Energy expenditure and protein turnover in three species of wallabies (Marsupialia: Macropodidae)

Summary Relationships between basal and fed metabolic rates and whole-body protein turnover rates were examined in three species of wallabies, the red-necked pademelon (Thylogale thetis), parma wallaby (Macropus parma) and tammar wallaby (M. eugenii).There were no significant differences among wallaby species in basal metabolic rate (BMR) which was 30% below eutherian mammals. However, the fed metabolic rate of the tammar was lower than that of the other two species (P<0.05), as was the protein turnover rate (P<0.01) which is consistent with its lower voluntary feed intake and with its lower maintenance nitrogen requirement.Protein turnover rates in the wallabies were 23–47% lower than in eutherian mammals. Similarly, protein synthesis made a lower contribution to fed metabolic rates in the wallabies (7–8%) than in eutherians (17–25%).Thus, compared with several eutherian species, macropodid marsupials have low rates of both energy and protein metabolism, but within the macropodids there is not necessarily a close link between basal metabolic rate and whole-body protein turnover.Abbreviations BMR basal metabolic rate - DEE daily energy expenditure - EE energy expenditure - LSD least significant difference - RQ respiratory quotient     

Nitrogen metabolism and urea kinetics in the rock hyrax (Procavia habessinica)

Summary Nitrogen metabolism and urea kinetics were studied in rock hyraxes (Procavia habessinica) fed diets of different protein content.The maintenance nitrogen (N) requirement of the rock hyrax (311 mg·kg^–0.75·24 h^–1 of dietary N, or 209 mg·kg^–0.75·24 h^–1 of truly digestible N) is similar to that of several marsupial species, and thus lower than that of other eutherians.Urea recycled to the gut, measured with single injections of¹⁴C-urea, was 63% and 60% of urea entry rate on diets with 14.6% and 8.2% crude protein, respectively. Urea recycling increased to 70% when water intake was restricted, but decreased to 40% on a low (5.3%) protein diet, presumably because of a low energy intake.Urea utilization in the gut, measured with single injections of¹⁵N-urea, was 59% and 53% of urea degradation on the 14.6% and 8.2% protein diets, respectively. Urea utilization increased to 71% on the low protein diet, and increased to 98% with water restriction.The low maintenance nitrogen requirement appears to be the main physiological attribute of the rock hyrax enabling it to survive periods of low dietary protein availability. However, this low requirement can be related to the low basal metabolic rate of the Hyracoidea in general, and thus is not necessarily a primary adaptation to the environment.     

Effects of water deprivation on urea metabolism in camels, desert sheep and desert goats fed dry desert grass

1. The effects of water restriction and complete water deprivation on nitrogen retention and urea recycling were examined in camels, desert sheep and desert goats fed only dry desert grass of 3.2% crude protein content. 2. All three species were in negative nitrogen balance when water was available ad libitum. 3. In camels urea recycling was consistently high (94-97%) and nitrogen balance did not change with treatment. 4. In sheep and goats urea recycling increased from 75% to 94% (sheep) and from 79% to 95% (goats) during water deprivation, and nitrogen balance improved to positive values. 5. In all species water deprivation depressed dry matter intake and increased apparent digestibility of dry matter and nitrogen. 6. The effects of water restriction were generally intermediate. 7. These results suggest that nitrogen metabolism in adult domestic livestock raised by nomadic pastoralists in Sudan is not harmed, and may in fact be improved by short periods of water deprivation.     

The complete primary structure of late lactation protein from quokka (Setonix brachyurus)

The complete primary structure of the late lactation protein from the milk of quokka (Setonix brachyurus) is presented. The amino acid sequence was established by N-terminal sequence analysis of high-performance liquid chromatography purified intact protein and peptides isolated from chemical and enzymatic digests of the protein. The protein contains 158 residues including four cysteines. The sequence comparison with the tamar wallaby (Macropus eugenii) late lactation protein shows only five differences. The protein is identified as a new member of a novel late lactation protein family present in the milk of marsupials.     

