The hormonal control of scent marking and precopulatory behavior in male gray short-tailed opossums (Monodelphis domestica)

Little is known about the hormonal control of behavior in marsupials. In the present study, the effects of castration and of testosterone or estradiol replacement therapy on scent marking and precopulatory behavior in male gray short-tailed oppossums (Monodelphis domestica) were examined. It was found that castration resulted in decreases in chest and flank/hip marking displayed by male gray opossums. Testosterone but not estradiol stimulated chest marking in castrates. Males treated with either estradiol or testosterone displayed more flank/hip marking than control males. Highest levels of female aggression toward males were seen when the males had received testosterone treatment. These findings are discussed with respect to similarities and differences between marsupials and eutherians in the neural metabolism of testosterone and the hormonal control of scent marking behavior.     

Evidence for extended action of gonadal hormones on the organization of sexually dimorphic behavior and morphology in gray short-tailed opossums (Monodelphis domestica)

Male and female gray short-tailed opossums were gonadectomized (GDX), or treated with the estrogen receptor antagonist tamoxifen citrate (TX), or corn oil (OIL) (control) during the 5th postnatal week, a time period equivalent to the 3rd postnatal week in rats and associated with high levels of circulating gonadal hormones and neural aromatase activity in this marsupial species. In adulthood following gonadectomy (for animals not previously gonadectomized) and replacement therapy with estradiol or testosterone, GDX males showed less male-typical scent marking and had shorter phalluses than OIL and TX males. Following replacement therapy with estradiol, GDX females were more likely to fight with and less likely to mate with stimulus males than TX females; OIL females were intermediate in these measures. Along with previous findings, these results suggest that gonadal hormones act over an extended postnatal period to organize sexually dimorphic behavior and morphology in male gray opossums and may have some effect on the organization of aggressive behavior in females of this species.     

Partner preference of intact and ovariectomized female gray short-tailed opossums (Monodelphis domestica)

Intact, ovariectomized and ovariectomized estradiol (E)-treated female gray short-tailed opossums were placed in a test situation in which they could choose between an intact and a castrated male. Intact females chose to visit intact males first and visited them more frequently and spent more time with intact than with castrated males. Ovariectomized (OVX) females did not show this preference for visiting intact males over castrates. When compared to OVX females with blank implants, OVX females with E implants spent less time with castrated males. Like intact females, OVX and OVX-E-treated females preferred to stay in close proximity to but not actually in the cage of intact rather than castrated males. To our knowledge, this is the first experimental study of partner preference and its relationship to hormonal condition in a female marsupial.     

Food preferences of captive gray short-tailed opossums (Monodelphis domestica)

The gray short-tailed opossum has been a subject in behavioral and biomedical studies for the last quarter century, but researchers know little about its preferred diet. The authors describe a study designed to determine food preferences of this species for the purpose of identifying suitable rewards to be used in behavioral studies.     

The effects of pheromonal stimuli on estrus and peripheral plasma estradiol in female gray short-tailed opossums (Monodelphis domestica)

Exposure to male pheromones is associated with the activation of vaginal estrus in gray short-tailed opossums. The effects of such exposure on peripheral plasma estradiol-17 beta (E) levels in this marsupial species were examined in this study. Mean E levels of 27.8 pg/ml +/- 4.4 in diestrous females living in a room containing only females were similar to those seen in other marsupials. Direct naso/oral exposure to pheromonal cues provided by males resulted in vaginal estrus in 75% of these females within 4 11 days. None of the females exposed to clean cages came into vaginal estrus. Animals that were in estrus at the time of blood sampling or came into estrus over the experimental period had significantly higher E levels (58.1 +/- 12.6 pg/ml) than females in the pheromone-exposed and control groups that did not come into estrus (23.3 +/- 8.2 pg/ml). These findings are discussed with respect to other marsupial and eutherian species.     

Age effects on hematologic and serum chemical values in gray short-tailed opossums (Monodelphis domestica)

Hematologic and serum chemical values were determined in groups of gray short-tailed opossums (Monodelphis domestica) that differed in age by increments of 6 months throughout the normal adult lives of M. domestica in captivity (6 months to 3.5 years). Significant age related differences were observed for some characteristics in both sexes. Previously reported sex differences in hematologic characteristics were confirmed. The means, ranges and standard deviations for the values presented here can be used as age-specific reference values for healthy M. domestica.     

Dietary effects on hematologic and serum chemical values in gray short-tailed opossums (Monodelphis domestica)

At the time of weaning (8 weeks), 57 gray short-tailed opossums (Monodelphis domestica) were placed in four dietary groups. One group was fed a horsemeat-based diet that had been used for 2.5 years in our colony, and three groups were fed three different commercial fox food diets. After the animals had reached sexual maturity (6 months), blood samples were collected and subjected to standard hematologic and serum chemical assays. Significant differences were observed among the dietary groups and between sexes in several values, but all animals appeared to be healthy and robust. The ranges, means, and standard deviations for the values presented here can be used as reference values for healthy young adult animals being fed these particular diets.     

