Phylogenetic differences of mammalian basal metabolic rate are not explained by mitochondrial basal proton leak.

Metabolic rates of mammals presumably increased during the evolution of endothermy, but molecular and cellular mechanisms underlying basal metabolic rate (BMR) are still not understood. It has been established that mitochondrial basal proton leak contributes significantly to BMR. Comparative studies among a diversity of eutherian mammals showed that BMR correlates with body mass and proton leak. Here, we studied BMR and mitochondrial basal proton leak in liver of various marsupial species. Surprisingly, we found that the mitochondrial proton leak was greater in marsupials than in eutherians, although marsupials have lower BMRs. To verify our finding, we kept similar-sized individuals of a marsupial opossum (Monodelphis domestica) and a eutherian rodent (Mesocricetus auratus) species under identical conditions, and directly compared BMR and basal proton leak. We confirmed an approximately 40 per cent lower mass specific BMR in the opossum although its proton leak was significantly higher (approx. 60%). We demonstrate that the increase in BMR during eutherian evolution is not based on a general increase in the mitochondrial proton leak, although there is a similar allometric relationship of proton leak and BMR within mammalian groups. The difference in proton leak between endothermic groups may assist in elucidating distinct metabolic and habitat requirements that have evolved during mammalian divergence.

The influence of reproductive hormones on the torpor patterns of the marsupial Sminthopsis macroura: bet-hedging in an unpredictable environment.

Seasonal cycles of reproduction are common in many mammals and these are combined with the necessary energy budgeting for thermoregulatory challenges. Many mammals meet the challenge of changing environmental temperatures in winter by using torpor, a controlled reduction in body temperature and metabolic rate. We aimed to determine the effects of photoperiod and reproductive hormones on the seasonal cycles of reproduction and torpor use in a marsupial that commences reproduction in winter, the stripe-faced dunnart, Sminthopsis macroura. Males and females were placed under LD 14:10 and natural reproductive hormones blocked by either flutamide (males) or mifepristone (females) or tamoxifen (females). Reproductive parameters, metabolic rate and torpor variables were determined. The same animals were then placed under LD 10:14 and given testosterone (males) or progesterone (females) or oestrogen (females). Reproductive parameters, metabolic rate and torpor variables were measured. Body mass and tail widths (fattening indicator) in males were significantly affected by testosterone, and the effects were reversed by hormone blockers. Reproductive parameters were unaffected. Resting metabolic rate and ability to use torpor were not affected by treatment in males, however torpor characteristics, especially torpor bout duration, were affected by presence of testosterone in males. In females, body mass was unaffected by hormone presence, although tail widths were affected. Disruption of reproductive cycles occurred with hormone blockers in females, however, resting metabolic rate was not affected, and only presence of progesterone affected torpor characteristics in females. Our results differ from those found for rodents, where presence of testosterone abolishes the use of torpor in males, and oestrogen inhibits torpor use in females. Our study suggests that, in this mammal, metabolic responses to the presence or absence of reproductive hormones differs between males and females, and there is no absolute endocrinologically-driven reproductive season demarcated from the torpor season.

Telomerase activity in the bats Hipposideros armiger and Rousettus leschenaultia.

Telomerase activity was examined in two species of bat, Hipposideros armiger and Rousettus leschenaultia, which have similar body mass and lifespan but differ in use of hibernation. We found that telomerase activity was present in all tissues sampled, but it was greater in metabolically active tissues such as liver, spleen, and kidney. Of special interest is the raised activity found in the heterothermic bat H. armiger, and the hibernating bats having raised values for spleen, heart, and kidney. These findings show that maintenance of high levels of telomerase is an essential part of the regulation of cellular activities during hibernation.

Marsupial uncoupling protein 1 sheds light on the evolution of mammalian nonshivering thermogenesis.

