Physiological and Biochemical Zoology最新文献

AbstractHibernators save energy during winter by expressing torpor bouts characterized by strongly reduced body temperature and metabolic rate. Polyunsaturated fatty acids (PUFAs), specifically n-6 PUFAs, are known to positively affect hibernation performance and thereby energy savings predominantly in fat-storing hibernators. Accordingly, hibernators usually retain PUFAs and mobilize monounsaturated fatty acids (MUFAs) or saturated fatty acids (SFAs) during hibernation. In food-storing common hamsters (Cricetus cricetus), however, we previously found that PUFA proportions in white adipose tissue (WAT) decreased during winter, indicating that individuals did mobilize PUFAs. To further investigate these patterns, we analyzed PUFA changes in WAT during hibernation as well as hibernation performance in free-ranging and captive common hamsters with lower prehibernation PUFA proportions compared to those in the previous study. Under controlled conditions, total PUFAs, n-6 PUFAs, and SFAs increased while n-3 PUFAs and MUFAs decreased during hibernation. Higher prehibernation n-6 PUFA proportions resulted in fewer torpor bouts and less time spent in torpor. In free-ranging hamsters, n-6 PUFAs increased while n-3 PUFAs and SFAs decreased during winter. Prehibernation n-6 PUFA proportions, however, did not affect hibernation performance. In summary, these results indicate that the mobilization or retention of n-6 PUFAs during hibernation could depend on their availability in WAT or in the diet before the onset of the hibernation period.

AbstractMaternal hormones, such as thyroid hormones (THs) transferred to embryos and eggs, are key signaling pathways for mediating maternal effects. To be able to respond to maternal cues, embryos must express the key molecular "machinery" of hormone pathways, such as enzymes and receptors. While altricial birds begin TH production only at or after hatching, experimental evidence suggests that their phenotype can be influenced by maternal THs deposited into the egg. However, it is not understood how or when altricial birds express genes in the TH pathway. For the first time, we measured the expression of key TH-pathway genes in altricial embryos by using two common altricial ecological model species, pied flycatcher (Ficedula hypoleuca) and blue tit (Cyanistes caeruleus). Deiodinase DIO1 gene expression could not be reliably confirmed in either species, but deiodinase enzyme genes DIO2 and DIO3 were expressed in both species. Given that DIO2 converts thyroxine to biologically active triiodothyronine and that DIO3 mostly converts triiodothyronine to inactive forms of THs, our results suggest that embryos may modulate maternal signals. TH receptors (THRA and THRB) and a monocarboxylate membrane transporter gene (SLC16A2) were also expressed, enabling TH responses. Our results suggest that altricial embryos may be able to respond to and potentially modulate maternal signals conveyed by THs in early development.

AbstractThe stress response is partially mediated by increased levels of circulating glucocorticoids. While the stress response may be adaptive in the short term, chronically elevated levels of glucocorticoids can be pathological. We aimed to verify that chronic stress causes metabolic dysregulation via increased corticosterone (Cort) exposure by monitoring free fatty acid (FFA) concentrations (evidence of fat breakdown), uric acid concentrations (evidence of protein breakdown), and organ weights (furcular fat, abdominal fat, liver, and pectoralis muscle) in chronically stressed juvenile house sparrows (Passer domesticus). The sparrows were chronically stressed for 3 wk by applying a series of rotating mild psychological stressors. One group of birds received injections of a glucocorticoid steroidogenesis inhibitor (mitotane) and a second group received injections of a glucocorticoid receptor antagonist (RU486) halfway through the chronic stress period to test whether glucocorticoids are responsible for protein and fat catabolism during chronic stress. Toward the end of the chronic stress period, mitotane birds increased weight compared to control and RU486 birds. Contrary to expectations, we saw no differences in FFA or uric acid levels between control and mitotane birds, but RU486 temporarily decreased stress-induced uric acid levels. Neither mitotane nor RU486 significantly altered organ weights at the end of the 3 wk. In conclusion, Cort does appear to negatively affect body weight, but the mechanism does not appear to involve increased protein or lipid metabolism.