Non‐invasive placentation in the marsupials Macropus eugenii (Macropodidae) and Trichosurus vulpecula (Phalangeridae) involves redistribution of uterine Desmoglein‐2

In mammalian pregnancy, the uterus is remodeled to become receptive to embryonic implantation. Since non‐invasive placentation in marsupials is likely derived from invasive placentation, and is underpinned by intra‐uterine conflict between mother and embryo, species with non‐invasive placentation may employ a variety of molecular mechanisms to maintain an intact uterine epithelium and to prevent embryonic invasion. Identifying such modifications to the uterine epithelium of marsupial species with non‐invasive placentation is key to understanding how conflict is mediated during pregnancy in different mammalian groups. Desmoglein‐2, involved in maintaining lateral cell–cell adhesion of the uterine epithelium, is redistributed before implantation to facilitate embryo invasion in mammals with invasive placentation. We identified localization patterns of this cell adhesion molecule throughout pregnancy in two marsupial species with non‐invasive placentation, the tammar wallaby (Macropus eugenii; Macropodidae), and the brushtail possum (Trichosurus vulpecula; Phalangeridae). Interestingly, Desmoglein‐2 redistribution also occurs in both M. eugenii and T. vulpecula, suggesting that cell adhesion, and thus integrity of the uterine epithelium, is reduced during implantation regardless of placental type, and may be an important component of uterine remodeling. Desmoglein‐2 also localizes to the mesenchymal stromal cells of M. eugenii and to epithelial cell nuclei in T. vulpecula, suggesting its involvement in cellular processes that are independent of adhesion and may compensate for reduced lateral adhesion in the uterine epithelium. We conclude that non‐invasive placentation in marsupials involves diverse and complementary strategies to maintain an intact epithelial barrier.     

A comparative proteomic analysis of skin secretions of the tammar wallaby (Macropus eugenii) and the wombat (Vombatus ursinus)

The secretome of the pouch skin of the model marsupial the tammar wallaby, Macropus eugenii has been investigated using techniques of two-dimensional gel electrophoresis, in-gel trypsin digestion followed by nanoliquid chromatography coupled tandem mass spectrometry (LC-MS/MS). Differences in the patterns of secreted proteins were observed in the female pouch at three stages of maturity — reproductively immature; reproductively mature and active and mature, postreproductively active. Skin from the underarm area of mature females had a markedly different secreted protein profile. The greatest diversity of proteins was seen in the mature reproductive pouch and from an opportunistic sample collected from the pouch another mature female marsupial, the common wombat, Vombatus ursinus. A total of 20 proteins were confidently identified from the pouch skin secretions of the tammar wallaby and wombats, whilst 20 proteins were tentatively identified. In all skin secretomes, globins were the most abundant proteins whilst the antimicrobial, dermcidin was detected in the wombat sample. Some proteins such as keratin and actin could be sourced to sloughed and degraded skin cells. A number of proteins were present at such low concentrations that confident identification was not possible. This was compounded by the lack of a comprehensive database of marsupial proteins which constrains the reliability of automated identification protocols.     

Effects of water restriction on digestive function in two macropodid marsupials from divergent habitats and the feral goat

The effects of water restriction on digestive function in the euro (Macropus robustus erubescens) found in the arid zone of inland Australia, the eastern wallaroo (M.r.robustus) from more mesic regions of eastern Australia, and the feral goat (Capra hircus) found throughout the range of M. robustus, were compared in order to examine some physiological adaptations required by herbivores for the exploitation of arid environments. Eight animals of each species were held in individual metabolism cages in temperature-moderated rooms and given a chopped hay diet ad libitum. Half the animals were restricted to 40 ml water·kg^-0.80·day^-1. This was 40%, 32% and 57% of voluntary drinking water intake in the euro, wallaroo and goat, respectively. All species responded to water restriction by reducing faecal, urinary and evaporative water losses in association with reductions in feed intake. All animals increased urine osmolality and electrolyte concentrations but not to maximal levels, while packed-cell volume and plasma osmolality and electrolyte concentrations were unaffected by water restriction. The euro displayed a suite of characteristics that separated it from the wallaroo in terms of physiological adaptation, including lower voluntary water intake, an increase in fibre digestibility and maintenance of nitrogen balance during water restriction, and lower faecal water efflux associated with a consistently lower faecal water content (54% versus 59% water in the wallaroo during water restriction, P<0.05). The euro's colon was 37% longer (P<0.01) than that of the wallaroo. The goat had the lowest faecal water efflux (P<0.05) and the longest colon (P<0.001). Water restriction did not affect water content in digesta, nor short-chain fatty acid concentrations or production rates in vitro. Total body water, as a proportion of body mass, was depressed (P<0.05) in the macropodids, but not in the goat. The reduction in dietary nitrogen intake, which accompanied water restriction, was partially compensated by an increase in urea degradation in the gut from 68% to 76% of urea synthesis water-restricted macropodids. These responses to water restriction are discussed in relation to those reported in other macropodid and ruminant species.Abbreviations BM body mass - SCFA short-chain fatty acids - DM dry matter - PCV packed-cell volume     

