A naturally variable life-history trait with underlying physiological variation is the photoperiodic response of many temperate-zone rodents, including white-footed mice (Peromyscus leucopus (Rafinesque, 1818)). Male P. leucopus were obtained from a short photoperiod responsive (R) line, artificially selected for reproductive suppression in short-day conditions (SD) and a nonresponsive (NR) line selected for reproductive maturity in SD. We tested for variation in metabolic rate between lines in SD and long-day conditions (LD). NR mice consumed 34% more food than R mice, without concomitant increase in body mass in SD. Basal metabolic rate (BMR) was found to be significantly greater in NR than R mice, and NR mice were found to engage in significantly more spontaneous (daily) locomotor activity. Energy-use estimates based on 24 h respirometry matched closely the level of intake reported for individual mice. The increased BMR and average daily metabolic rate in NR mice was correlated with testis size, but not with major central organs or digestibility. No significant difference in BMR or activity was found in mice from the same lines held in LD. Elevated intake in SD mice appears to be associated with differences in fertility and not other aspects of physiology in the respective lines.
Heritable variation in metabolic traits is likely to affect fitness. In this study, white-footed mice from wild-derived photoresponsive [R, infertile in short day length (SD)] and non-photoresponsive (NR, fertile in SD) selection lines were maintained under short-day (SD 8Light:16Dark), sub-thermoneutral conditions (22 or 12 °C). Mice had significantly higher levels of food intake and resting metabolic rates (RMR) at low temperature. RMR differed significantly between lines (greater in NR mice). In contrast to previous work under thermoneutral conditions, there was no significant difference in overall activity or average daily metabolic rates (ADMR) of mice from the two lines. Reduced activity may reflect behavioral changes under cooler conditions (e.g., nest building) reducing the overall energetic cost of fertility (for NR mice). There was no significant difference in maximal rate of oxygen consumption ([Formula: see text]) between lines. R mice had significantly greater brown adipose tissue and white abdominal fat mass due to both line and temperature. Reaction norms for intake, resting metabolism (RMR/BMR) and level of activity from current (12 and 22 °C) and previously published data (28 °C) demonstrate independent effects of genetics (line) and environment (temperature) for resting metabolism, but a clear interaction between these for activity. The results suggest that fertility under winter conditions imposes metabolic costs that are related to the level of reproductive development. Under the coldest conditions tested, however, mice that remained fertile in SD reduced activity, ADMR and food requirements, decreasing the differential between selection lines. Heritable variation in reaction norms suggests a genetic by environment effect that could be subject to selection.
A successful transition from a terrestrial to an aquatic environment requires the acquisition of unique adaptations that fit the distinct habitat. Pachyosteosclerosis is a combination of pachyostosis (thickening of cortical bone) and osteosclerosis (thickening of medullary bone). This accumulation of bone increases the density and weight of the animals that exhibit these characters. Pachyosteosclerosis is commonly found in aquatic and semiaquatic animals such as sirenians (sea cows), odobenids (walruses), otariids (sea lions), phocids (true seals) and cetaceans (whales). Increased bone density occurred early in the transition from a terrestrial to a marine environment, present before many of the other morphological characters such as flukes, flippers and fins and behavioral characters such as oscillatory swimming associated with aquatic life appear. Pachyosteosclerosis helps to counteract buoyancy, allowing taxa to remain submerged longer in order to feed while expending little energy. This is clearly demonstrated in sirenians, which are known to feed on sea grasses in shallow waters near the coastline. In most other aquatic mammals, such as modern cetaceans, pachyosteosclerosis is followed by an osteoporosis‐like bone lightening for increased swimming speeds that allows for the active hunting of fast‐moving prey. Pinnipeds live in a wide range of habitats (from cold to warm waters, from marine to fresh water) and demonstrate varying feeding mechanisms, ranging from filter feeding to bottom feeding to catching fast prey. Increased bone density is seen in multiple species of seals in different geological ages from the Miocene (~ 23mya) to present. The morphological and ecological significance for seals with thicker bones is that they swim slower and hunt in shallower water, while seals with lighter bones swim faster and hunt fast moving prey. Bone microstructural disparity allows different species of seals to successfully live in sympatric habitats due to their ecological and dietary specializations. Bone density is one vital character that can be used to predict the specific ecological niche and feeding preference for pinnipeds. Some early hominids, such as Homo erectus, have been shown to have increased density in the bones also. These heavier, thicker bones would make it easier for early Homo to hunt in coastal waters for littoral food sources and would compensate for the lack of stability from bipedalism. Support or Funding Information Funding Provided by Torres Advanced Enterprise Solutions LLC and Just‐Julian Fellowship
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