AbstractThe rise in temperature associated with climate change may threaten the persistence of stenothermal organisms with limited capacities for beneficial thermal acclimation. We investigated the capacity for within-generation and transgenerational thermal responses in brook trout (Salvelinus fontinalis), a cold-adapted salmonid. Adult fish were acclimated to temperatures within (10°C) and above (21°C) their thermal optimum for 6 mo before spawning, then mated in a full factorial breeding design to produce offspring from cold- and warm-acclimated parents and bidirectional crosses between parents from both temperature treatments. Offspring from families were subdivided and reared at two acclimation temperatures representing their current (15°C) and anticipated future (19°C) habitat temperatures. Offspring thermal physiology was measured as the rate of oxygen consumption (Mo₂) during an acute change in temperature (increase of 2°C h^-1) to observe their Mo₂-temperature relationship. We recorded resting Mo₂, peak (highest achieved, thermally induced) Mo₂, and critical thermal maximum (CTM) as performance metrics. Although limited, within-generation plasticity was greater than transgenerational plasticity, with offspring warm acclimation elevating CTM by 0.5°C but slightly lowering peak thermally induced Mo₂. Transgenerational plasticity was evident as a slightly elevated resting Mo₂ and a shift of the Mo₂-temperature relationship to higher rates overall in offspring from warm-acclimated parents. Furthermore, offspring whose parents were warm acclimated were in worse condition than those whose parents were cold acclimated. Both parents contributed to offspring thermal responses; however, the paternal effect was stronger. Despite the existence of within-generation and transgenerational plasticity in brook trout, it is unlikely that these will be sufficient for coping with long-term changes to environmental temperatures.

Living in social groups may exacerbate interindividual competition for territory, food, and mates, leading to stress and possible health consequences. Unfavorable social contexts have been shown to elevate glucocorticoid levels (often used as biomarkers of individual stress), but the downstream consequences of socially stressful environments are rarely explored. Our study experimentally tests the mechanistic links between social aggression, oxidative stress, and somatic maintenance in captive zebra finches (Taeniopygia guttata). Over 64 d, we measured the effects of aggression (received or emitted) on the individual oxidative status, body condition, and changes in relative telomere length (rTL) of birds living in high- and low-social-density conditions. Using path analyses, we found that birds living at high social density increased their aggressive behavior. Birds receiving the highest number of aggressions exhibited the strongest activation of antioxidant defenses and highest plasmatic levels of reactive oxygen metabolites. In turn, this prevented birds from maintaining or restoring telomere length between the beginning and the end of the experiment. Received aggression also had a direct negative effect on changes in rTL, unrelated to oxidative stress. In contrast, emitted aggression had no significant effect on individual oxidative stress or changes in rTL. Body condition did not appear to affect the physiological response to aggression or oxidative stress. At low density, we found trends that were similar to those at high density but nonsignificant. Our study sheds light on the causal chain linking the social environment and aggressive behavior to individual oxidative stress and telomere length. The long-term consequences of socially induced stress on fitness remain to be characterized.

Brown bears are obese when they enter the den, and after 6 mo of hibernation and physical inactivity, bears show none of the adverse consequences of a sedentary lifestyle in humans, such as cardiovascular disease, type 2 diabetes, and kidney failure. The metabolic mechanisms that drive hibernation physiology in bears are poorly defined, but systemic endocrine regulators are likely involved. To investigate the potential role of steroid hormones, we quantified the total levels of 12 steroid hormones, the precursor cholesterol, sex hormone-binding globulin (SHBG), and corticosterone-binding globulin (CBG) in paired serum samples from subadult free-ranging Scandinavian brown bears during the active and hibernation states. During hibernation, androstenedione and testosterone were significantly decreased in subadult female bears ($n=13$), whereas they increased in all males but one ($n=6$) and therefore did not reach a significant difference. Despite this difference, SHBG increased more than 20-fold during hibernation for all bears. Compared with SHBG concentrations in humans, bear levels were very low in the active state, but during hibernation, levels equaled high levels in humans. The increased SHBG levels likely maintain a state of relative quiescence of the reproductive hormones in hibernating bears. Interestingly, the combination of SHBG and testosterone levels results in similar free bioavailable testosterone levels of 70-80 pM in both subadult and adult sexually active male bears, suggesting a role for SHBG in controlling androgen action during hibernation in males. Dehydroepiandrosterone sulfate, dihydrotestosterone, and estradiol levels were below the detection limit in all but one animal. The metabolically active glucocorticoids were significantly higher in both sexes during hibernation, whereas the inactive metabolite cortisone was reduced and CBG was low approaching the detection limit. A potential caveat is that the glucocorticoid levels might be affected by the ketamine applied in the anesthetic mixture for hibernating bears. However, increased hibernating cortisol levels have consistently been reported in both black bears and brown bears. Thus, we suggest that high glucocorticoid activity may support the hibernation state, likely serving to promote lipolysis and gluconeogenesis while limiting tissue glucose uptake to maintain a continuous glucose supply to the brain.

AbstractMammalian hibernation in ground squirrels is characterized by periods of torpor wherein body temperature approaches ambient temperature and metabolism is reduced to as low as 1/100th of active rates. It is unclear how hibernation affects long-term spatial memory, as tremendous remodeling of neurons is associated with torpor use. Given the suspected links between remodeling and memory formation and retention, we examined long-term spatial memory retention throughout a hibernation season. Animals were trained on a Barnes maze before entering torpor. Animals were tested for memory retention once a month throughout a hibernation season. Results indicate marked variation between individuals. Some squirrels retained memory across multiple torpor bouts, while other squirrels did not. No relationship was found between the number of torpor bouts, duration of bouts, or time spent torpid on long-term memory retention. However, that some squirrels successfully retain memory suggests that the profound remodeling of dendritic spines during torpor does not always lead to memory loss.