The complete primary structure of late lactation protein from quokka (Setonix brachyurus)

The complete primary structure of the late lactation protein from the milk of quokka (Setonix brachyurus) is presented. The amino acid sequence was established by N-terminal sequence analysis of high-performance liquid chromatography purified intact protein and peptides isolated from chemical and enzymatic digests of the protein. The protein contains 158 residues including four cysteines. The sequence comparison with the tamar wallaby (Macropus eugenii) late lactation protein shows only five differences. The protein is identified as a new member of a novel late lactation protein family present in the milk of marsupials.     

Mate Choice in Female Red-necked Pademelons,Thylogale thetis (Marsupialia: Macropodoidea)

Changes in the behaviour of captive female red-necked pademelons, Thylogale thetis, before, during and after oestrus were observed relative to the location of potential male mates. A penned female was presented with four adjacent, individually caged males representing the size classes large, medium, small and subadult. The female's behaviour was videotaped for 8–9 d spanning oestrous (3 d), post-oestrous (3 d) and normal (3 d) sessions of time. Changes in the activity, orientation and location of six penned females, relative to the caged males, were transcribed from videotape. A model of female pademelon behaviour pattern was developed. Results suggest that the females spent a significantly greater amount of time directly in front of the largest available male, relative to the other three smaller males during the oestrous period only, and that a significant proportion of this time was spent involved in active behaviours (grooming and locomotion, compared with inactive behaviours of sitting and scan/sniffing). Although there was a significant preference for females to orientate their bodies away from the location of the males, they faced the males significantly more during oestrus than any other session. Furthermore, the females faced the large male significantly more frequently than any other male. This study provides evidence for mate choice by female red-necked pademelons.     

Energy and water balance in the lesser double-collared sunbird (Nectarinia chalybea) feeding on different nectar concentrations

The water balance of nectarivores is tightly linked to their energy balance. When nectar is dilute, consumption of a large water excess is inevitable. We investigated energy and water balance in lesser double-collared sunbirds, Nectarinia chalybea (8 g), kept at 20 °C and fed different nectar concentrations (0.4, 0.8 M sucrose or 1.2 M sucrose). The mass of sucrose consumed, body mass, day-time mass gain and night-time mass loss were the same irrespective of diet, the birds compensating energetically for changes in sucrose concentration by drinking greater volumes of the more dilute solutions. Sunbirds consumed between 0.5 times and 1.8 times their body mass in preformed water per day, depending on sucrose concentration, and excreted around 75% of the water. The difference between water gain (preformed and metabolic water) and excreted water is assumed to equal evaporative water loss, and was similar on 1.2 M and 0.8 M sucrose, but was higher on a diet of 0.4 M sucrose. The osmolalities and K⁺ and Na⁺ concentrations of the excreted fluid were extremely low, so that sunbird urine resembled that of hummingbirds and freshwater vertebrates rather than that of typical terrestrial vertebrates. N. chalybea is able to maintain energy and water balance over a range of nectar concentrations by adjusting the volume of solution consumed and by excreting copious, dilute fluid. Accepted: 2 January 1999     

Insular tammar wallabies (Macropus eugenii) respond to visual but not acoustic cues from predators