Brown adipose tissue expressing uncoupling protein 1 (UCP1) is responsible for adaptive nonshivering thermogenesis giving eutherian mammals crucial advantage to survive the cold. The emergence of this thermogenic organ during mammalian evolution remained unknown as the identification of UCP1 in marsupials failed so far. Here, we unequivocally identify the marsupial UCP1 ortholog in a genomic library of Monodelphis domestica. In South American and Australian marsupials, UCP1 is exclusively expressed in distinct adipose tissue sites and appears to be recruited by cold exposure in the smallest species under investigation (Sminthopsis crassicaudata). Our data suggest that an archetypal brown adipose tissue was present at least 150 million yr ago allowing early mammals to produce endogenous heat in the cold, without dependence on shivering and locomotor activity.

Sex, season and melatonin administration affects daily activity rhythms in a marsupial, the brown antechinus, Antechinus stuartii.

The carnivorous marsupial Antechinus stuartii relies on photoperiodic changes to time reproductive activities, including behaviour, in spring. Similar to other mammals, the administration of the hormone melatonin is known to affect the synchronisation of reproduction in A. stuartii. The present study sought to explore the alterations in locomotor activity from the winter solstice in both males (body mass 35 g) and females (body mass 20 g) as a result of the influences of the changes in the natural photocycle and also of melatonin administration while under the natural photocycle. The total daily activity was found to differ between sexes, with males more active than females, irrespective of melatonin or control treatments. Daily activity patterns were significantly different between male groups but not female treatment groups. Activity patterns were also found to differ between males and females. The significance of these differences is discussed with relation to the profound physiological differences between the sexes, in this mammal where an irreversible stress response is part of the complete post-mating mortality of all males, but not females.

Cortical cyto- and chemoarchitecture in three small Australian marsupial carnivores: Sminthopsis macroura, Antechinus stuartii and Phascogale calura.

The cyto- and chemoarchitecture of the cerebral cortex has been examined in three small (mouse-sized) polyprotodont marsupial carnivores from Australia (the stripe-faced dunnart, Sminthopsis macroura; the brown antechinus, Antechinus stuartii; and the red-tailed phascogale, Phascogale calura) in order to compare the cortical topography of these marsupials with that of diprotodontids, didelphids and eutherians. All three species studied had similar cortical cytoarchitecture. The isocortical surface was dominated by primary somatosensory (S1) and visual (V1) areas. Putative secondary sensory areas (S2, V2M, V2L) were also identified. The primary somatosensory cortex demonstrated clumps of granule cells in the presumptive mystacial field, whereas the primary visual area showed a distinctive chemical signature of intense calbindin immunoreactivity in layer IV. On the other hand, the primary auditory area was small and indistinct, but flanked by a temporal association area (TeA). A cytoarchitecturally distinct primary motor cortex (M1) with prominent pyramidal neurons in layer V and poor layer IV was identified medially to S1, and at rostral levels a putative secondary motor area was identified medial to M1. Transitional areas between isocortex and allocortical regions showed many cyto- and chemoarchitectural similarities to those reported for eutherian (and in particular rodent) cortex. Medially, two cingulate regions were found at rostral levels, with dysgranular and granular 'retrosplenial' areas identified caudally. Laterally, granular and agranular areas surrounded the rostral rhinal fissure, to be replaced by ectorhinal and perirhinal areas caudally. The findings indicate that the cyto- and chemoarchitectural features which characterize the iso- and allocortex in these small marsupial carnivores are similar to those reported in didelphids and eutherians and our findings suggest the existence of putative dedicated motor areas medial to the S1 field.

The role of photoperiod in the timing of reproduction in the Dasyurid Marsupial Antechinus stuartii.

Antechinus stuartii has one highly synchronized mating period which occurs at the same time each year. An analysis of the time of reproduction in 162 populations of A. stuartii shows that the onset of the mating period is correlated with the rate of change of photoperiod, rather than with critical photoperiodic length. The rate of change of photoperiod is different for two designated forms of this species and can be used as a predictor of these animals' reproductive timing. A rate of change model further explains the rigid and highly sunchronized nature of the mating period and, by providing a mechanism for reproductive isolation, offers an explanation for the evolution of the two forms of this species.

