We have a limited understanding of the proximate mechanisms that are responsible for the development of variation in animal performance and life-history strategies. Provided that components of an organism's successful life historyfor example, mate competition, gestation, lactation, etc.are energetically demanding, increased energy production within mitochondria is likely the foundation from which organisms are able to perform these tasks. Mitochondrial behaviour (positioning within the cell and communication between mitochondria) and morphology affect variation in energy production at the molecular, cellular, and organismal levels. Therefore, adaptations in mitochondrial behaviour and morphology that favour efficient energy production likely influence variation in animal performance. Previous work has linked greater proportions of inter-mitochondrial junctions and density of the inner mitochondrial membrane, among other traits, with increased energetic demand. Future research should focus on how inter-mitochondrial junctions and morphology of the inner mitochondrial membrane, in particular, influence animal performance in accordance with mitochondrial density, fission, and fusion. Biological Reviews 95 (2020) 730-737

Exponential increase in respiration rate with increasing temperature in poikilotherms is well documented, however, the rate of change varies greatly across copepod taxa. Studies often report magnitude of change, but the rate of change in respiration across multiple temperatures is equivocal. We used 32 studies spanning 78 yrs of research and 50 copepod species (three orders) to quantify percent change in respiration rates per one‐unit change in temperature. We found that copepod respiration rates increased by approximately 7% per °C increase in water temperature across the orders Calanoida, Cyclopoida, and Harpacticoida. Neither food availability nor scaling respiration to copepod dry weight affected the rate of change of respiration rates. Studies using Winkler titration to measure oxygen consumption produced significantly larger percent changes in respiration, whereas newer methods such as fiber optics produced smaller effects. These results have far reaching implications for understanding how copepod respiration responds to increasing water temperatures.

Our current understanding of variation in mitochondrial performance is incomplete. The production of ATP via oxidative phosphorylation is dependent, in part, upon the structure of the inner mitochondrial membrane. Morphology of the inner membrane is crucial for the formation of the proton gradient across the inner membrane and, therefore, ATP synthesis. The inner mitochondrial membrane is dynamic, changing shape and surface area. These changes alter density (amount per volume) of the inner mitochondrial membrane within the confined space of the mitochondrion. Because the number of electron transport system proteins within the inner mitochondrial membrane changes with inner mitochondrial membrane area, a change in the amount of inner membrane alters the capacity for ATP production within the organelle. This review outlines the evidence that the association between ATP synthases, inner mitochondrial membrane density, and mitochondrial density (number of mitochondria per cell), impact ATP production by mitochondria. Further, we consider possible constraints on the capacity of mitochondria to produce ATP by increasing inner mitochondrial membrane density.

We assessed responses in grooming behavior to ectoparasite densities in naturally occurring white-tailed deer (Odocoileus virginianus) in southwest Alabama, and tested predictions of the stimulus-driven and programmed grooming models with respect to intraspecific body size and vigilance. Stimulus-driven grooming predicts greater tick densities would lead to an increase in grooming, whereas the programmed model predicts a higher rate of grooming would decrease tick densities. Within the programmed model, smaller individuals are predicted to groom more and host fewer ticks, and, due to increased vigilance, breeding males will groom less than females and bachelor males, and thus host more ticks during the rut. We used generalized linear models to determine males had a higher average tick density than females and exhibited complete separation of tick parasitism between nonrutting and rutting periods. Our results support the stimulus-driven grooming model as both fawns and yearlings had significantly higher deer ked and combined deer ked/tick densities than adults. We used Mann-Whitney U tests to conclude fawns oral groomed at a significantly higher rate than adults, even in the absence of allogrooming. Programmed and stimulus-driven grooming were not mutually exclusive, but rather ectoparasite and host dependent. Although individuals observed grooming were analyzed separately from harvested individuals, we infer that heavy deer ked densities may lead to higher grooming rates. We suggest white-tailed deer grooming for ticks should be considered within an evolutionary framework, and grooming for deer keds should be viewed as a proximate response to agitation. This is the first study to show that deer keds may have a greater influence in overall grooming behavior of deer than do ticks. We recommend that future studies should consider other ectoparasites along with ticks to understand their effects on grooming behavior in dimorphic terrestrial mammals.

An important component of life history theory is understanding how natural variation arises in populations. Both endogenous and exogenous factors contribute to organism survival and reproduction, and therefore, it is important to understand how such factors are both beneficial and detrimental to population dynamics. One ecologically relevant factor that influences the life history of aquatic organisms is ultraviolet (UV) radiation. While the majority of research has focused on the potentially detrimental effects that UV radiation has on aquatic organisms, few studies have evaluated hormetic responses stimulated by radiation under select conditions. The goal of this study was to evaluate the impact of UV‐A/B irradiation on life history characteristics in Tigriopus californicus copepods. After exposing copepods to UV‐A/B irradiation (control, 1‐, and 3‐hr UV treatments at 0.5 W/m2), we measured the impact of exposure on fecundity, reproductive effort, and longevity. We found that UV irradiation increased the size of the first clutch among all reproducing females in both the 1‐ and 3‐hr experimental groups and decreased longevity among all females that mated in the 1‐hr treatment. UV irradiation had no effect on the number of clutches females produced. These findings indicate a potential benefit of UV irradiation on reproductive performance early in life, although the same exposure came at a cost to longevity.