We studied the way in which a population of tammar wallabies(Macropus eugenii), which have been isolated from mammalianpredators since the last ice age, responded to the sight andsound of historical and ontogenetically and evolutionarilynovel predators. Tammars were shown a range of visual stimuli,including taxidermic mounts of two evolutionarily novel predators,a red fox (Vulpes vulpes) and a cat (Felis catus), and a modelof an extinct predator, the thylacine (Thylacinus cynocephalus).Controls were a conspecific, the cart on which all mounts werepresented, and blank trials in which spontaneous change in behaviorwas measured. We played back recorded sounds to characterizeresponses to acoustic cues from predators and to a putativeconspecific antipredator signal. Treatments included the howlsof dingoes (Canis lupus dingo), an evolutionarily novel predator;calls of a wedge-tailed eagle (Aquila audax), a historicaland current predator; and wallaby foot thumps. Controls werethe song of an Australian magpie (Gymnorhina tibicen) and ablank trial. After seeing a fox, wallabies thumped their hindfeet in alarm, suppressed foraging, and increased looking.The sight of a cat similarly suppressed foraging and increasedlooking. The sounds of predators did not influence responsiveness,but wallabies foraged less and looked more after thump playbacks.Our results suggest that tammars respond to the sight, butnot the sounds, of predators. In contrast, the response to footthumps demonstrates that this particular sound functions asan antipredator signal. We suggest that responsiveness to visualcues has been preserved under relaxed selection because predatormorphology is convergent, but vocalizations are not.     

Resolution of portions of the kangaroo phylogeny (Marsupialia: Macropodidae) using DNA hybridization

We generated a DNA hybridization matrix comparing eleven 'true' kangaroos (Macropodinae) and two outgroup marsupials, the rufous rat-kangaroo Aepyprymnus rufescens (Potoroinae) and the brush-tailed phalanger Trichosurus vulpecula (Phalangeridae). A small matrix included additional species of the genus Macropus (large kangaroos and wallabies). The results indicate that the New Guinean forest wallaby Dorcopsulus vanheurni, and the quokka Setonix brachyurus, represent successively closer sister-groups of other macropodines. The remaining taxa examined form two clades: the tree kangaroo Dendrolagus matschiei with die pademelons Thylogale and rock wallabies Petrogale, and Macropus including the swamp wallaby Wallabia bicolor. The smaller matrix of five Macropus species and Wallabia (with Dorcopsulus as an outgroup) pairs the red-necked wallaby M. rufogriseus and Parry's wallaby M. parryi, with the eastern grey kangaroo M. giganteus as their nearest relative; and associates the red kangaroo M. rufus and wallaroo M. robustus, with Wallabia as their sister-taxon. In the larger study, we found mat inclusion of both outgroups provided little resolution among the macropodines, judging by jackknife and bootstrap tests. When Aepyprymnus was deleted, the Dendrolagus-Thylogale-Petrogale association obtained; with Trichosurus eliminated instead, the Wallabia-Macropus group was recovered. Only analysis of the eleven ingroup taxa by themselves gave a topology which supported both major clades. Our findings suggest that, at least for DNA hybridization studies, when ingroup taxa are separated by very short internodes experimental error in outgroup-to-ingroup distances may seriously compromise determination of ingroup affinities as well as the position of the root. We recommend that in such cases separate analyses with the outgroups sequentially eliminated and rigorous validation of die topology at each step should be conducted.     

Effect of water restriction on energy and water balance and osmoregulation of the fruit bat Rousettus aegyptiacus

The energy budget, water balance and osmoregulation of the fruit bat, Rousettus aegyptiacus, were studied during normal hydration and during water restriction (oven-dried apple diet). The water input and output were balanced during both normal hydration and water restriction. The kidney of the fruit bat is well adapted to handle the water load from its fruit diet by excreting large volumes (14% of the body mass per day) of dilute urine (113±25 mosmol·kg H₂P^-1) as well as reducing urine volume (-95%) and increasing urine osmotic concentration (555±280 mosmol·kg H₂O^-1) during water restriction. The haematocrit, plasma haemoglobin and total protein concentrations did not increase during water restriction and heat exposure, suggesting the conservation of plasma volume. Gross energy intake was not alfected by water restriction. However, digested energy intake and digestibility were significantly reduced. The effective regulation of energy and water budgets during water restriction suggests that the fruit bat can cope with seasonal climatic changes and with variable fruit supply during various seasons.Abbreviations BM body mass - DEI digested energy intake - EWL evaporative water loss - GEL gross energy intake - NH normal hydration - T _a ambient temperature - WR water restriction     

Desiccation independence of terrestrialNostoc commune ecotypes (cyanobacteria)

The subspeciesNostoc commune var.flagelliforme andN. commune var.commune are found in China (Ningxia Province, Inner Mongolia) as two morphologically different ecotypes of the desiccation-independent cyanobacteriumN. commune. The first ecotype, but not the second, colonizes arid areas. Various biochemical parameters and water dependence of photosynthesis and nitrogen fixation were compared for both ecotypes. Different patterns of water stress proteins were found in the two ecotypes. Repeated desiccation resulted in an enhanced desiccation independence for photosynthesis and, in the case of the ecotypecommune, for nitrogen fixation. The different response of nitrogenase of both ecotypes towards repeated cycles of rewetting and desiccation under conditions simulating the natural environment is discussed in terms of the energy balance of the colonies that are adapted to different environmental conditions.     