Torpor during reproduction in mammals and birds: dealing with an energetic conundrum.

Torpor and reproduction in mammals and birds are widely viewed as mutually exclusive processes because of opposing energetic and hormonal demands. However, the reported number of heterothermic species that express torpor during reproduction is ever increasing, to some extent because of recent work on free-ranging animals. We summarize current knowledge about those heterothermic mammals that do not express torpor during reproduction and, in contrast, examine those heterothermic birds and mammals that do use torpor during reproduction. Incompatibility between torpor and reproduction occurs mainly in high-latitude sciurid and cricetid rodents, which live in strongly seasonal, but predictably productive habitats in summer. In contrast, torpor during incubation, brooding, pregnancy, or lactation occurs in nightjars, hummingbirds, echidnas, several marsupials, tenrecs, hedgehogs, bats, carnivores, mouse lemurs, and dormice. Animals that enter torpor during reproduction often are found in unpredictable habitats, in which seasonal availability of food can be cut short by changes in weather, or are species that reproduce fully or partially during winter. Moreover, animals that use torpor during the reproductive period have relatively low reproductive costs, are largely insectivorous, carnivorous, or nectarivorous, and thus rely on food that can be unpredictable or strongly seasonal. These species with relatively unpredictable food supplies must gain an advantage by using torpor during reproduction because the main cost is an extension of the reproductive period; the benefit is increased survival of parent and offspring, and thus fitness.

Boom and bust: a review of the physiology of the marsupial genus Antechinus.

The marsupial genus Antechinus is a group of small carnivorous marsupials from the order Dasyuromorphia (Family Dasyuridae) and is found in eastern Australia. The life history of all species in the genus is characterized by a complex, but highly synchronized life cycle in both sexes, culminating in a short mating period followed by total male mortality (semelparity). The breeding season is defined by a specific rate of increase in photoperiod, which is different for each species. In Antechinus spp., male mortality is due to the effects of high free testosterone and cortisol levels on many organ systems. Unusually, spermatogenesis is complete before testosterone levels begin to rise at the winter solstice. In males, low sperm counts have been compensated for by high proportions of sperm reaching the isthmus of the female reproductive tract and long-term storage in the crypts. The females survive to rear their young and may mate again in their second year. Gestation lasts from 26 to 34 days, depending on the species. However, developmental arrest can occur at several stages during embryogenesis, elongating the apparent gestation duration by several days. Several species have strong female sex biases in their litters. The high degree of life history synchrony and the cascade of endocrine-driven physiological events that result in male death are unusual physiological characteristics for mammals. Suggestions why semelparity may have evolved in Antechinus are discussed.

Photoperiod affects daily torpor and tissue fatty acid composition in deer mice.

Photoperiod and dietary lipids both influence thermal physiology and the pattern of torpor of heterothermic mammals. The aim of the present study was to test the hypothesis that photoperiod-induced physiological changes are linked to differences in tissue fatty acid composition of deer mice, Peromyscus maniculatus ( approximately 18-g body mass). Deer mice were acclimated for >8 weeks to one of three photoperiods (LD, light/dark): LD 8:16 (short photoperiod), LD 12:12 (equinox photoperiod), and LD 16:8 (long photoperiod). Deer mice under short and equinox photoperiods showed a greater occurrence of torpor than those under long photoperiods (71, 70, and 14%, respectively). The duration of torpor bouts was longest in deer mice under short photoperiod (9.3 +/- 2.6 h), intermediate under equinox photoperiod (5.1 +/- 0.3 h), and shortest under long photoperiod (3.7 +/- 0.6 h). Physiological differences in torpor use were associated with significant alterations of fatty acid composition in approximately 50% of the major fatty acids from leg muscle total lipids, whereas white adipose tissue fatty acid composition showed fewer changes. Our results provide the first evidence that physiological changes due to photoperiod exposure do result in changes in lipid composition in the muscle tissue of deer mice and suggest that these may play a role in survival of low body temperature and metabolic rate during torpor, thus, enhancing favourable energy balance over the course of the winter.