Evidence for spontaneous postlactational estrus in gray short-tailed opossums (Monodelphis domestica)

Previous studies of the gray short-tailed opossum have shown that ovarian activity and estrus are induced by male pheromones, but we recently documented urogenital sinus (UGS) estrus in postlactational females despite their isolation from the male stimuli known to be associated with induced estrus. Body weights and UGS smears were collected after removal of pups in midlactation (19-37 days postpartum), after weaning (55-61 days postpartum), or after pheromone exposure. Estradiol was measured by RIA in plasma samples collected from dams during lactation, after separation from pups, and at estrus. Average days to UGS estrus from pup removal or initial pheromone exposure differed (P<0.05) only between the midlactation and pheromone exposure groups. Postlactational females showed a decrease in body weight from the time of pup removal or weaning to estrus, which contrasts with the increase seen in pheromonally stimulated females. Plasma estradiol was elevated at estrus in all groups, and females that were paired with males at postlactational estrus mated and produced litters. This study demonstrates that gray short-tailed opossums consistently experience estrus within 2 wk of weaning their young and that postlactational estrus appears to be hormonally and behaviorally equivalent to estrus induced by direct exposure to male pheromones.     

Perinatal exposure to estradiol masculinizes aspects of sexually dimorphic behavior and morphology in gray short-tailed opossums (Monodelphis domestica)

The effects on adult sexually dimorphic behavior of perinatal exposure to estrogen were examined by treating male and female gray opossums with estradiol (EST), an estrogen receptor antagonist (tamoxifen:TX) or oil control (OIL) during the first week of life, a time period corresponding in this marsupial to late gestation in rodent species. Following gonadectomy and replacement therapy with testosterone in adulthood, males showed more scent-marking behavior than females and EST animals showed more scent marking than TX or OIL animals. Also, phalluses were longer and body weight was higher in males than in females and in EST-treated animals than in TX-treated animals; OIL animals were intermediate in these morphological measures. EST animals of both sexes showed less female-typical screeching threat behavior than OIL or TX animals. Because these hormone manipulations were conducted on the "fetus" directly in this marsupial (rather than via the maternal circulation as in previously studied eutherian species), these findings provide unique confirming evidence for masculinization of aspects of behavior and morphology by early exposure to estradiol in mammals.     

The cremasteric neuromuscular complex in male and female grey short-tailed opossums (Monodelphis domestica)

In marsupials, both sexes possess a cremaster muscle, that of the female being attached to the mammary gland and surrounding subcutaneous tissues. In the grey short‐tailed opossum, the muscle in both sexes has a distinct proximal origin from the anterior superior iliac spine. The number of muscle fibres is higher in male than in female opossums on average, but the difference is not statistically significant. Retrograde tracing, achieved by injecting B‐cholera toxin into the muscle, showed that labelled neurons occupied several spinal cord segments, the bulk being in L2–L4. There was no overall sex difference in the number of labelled neurons. It is concluded that cremaster muscles are present in both sexes of the grey short‐tailed opossum Monodelphis domestica, contain approximately equal numbers of fibres, and differ only in their insertion target. Since the motor innervation also shows no dimorphisms in this species, it is concluded that the whole neuromuscular complex is homologous in the two sexes. This is in marked contrast to the situation in adult eutherian mammals where the muscle is possesed by males only, and where the cremasteric nucleus is highly dimorphic in terms of motor neuron numbers.     

Sexual maturation in female gray short-tailed opossums, Monodelphis domestica, is dependent upon male stimuli.

The development of physiological responsiveness to male stimuli in peripubertal female gray short-tailed opossums was examined in this study. Prepubertal females were exposed directly or indirectly to male chemicals (odors) or to no male stimuli. All females exposed directly to deposited male stimuli (22/22) showed estrus at a mean (+/- SEM) age of 127 +/- 2 days. Only 59% (13/22) of females exposed indirectly showed estrus, and at an older mean age of 162 +/- 5 days (p less than 0.01). Vaginal estrus was not observed in any of the females isolated from male stimuli (0/11). Direct exposure of prepubertal females to male odors was associated with higher uterine weights compared to those of indirectly exposed and isolated females. Reproductive success, measured as litter production, of peripubertal females greater than 140 days old was comparable to adult reproductive success. This study has shown that reproductive maturation in pubertal female opossums requires male stimuli, that this stimulus appears to be pheromonal, and that direct exposure to deposited male odors is the most effective stimulus for female reproductive maturation.     