Vigilance and its complex synchrony in the red-necked pademelon, Thylogale thetis

Several adaptive functions, including gaining information fromother group members and detecting predators, are generally ascribedto vigilance in groups of animals subject to predation. Moststudies of the effects of neighbors on vigilance have focusedon individual vigilance. We investigated the effects of neighborson vigilance in wild red-necked pademelons Thylogale thetisforaging at night in nonpersistent aggregations in a clearingin rain forest. Neither the total number of pademelons in theclearing nor the numbers at various distances around focal individualsaffected the individual vigilance of focal animals. However,focal animals’ individual vigilance did change with thedistance to their nearest neighbor and also with distance tocover. Pairs of individuals closer than 10 m apart tended tosynchronize their bouts of individual vigilance and foraging.The degree of synchrony within pairs increased with both distanceto cover and the total number of pademelons foraging in thearea and decreased with increasing distance to the pair's nearestneighbor but did not vary with the distance separating the membersof the pair. Thus, despite their individual vigilance beingunaffected by the number of other pademelons in the feedingaggregation, pademelons were nonetheless sensitive to the presenceof conspecifics and adjusted their behavior in relation to theirseparation from neighbors. Thus, some vigilance benefits maybe obtained from the presence of conspecifics even in speciesthat aggregate only temporarily on food patches without formingmore permanent social groups.     

Chromosome‐specific microsatellites from the tammar wallaby X chromosome and chromosome 2

Microsatellites were identified from fully sequenced bacterial artificial chromosome clones from the X chromosome and chromosome 2 of the tammar wallaby, Macropus eugenii. A total of 20 microsatellites were genotyped. Seven X chromosome loci and eight loci from chromosome 2 were polymorphic, with between two and 11 alleles per locus. These markers will facilitate genetic mapping in the tammar wallaby and will have wider applications in macropod population genetics. In particular, X chromosome microsatellites will provide a novel evolutionary perspective from which to view macropod mating systems and gene flow.     

Pulmonary cytochrome P450 enzymes belonging to the CYP4B subfamily from an Australian marsupial,the tammar wallaby (Macropus eugenii)

Cytochromes P450 (CYPs) are critically important in the oxidative metabolism of a diverse array of xenobiotics and endogenous substrates. We have previously reported the cloning and characterisation of the koala CYP4A15, the first reported member of the CYP4 family from marsupials, and have demonstrated important species differences in CYP4A activity and tissue expression. In the present study, the cloning of CYP4B1 in the wallaby (Macropus eugenii) and their expression across marsupials is described. Rabbit anti-mouse CYP4B1 antibody detected immunoreactive proteins in lung and liver microsomes from all test marsupials, with relative weak signal detected from the koala, suggesting a species-specific expression. Microsomal 2-aminofluorene bio-activation (a CYP4B1 marker) in wallaby lung was comparable to that of rabbit, with significant higher activities detected in wallaby liver and kidneys compared to rabbit. A 1548 bp wallaby lung CYP4B complete cDNA, designated CYP4B1, which encodes a protein of 510 amino acids and shares 72% nucleotide and 69% amino acid sequence identity to human CYP4B1, was cloned by polymerase chain reaction approaches. The results demonstrate the presence of wallaby CYP4B1 that shares several common features with other published CYP4Bs; however the wallaby CYP4B1 cDNA contains four extra amino acid residues at the NH₂-terminal, a fundamentally conserved transmembrane anchor of all eukaryote CYPs.     