Effect of torpor on the water economy of an arid-zone marsupial, the stripe-faced dunnart (Sminthopsis macroura).

Metabolic rate and evaporative water loss (EWL) were measured for a small, arid-zone marsupial, the stripe-faced dunnart (Sminthopsis macroura), when normothermic and torpid. Metabolic rate increased linearly with decreasing ambient temperature (T(a)) for normothermic dunnarts, and calculated metabolic water production (MWP) ranged from 0.85+/-0.05 (T(a)=30 degrees C) to 3.13+/-0.22 mg H2O g(-1) h(-1) (T(a)=11 degrees C). Torpor at T(a)=11 and 16 degrees C reduced MWP to 24-36% of normothermic values. EWL increased with decreasing T(a), and ranged from 1.81+/-0.37 (T(a)=30 degrees C) to 5.26+/-0.86 mg H2O g(-1) h(-1) (T(a)=11 degrees C). Torpor significantly reduced absolute EWL to 23.5-42.3% of normothermic values, resulting in absolute water savings of 50-55 mg H2O h(-1). The relative water economy (EWL/MWP) of the dunnarts was unfavourable, remaining >1 at all T(a) investigated, and did not improve with torpor. Thus torpor in stripe-faced dunnarts results in absolute, but not relative, water savings.

Effect of testosterone and cortisol administration on the reproductive tract of male Antechinus stuartii (Marsupialia).

The life history of Antechinus stuartii, a marsupial, is highly synchronized and culminates in a brief mating period that is followed by complete male mortality. The accessory reproductive tracts of male A. stuartii enlarge in association with testosterone and cortisol hormone concentrations, but this appears to be unrelated to the spermatogenic cycle. The present study examined the effects of testosterone and cortisol on the male reproductive tract. Four groups of adult males from May (when plasma testosterone and cortisol concentrations are low) were given depot injections of testosterone esters or synthetic cortisol in doses that mimic concentrations found in males in the breeding period (August). Males were given either saline, testosterone only, cortisol only, or testosterone plus cortisol. Experimental groups did not differ in the seminiferous tubule morphology. However, the cells from the caudal end of the epididymides of both testosterone groups were considerably hypertrophied compared with males treated with saline or cortisol only. Testosterone treatment significantly increased prostate and bulbourethral gland mass, although addition of cortisol to the testosterone administration diminished this effect. The morphology of the accessory reproductive tract of males treated with either saline or cortisol only was similar to that of untreated males at the same time of year, and the morphology of the accessory reproductive tract of males treated with testosterone plus cortisol was similar to that of untreated males in the breeding season. Like some other marsupials, the spermatogenic cycle in A. stuartii is apparently not correlated with androgen activity, while the accessory reproductive tract is affected by androgens.

The degree of dietary fatty acid unsaturation affects torpor patterns and lipid composition of a hibernator.

Diets rich in unsaturated and polyunsaturated fatty acids have a positive effect on mammalian torpor, whereas diets rich in saturated fatty acids have a negative effect. To determine whether the number of double bonds in dietary fatty acids are responsible for these alterations in torpor patterns, we investigated the effect of adding to the normal diet 5% pure fatty acids of identical chain length (C18) but a different number of double bonds (0, 1, or 2) on the pattern of hibernation of the yellow-pine chipmunk, Eutamias amoenus. The response of torpor bouts to a lowering of air temperature and the mean duration of torpor bouts at an air temperature of 0.5 degree C (stearic acid C18:0, 4.5 +/- 0.8 days, oleic acid C18:1, 8.6 +/- 0.5 days; linoleic acid C18:2, 8.5 +/- 0.7 days) differed among animals that were maintained on the three experimental diets. The mean minimum body temperatures (C18:0, +2.3 +/- 0.3 degrees C; C18:1, +0.3 +/- 0.2 degree C; C18:2, -0.2 +/- 0.2 degrees C), which torpid individuals defended by an increase in metabolic rate, and the metabolic rate of torpid animals also differed among diet groups. Moreover, diet-induced differences were observed in the composition of total lipid fatty acids from depot fat and the phospholipid fatty acids of cardiac mitochondria. For depot fat 7 of 13 and for heart mitochondria 7 of 14 of the identified fatty acids differed significantly among the three diet groups. Significant differences among diet groups were also observed for the sum of saturated, unsaturated and polyunsaturated fatty acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of testosterone and cortisol on the renal morphology of male Antechinus stuartii (Marsupialia).