The hormonal control of birth behavior in the gray short-tailed opossum (Monodelphis domestica)

In all major groups of Australian marsupials, prostaglandin F2alpha (PGF) or oxytocin injection initiates birth behavior in adult females, adult males and pouch young. Because inhibitors of PGF synthesis block this initiation, oxytocin may activate birth behavior via the stimulation of PGF synthesis. In this study, the role of PGF and oxytocin in the activation of birth behavior was examined in an American marsupial, the gray short-tailed opossum (Monodelphis domestica). Adult male and female gray opossums were given PGF, oxytocin, or saline (control) before behavioral observation. On the next day, the animals in the oxytocin group were injected with the PGF inhibitor flunixin meglumide (Finadyne, Schering Corp., U.S.A.) before oxytocin reinjection and behavioral observation. Both males and females showed birth behavior in response to PGF but only females responded to oxytocin. There was no significant difference in the latency of response of females to oxytocin alone versus response to oxytocin after receipt of the PGF inhibitor. These results suggest that, in contrast to Australian species, in this American marsupial, oxytocin initiates birth behavior only in females and does not operate via stimulation of prostaglandin secretion.     

Genetic diversity of laboratory gray short-tailed opossums (Monodelphis domestica): effect of newly introduced wild-caught animals.

The colony of gray, short-tailed opossums (Monodelphis domestica) at the Southwest Foundation for Biomedical Research, the primary supplier of this species for research purposes, was founded with nine animals trapped in 1978 in the state of Pernambuco, Brazil. Since 1984, 14 newly acquired founders from the state of Paraiba, Brazil have contributed to the gene pool of the colony. The animals from Paraiba and their descendants are significantly larger than the founders from Pernambuco and their descendants. The two groups also differ significantly in several measurements of morphologic traits. The changes in proportional contribution of each founder to the colony, and changes in inbreeding coefficients during the colony's history, are evaluated. Using previously established markers and three newly identified markers (ACP2, APRT, and DIA1), we show that the Paraiba-derived animals differ significantly from the original founders in allele frequencies and heterozygosity. The genetic diversity of the colony has been substantially increased by acquisition of the new founders from Paraiba. The colony is highly polymorphic, with 22.2% of loci surveyed by protein electrophoresis being variable. We conclude that the genetic differences between populations and among projects within the colony should be considered in future colony management procedures and in selection of experimental subjects.     

Reproduction, development and physiology of the gray short-tailed opossum (Monodelphis domestica)

The gray short-tailed opossum (Monodelphis domestica) has become a useful marsupial for biomedical research since it breeds reliably in the laboratory. During a 3 year period (1983-1986), a total of 129 litters were born in the colony at NJMS resulting in five generations of captive-born animals. Litter size ranged from 2 to 13 and the average sex ratio at weaning was 1:1. Mean body temperature of M. domestica was approximately 32.3 degrees C and respiratory rate approximately 54 breaths/minute. Systolic blood pressures of approximately 188 mmHg and heart rates of 345 beats/minute were found. Techniques of blood collection, anesthesia, implanting of hormones, ovariectomies and castrations, and treatment of neonates are described as well as current practices of colony management.     

An evaluation of three anaesthetic regimes in the gray short-tailed opossum (Monodelphis domestica)

The effect of several anaesthetic agents on the gray short-tailed opossum (Monodelphis domestica) was investigated. Pentobarbitone sodium at a dose of 50 mg/kg sedated the animals but did not produce analgesia or anaesthesia. A combination of ketamine hydrochloride and xylazine at 40 mg/kg and 5 mg/kg, respectively, sedated the animals, but anaesthetic levels were not attained. Halothane was most effective in producing anaesthesia in Monodelphis domestica. Hypothermia was a major side effect with all three anaesthetic regimes.     

Characteristics of the oestrous cycle and influence of social factors in grey short-tailed opossums (Monodelphis domestica)

Characteristics of the oestrous cycle of grey short-tailed opossums were studied by vaginal smears. The period of oestrus was identified by a sudden proliferation of epithelial cells which lasted about 6 days (range 3-12 days), followed by a leucocytic infiltration. Oestrous cycle length showed a bimodal distribution of 14.4 days (5 cycles, range 11-17 days) and 32.3 days (10 cycles, range 28-39 days). Females housed with males showed more days of epithelial cell proliferation than did females housed alone, and oestrous periods tended to occur in synchrony, suggesting that social factors may influence the oestrous cycle in this species.     

