to global warming, and increase risk of overheating.

Trait-Based Variation in the Foraging Performance of Individuals

Although average, species-level interaction strength plays a key role in driving population dynamics and community structure, predator-prey interactions occur among individuals. As a result, individual variation in foraging rates may play an important role in determining the effects of predator-prey interactions on communities. Such variation in foraging rates stems from individual variation in traits that influence the mechanistic components of the functional response, such as movements that determine encounters and behaviors such as decisions to attack. However, we still have little information about individual-level variation in functional responses or the traits that give rise to such variation. Here we combine a standard functional response experiment with wolf spiders foraging on fruit flies with a novel analysis to connect individual morphology, physiology, and movement to individual foraging performance. We found substantial variation in traits between males and females, but these were not clearly linked to the differences in the functional response between males and females. Contrary to expectations, we found no effect of body velocity, leg length, energetic state, or metabolic rate on foraging performance. Instead, we found that body mass interacted with body rotations (clockwise turns), such that larger spiders showed higher foraging performance when they turned more but the reverse was true for smaller spiders. Our results highlight the need to understand the apparent complexity of the links between the traits of individuals and the functional response.

Do colour morphs of wall lizards express different personalities?

Colour morphs sometimes have different behavioural strategies which may be maintained by frequency or density dependence mechanisms. We investigated temporal changes in behavioural reaction to a novel environment among colour morphs (yellow, orange, white) of the European wall lizard (Podarcis muralis). Adult males were given two 15 min experimental trials, and their locomotion was highly consistent between the two trials. Boldness, freezing and escape behaviour were less repeatable. Colour morphs differed in their locomotion and freezing behaviour. Boldness was similar among the morphs, whereas escape behaviour was lowest in yellow morph. Consequently, yellow morph males tended to explore novel environments quickly and thus were more likely to move to potentially safe areas. Orange and white males showed more fear when exposed to a novel environment. Whether such alternative behavioural strategies can contribute to the maintenance of variable fitness optima among the morphs and ultimately to the maintenance of polymorphism remains open to further investigation.

Energetic constraints on mammalian distribution areas

Energy is a universal resource essential for all life functions. The rate of transformation of energy into an organism, and the energetic investment into reproduction, determines population and ecological-level processes. Several hypotheses predicted that the ecological expansion and size of the geographic distribution of a species are shaped by, among other factors, metabolic performance. However, how organismal energetic characteristics contribute to species geographic range size is poorly understood. With phylogenetic comparative methods whether energetic maintenance costs (basal metabolic rate, BMR), aerobic capacity (maximum exercise metabolic rate, VO₂ max), summit thermoregulation (summit metabolic rate, VO₂ sum) and the ability to sustain energy provisioning (daily energy expenditure, DEE) determine the distribution of mammalian species range sizes was tested. Both basal and maximum exercise metabolic rates (accounting for body mass), but not summit thermogenic metabolic rate, were positively associated with species range sizes. Furthermore, daily energy expenditure (accounting for body mass) was positively associated with species ranges. Body mass (accounting for energetic maintenance) was negatively related to range sizes. High aerobic exercise capacity, aiding mobility such as running and dispersal, and high sustained energy provisioning, aiding reproductive effort such as pregnancy, lactation and natal dispersal, can facilitate the establishment of large mammalian geographic ranges. Consequently, the pace of organismal physiological processes can shape important ecological and biodiversity patterns by setting limits to species' range sizes.