The life cycle of the marsupial Antechinus stuartii includes postmating mortality of all males. A previous study found that renal morphology changes during the yearly cycle when endogenous concentrations of testosterone and cortisol are elevated in males. The present study determined whether administration of testosterone only, cortisol only, or testosterone plus cortisol affects renal structure in males at a time of year when endogenous hormones are low (May). Saline administration was used as the control. Gross morphometrics of the kidney did not show significant differences between groups. However, some pathological changes were observed in distal tubules and collecting ducts from cortisol-treated groups, and in the glomeruli of testosterone-treated males. Hypertrophy of the proximal tubules, distal straight tubules, and the cells of the cortical collecting duct occurred with the administration of testosterone. Distension of the distal convoluted tubules, cortical collecting ducts, and outer medullary collecting ducts occurred with the administration of cortisol, and there was some interaction with testosterone. Glomerular volumes increased with cortisol administration, although cortisol interacted with testosterone in the superficial glomeruli. Many of these changes mimicked those seen in the seasonal study, with the kidneys of the testosterone plus cortisol group closely resembling those of males in August, just prior to male mortality. The present study demonstrates that testosterone administration causes hypertrophy of renal tissue, whereas cortisol administration can cause tubular disruption in male A. stuartii.

Seasonal changes in the reproductive anatomy of male Antechinus stuartii (Marsupialia: Dasyuridae).

Male Antechinus stuartii (Marsupialia: Dasyuridae) undergo significant endocrinological changes during their life history. The endocrine changes are associated with complete male mortality. The accessory reproductive tract of males is important for seminal plasma production, and changes associated with the reproductive cycle were not described in detail by earlier studies. The present study sought to describe some of the seasonal changes in structure of the male accessory reproductive tract in relation to the known hormonal changes. The epididymis, prostate, and bulbourethral glands are relatively undifferentiated in February and May, a time when plasma concentrations of testosterone are known to be low. By July, considerable hypertrophy and differentiation of the accessory reproductive tract are observed. This is most obvious in the bulbourethral glands, which change from being indistinguishable from one another in February and May to being three large morphologically and histologically distinct glands in July. The hypertrophy and secretory activity continue into August, the breeding season. These findings correlate with the hormonal profiles found in other studies of A. stuartii. J Morphol 231:261-275, 1997. © 1997 Wiley-Liss, Inc.

The seasonal reproductive cycle of a marsupial, Antechinus stuartii: effects of oral administration of melatonin.

Antechinus stuartii is a small marsupial with a brief, highly synchronised mating period believed to be controlled by the rate of change of photoperiod. Two experiments were performed to explore aspects of photoperiodic control of the seasonal cycle. In the first experiment the pineal hormone, melatonin, administered in the drinking water from the winter solstice, changed the normal response of A. stuartii to increasing rate of change of photoperiod. Melatonin administration shifted the induction of estrus in the females from the first week of August (controls) to an earlier time of mid-July and the consequent pouch changes associated with pregnancy and pseudo-pregnancy were also shifted by the same length of time. Post-mating decline and consequent death of males were also accelerated. In the second experiment melatonin was administered from the autumnal equinox, and this experimental protocol resulted in a desynchronisation of reproductive events. Melatonin administration desynchronised the female reproductive cycle, such that the mating period was extended to eight weeks, instead of the two weeks displayed by control females. Pouch changes and birth of young reflected this desynchronisation. Melatonin administration in males resulted in desynchronisation of reproductive parameters. While the normal yearly reproductive cycle was approximated in these males, the high syncronisation of reproductive maturation and male mortality events observed in control males, was not evident in melatonin-treated males. These results indicate that the pineal gland by way of the hormone melatonin is important in the synchronisation of the unusual life history of this marsupial mammal.