Biochemical genetics of alcohol dehydrogenase isozymes in the gray short-tailed opossum (Monodelphis domestica)

Polyacrylamide gel-isoelectric focusing (PAGE-IEF) methods were used to examine the multiplicity, tissue distribution, and biochemical genetics of alcohol dehydrogenase (ADH) isozymes among gray short-tailed opossums (Monodelphis domestica). Seven ADH isozymes were resolved and distinguished on the basis of their isoelectric points, tissue distributions, and substrate and inhibitor specificities. ADH1 and ADH2 exhibited Class I properties and were observed in liver (and intestine) extracts. ADH3, ADH4, and ADH5 showed “high-K _m ” (possibly Class IV) properties, with ADH3 and ADH4 exhibiting high activity in cornea, ear, stomach, and esophagus extracts. ADH6 and ADH7 exhibited Class III properties, including activities as formaldehyde dehydrogenases, with each showing different tissue distribution characteristics; ADH6 was widely distributed, and ADH7 was restricted to prostate extracts. An additional form of formaldehyde dehydrogenase (FDH) was observed, which was inactive with hexenol and ethanol as substrates. Isoelectric point variants were observed for ADH3 (three forms) and for ADH4 (two forms), and the inheritance of ADH3 was studied in 15 families ofM. domestica. The data were consistent with codominant inheritance of two alleles (ADH3*A andADH3*B) at a single autosomal locus (designatedADH3) and with a model involving a dimeric ADH isozyme: ADH3 (γ₂ isozyme, forming three dimers designated γ ₂ ¹ , γ¹ γ², and γ ₂ ² in heterozygous individuals).     

Evidence for the activation of female reproduction by males in a marsupial, the gray short-tailed opossum (Monodelphis domestica)

Although social factors are known to influence reproduction in a number of eutherian mammals, the effect of these factors on marsupial reproduction has not heretofore been studied. In this study, the temporal pattern of births and of the appearance of vaginal estrus following pairing with a male were examined in the gray short-tailed opossum, a small Brazilian didelphid. Although the estrous cycle is approximately 32 days long in this species, 75% of conceptions occurred within 9 days and 91.6% within 13 days following pairing. When sensory contact was permitted although mating was prevented, females tended to show vaginal estrus 3 to 7 days following pairing with a male. These findings are discussed with respect to possible mechanism, adaptive significance, and evolutionary perspective.     

The organization of somatosensory cortex in the short-tailed opossum (Monodelphis domestica)

The organization of neocortex in the short-tailed opossum (Monodelphis domestica) was explored with multiunit microelectrode recordings from middle layers of cortex. Microelectrode maps were subsequently related to the chemoarchitecture of flattened cortical preparations, sectioned parallel to the cortical surface and processed for either cytochrome oxidase (CO) or NADPH-diaphorase (NADPHd) histochemistry. The recordings revealed the presence of at least two systematic representations of the contralateral body surface located in a continuous strip of cortex running from the rhinal sulcus to the medial wall. The primary somatosensory area (S1) was located medially while secondary somatosensory cortex (S2) formed a laterally located mirror image of S1. Auditory cortex was located in lateral cortex at the caudal border of S2, and some electrode penetrations in this area responded to both auditory and somatosensory stimulation. Auditory cortex was outlined by a dark oval visible in flattened brain sections. A large primary visual cortex (V1) was located at the caudal pole of cortex, and also consistently corresponded to a large chemoarchitecturally visible oval. Cortex just rostral and lateral to V1 responded to visual stimulation, while bimodal auditory/visual responses were obtained in an area between V1 and somatosensory cortex. The results are compared with brain organization in other marsupials and with placentals and the evolution of cortical areas in mammals is discussed.     

The organization of somatosensory cortex in the short-tailed opossum ( Monodelphis domestica )

The organization of neocortex in the short-tailed opossum ( Monodelphis domestica ) was explored with multiunit microelectrode recordings from middle layers of cortex. Microelectrode maps were subsequently related to the chemoarchitecture of flattened cortical preparations, sectioned parallel to the cortical surface and processed for either cytochrome oxidase (CO) or NADPH-diaphorase (NADPHd) histochemistry. The recordings revealed the presence of at least two systematic representations of the contralateral body surface located in a continuous strip of cortex running from the rhinal sulcus to the medial wall. The primary somatosensory area (S1) was located medially while secondary somatosensory cortex (S2) formed a laterally located mirror image of S1. Auditory cortex was located in lateral cortex at the caudal border of S2, and some electrode penetrations in this area responded to both auditory and somatosensory stimulation. Auditory cortex was outlined by a dark oval visible in flattened brain sections. A large primary visual cortex (V1) was located at the caudal pole of cortex, and also consistently corresponded to a large chemoarchitecturally visible oval. Cortex just rostral and lateral to V1 responded to visual stimulation, while bimodal auditory/visual responses were obtained in an area between V1 and somatosensory cortex. The results are compared with brain organization in other marsupials and with placentals and the evolution of cortical areas in mammals is discussed.