Woodrat (<Emphasis Type="Italic">Neotoma</Emphasis>) herbivores maintain nitrogen balance on a low-nitrogen,high-phenolic forage, <Emphasis Type="Italic">Juniperus monosperma</Emphasis>

The acquisition of adequate quantities of nitrogen is a challenge for herbivorous vertebrates because many plants are in low nitrogen and contain secondary metabolites that reduce nitrogen digestibility. To investigate whether herbivores maintain nitrogen balance on plant diets low in nitrogen and high in secondary compounds, we studied the effect of juniper (Juniperus monosperma) ingestion on the nitrogen balance of two species of herbivorous woodrats (Neotoma stephensi and N. albigula). These woodrat species feed on the foliage of juniper: N. stephensi is a juniper specialist, whereas N. albigula is a generalist that incorporates some juniper in its diet. Based on the nitrogen contents of the natural diets of these woodrats, we predicted that the generalist would be in negative nitrogen balance on a juniper diet whereas the specialist would not be affected. We found that both species of woodrat had low-nitrogen requirements (334.2 mg N/kg^0.75/day) and that a diet of 50% juniper did not result in negative nitrogen balance for either species. However, excretion patterns of nitrogen were altered; on the 50% juniper diet, fecal nitrogen losses increased ~38% and urinary nitrogen losses were half that of the control diet. The results suggest that absorption and detoxification of juniper secondary compounds may be more important for restricting juniper intake by the generalist than nitrogen imbalance.     

Development of urine concentrating ability in pouch young of a marsupial,the tammar wallaby (Macropus eugenii)

Summary During the first 28–30 weeks after birth, pouch young of the tammar wallaby (Macropus eugenii) normally produce urine less than 500 mOsm/kg and elevate their urine concentration by less than 20% when dehydrated by about 10% of body weight. The adult tammar, in contrast, can produce urine in excess of 3,000 mOsm/kg. The aim of this study was to determine when the various processes involved in urine concentration become mature in the tammar.Vasopressin was detectable in the pituitary of week-old tammars and pituitary vasopressin content decreased significantly after dehydration. Plasma vasopressin did not vary with age and dehydration was associated with an increase in plasma vasopressin levels. By 15 weeks of age at least, tammar kidney slices were able to bind vasopressin as indicated by a rise in tissue cAMP level following hormone treatment.The sodium and urea content of the renal medulla increased with age and significant gradients of these solutes were established by 25 weeks of age. Pouch young older than 25 weeks showed increased medullary sodium and urea levels following dehydration.The inability of pouch young less than 20 weeks of age to produce a highly concentrated urine does not result from any inadequacy in perception of osmotic stimuli or release of vasopressin by the pituitary or of binding of hormone by the kidney. Rather, it appears to be largely attributable to an insufficient medullary hypertonicity, particularly with respect to urea, which is consequent upon structural immaturity of the loop of Henle.Abbreviations cAMP cyclic AMP adenosine 3,5-monophosphate - AVP arginine vasopressin - LVP lysine vasopressin     

Testosterone dehydrogenase activity in koala liver: characterisation of cofactor and steroid substrate differences

We have studied the hepatic microsomal 17β-hydroxysteroid dehydrogenase (17β-HSD) capacity of koala (Phascolarctos cinereus) and tammar wallaby (Macropus eugenii). A detailed comparison of the activity in hepatic fractions from koala and rat was made. Hepatic microsomal NADP-supported 17β-HSD activity was significantly higher in koala (11.64±3.35 nmoles/mg protein/min), (mean±S.D.) than in tammar wallaby liver (1.52±0.79 nmoles/mg protein/min). However, when NAD was utilised as cofactor the activity was similar in both marsupial species (2.83±2.03 nmoles/mg protein/min, koala; 0.70±0.71 nmoles/mg protein/min, tammar wallaby). Data for rat indicated a cofactor preference for NAD rather than NADP (17.94±6.40 nmoles/mg protein/min, NAD; 2.18±1.04 nmoles/mg protein/min, NADP). Michaelis–Menten parameters for the kinetics of 17β-HSD testosterone oxidation by NADP and NAD were determined in the koala. The Km for testosterone was of the order of 10.0–24.0 μM (n=6) irrespective of the cofactor used, whilst the Km for NADP was 0.28–0.43 μM (n=2) and for NAD was 13.9–18.5 μM (n=2). 17β-estradiol was found to be an inhibitor of both NAD- and NADP- supported 17β-HSD activity. These findings indicate that NADP-mediated, but not NAD-mediated testosterone dehydrogenation is a major pathway of steroid biotransformation in koala liver; the reaction is less extensive in fractions from wallaby, human and rat. Such species-related differences in cofactor preference may contribute along with species differences in gene expression to observed rates of 17β-HSD activity in mammals.