The effects of cortisol and testosterone on renal function in male Antechinus stuartii (Marsupialia).

Seasonal changes in the physiology of Antechinus stuartii result in complete male mortality after mating. The most important endocrine changes in males are large rises in plasma testosterone and cortisol concentrations. Glomerular filtration rate (GFR) in males declines coincident with high plasma testosterone and cortisol. In the present study GFRs were measured in males captured in May (when endogenous plasma testosterone and cortisol levels are low) and given depot injections of either saline, testosterone-only, cortisol-only or testosterone plus cortisol at doses designed to mimic plasma levels during the mating period. GFR decreased significantly with testosterone injection, independent of cortisol treatment. Urinary concentrations of sodium and chloride, and osmolality decreased significantly with cortisol treatment, although the addition of testosterone reversed the effect. Total urinary excretion of electrolytes was similar between groups. Plasma potassium levels significantly increased in testosterone plus cortisol treated males. Plasma sodium levels significantly increased and plasma chloride significantly decreased in all groups treated with cortisol. Water consumption significantly increased in all cortisol-treated males and food consumption significantly increased in all testosterone-treated males. The seasonal renal functional changes observed in A. stuartii were mimicked by testosterone administration.

Seasonal changes in glomerular filtration rate in Antechinus stuartii (Marsupialia: Dasyuridae).

The small marsupial Antechinus stuartii experiences a synchronised life cycle that culminates in complete male mortality (within 3 weeks) following the 1 week mating period in mid-August (late winter). There are pronounced physiological changes in male A. stuartii over the life cycle and renal function was assessed for correlation with these changes. Glomerular filtration rate and urine and plasma electrolytes were determined in male and female A. stuartii in February, May, July and August. Females showed little change in glomerular filtration rate, except for pre-mating values in August which decreased. In contrast, glomerular filtration rate of males decreased significantly in July and August. Plasma sodium and chloride levels were higher in males than females and were higher in animals in July and August than in February and May. Plasma potassium levels dropped in both males and females in July and August. Plasma osmolality was higher in animals in February compared to animals from May and August. However, there were no significant sex or seasonal differences in urine electrolytes, although urea concentration was higher in females than males. Urine osmolality was higher in both sexes in July and August. There were no significant differences in total excretory rates of sodium, potassium or chloride between sexes or between seasons. Many of the alterations in renal function are correlated with known physiological and hormonal profiles in A. stuartii. This is the first observation of seasonal changes in glomerular filtration rate that are unrelated to dietary and water stresses.

Effects of temperature acclimation on maximum heat production, thermal tolerance, and torpor in a marsupial.

Marsupials, unlike placental mammals, are believed to be unable to increase heat production and thermal performance after cold-acclimation. It has been suggested that this may be because marsupials lack functional brown fat, a thermogenic tissue, which proliferates during cold-acclimation in many placentals. However, arid zone marsupials have to cope with unpredictable, short-term and occasionally extreme changes in environmental conditions, and thus they would benefit from an appropriate physiological response. We therefore investigated whether a sequential two to four week acclimation in Sminthopsis macroura (body mass approx. 25 g) to both cold (16 degrees C) and warm (26 degrees C) ambient temperatures affects the thermal physiology of the species. Cold-acclimated S. macroura were able to significantly increase maximum heat production (by 27%) and could maintain a constant body temperature at significantly lower effective ambient temperatures (about 9 degrees C lower) than when warm-acclimated. Moreover, metabolic rates during torpor were increased following cold-acclimation in comparison to warm-acclimation. Our study shows that, despite the lack of functional brown fat, short-term acclimation can have significant effects on thermoenergetics of marsupials. It is likely that the rapid response in S. macroura reflects an adaptation to the unpredictability of the climate in their habitat.