AbstractThe body mass (M) scaling of resting metabolic rate (RMR) may vary significantly throughout ontogeny for multiple reasons that are not perfectly understood. To compare two major geometric theories of metabolic scaling, surface area (SA) theory and resource transport network (RTN) theory, we tested whether ontogenetic shifts in metabolic scaling relate to changes in body shape in the American eel (Anguilla rostrata). To do so, we compared the log-linear scaling exponents of RMR to M (b_R) and M to body length (b_L) in juvenile and subadult eels (glass and yellow eel life stages, respectively). Glass eels exhibited a $b_L>3$ and b_R significantly <2/3, as predicted by SA theory. Yellow eels also had a $b_L>3$, but their b_R was not significantly different from 2/3 or 3/4. We hypothesize that two developmental changes contribute to b_R being higher for yellow eels: (1) a greater reliance on branchial respiration than body-surface-dependent cutaneous respiration and (2) a lower rate of thickening during subadult growth. An ontogenetic decrease in the ratio of cutaneous to gill respiration may have increased the relative importance of the physical constraints of a single-pump, closed circulatory system on the body-size-dependent rate of resource supply to metabolizing tissues (as predicted by RTN theory) in subadult eels. Future research is needed to quantify these developmental changes and their potential mechanistic effects on metabolic scaling, especially in the elver, a critical life stage between the glass and yellow eel stages, that was not analyzed in this study.

Allometry predicts that the 12-17-g American water shrew (Sorex palustris)-the world's smallest mammalian diver-will have the highest diving metabolic rate coupled with the lowest total body oxygen storage capacity, skeletal muscle buffering capacity, and glycolytic potential of any endothermic diver. Consistent with expectations, and potentially owing to their low thermal inertia, water shrews had a significantly higher diving metabolic rate in 10°C water (8.77 mL O₂ g^-1 h^-1) compared with 30°C water (6.57 mL O₂ g^-1 h^-1). Unlike larger-bodied divers, muscle myoglobin contributed minimally (7.7%-12.4%) to total onboard O₂ stores of juvenile and adult water shrews, respectively, but was offset by high blood O₂ carrying capacities (26.4%-26.9% v/v). Diving was predominantly aerobic, as only 1.2%-2.3% of dives in 10°C and 30°C water, respectively, exceeded the calculated aerobic dive limits at these temperatures (10.8-14.4 s). The mean voluntary dive time of water shrews during 20-min trials in 3°C-30°C water was $5.0\pm 0.1$ s ($N=25$, $n=\mathrm{1,628}$), with a mean maximum dive time of $10.1\pm 0.4$ s. However, the average dive duration ($6.9\pm 0.2$ s, $n=257$) of radio-telemetered shrews exclusively foraging in a simulated riparian environment (3°C water) for 12-28 h suggests that mean (but not maximum) dive times of water shrews in the wild may be longer. Mean dive duration, duration of the longest dive, and total time in water all decreased significantly as water temperature declined, suggesting that shrews employed behavioral thermoregulation to defend against immersion hypothermia. Additionally, free-diving shrews in the 24-h trials consistently elevated core body temperature by ∼1°C immediately before initiating aquatic foraging bouts and ended these bouts when body temperature was still at or above normal resting levels (∼37.8°C). We suggest that this observed predive hyperthermia aids to heighten the impressive somatosensory physiology, and hence foraging efficiency, of this diminutive predator while submerged.

AbstractInvestigations of thermally adaptive behavioral phenotypes are critical for both understanding climate as a selective force and predicting global species distributions under climate change conditions. Cooperative nest founding is a common strategy in harsh environments for many species and can enhance growth and competitive advantage, but whether this social strategy has direct effects on thermal tolerance was previously unknown. We examined the effects of alternative social strategies on thermal tolerance in a facultatively polygynous (multiqueen) desert ant, Pogonomyrmex californicus, asking whether and how queen number affects worker thermal tolerances. We established and reared lab colonies with one to four queens, then quantified all colony member heat tolerances (maximum critical temperature [CT_max]). Workers from colonies with more queens had higher and less variant CT_max. Our findings resemble weak link patterns, in which colony group thermal performance is improved by reducing frequencies of the most temperature-vulnerable individuals. Using ambient temperatures from our collection site, we show that multiqueen colonies have thermal tolerance distributions that enable increased midday foraging in hot desert environments. Our results suggest advantages to polygyny under climate change scenarios and raise the question of whether improved thermal tolerance is a factor that has enabled the success of polygyne species in other climatically extreme environments.