Aust. J. Zool., 1978,26,43547 Observations on Spontaneous Stress-related Mortality among Males of the Dasyurid Marsupial Antechinus stuartii Macleay I. K. Ian BeveridgeAC, A. J. BradleyD and A. K. LeeE A Department of Veterinary Paraclinical Sciences, University of Melbourne, Veterinary Clinical Centre, Werribee, Vic. 3030. Present address: Department of Pathology, Ontario Veterinary College, Guelph, Ontario, Canada N1G 2W1. Present address: Department of Tropical Veterinary Science, James Cook University of North Queensland, Townsville, Qld. 48 1 1. Department of Physiology, Monash University, Clayton, Vic. 3168. Department of Zoology, Monash University, Clayton, Vic. 3168. Abstract Splenic follicle sizes in male A. stuartii killed during the period of male mortality in 1973 were smaller than those of females killed at the same time. In 1974, all 17 males and two of four females held in the laboratory died during the period of male mortality in the field. Significant findings in some moribund animals included moderate anaemia, associated with heavy parasitaemias by Babesia sp. and elevated plasma corticosteroid levels. At autopsy, a high proportion of animals had haemoglobinuria, focal hepatic necrosis, and gastrointestinal haemorrhage due to gastric and dundenal ulcers. Males dying spontaneously had severely involuted splenic follicles. Listeria monocytogenes was recovered from four livers with focal necrosis but not from six livers with no necrotic foci. Splenic follicles were smaller in one group of males treated experimentally with a high level of exogenous corticosteroid. Deaths were related mainly to gastrointestinal haemorrhage, listeriosis and possibly babesiosis, considered to be associated with an adrenocortical response to stress, and concomitant reduction in resistance to infec- tion or latent disease. The probability that this syndrome is involved in mortality in the field is discussed. Introduction The brown antechinus, Antechinus stuartii Macleay, is a small insectivorous dasyurid marsupial inhabiting the forests of eastern Australia. An abrupt and total mortality of males occurs annually within a 3-week period following the onset of breeding in late winter or spring, i.e. August-September (Woolley 1966; Wood 1970; Lee et al. 1977). This mortality occurs at approximately the same time each year in the same locality, but may be displaced by as much as a month in geographi- cally remote populations (Woolley 1966; Wood 1970; Leonard 1972). The consist- ency, synchrony and sex-related nature of the mortality, and the occasional discovery of uninjured moribund males in the field, suggest that the die-off is intern- ally governed, and is an integral part of the life history of this marsupial. During the last weeks of life, males show increased total and free plasma cortico- steroid concentrations (Bradley et al. 1976), hypertrophy of the adrenal glands, par- ticularly the zona fasciculata (Barnett 1973), a negative nitrogen balance (Woollard 1971) and a lymphopenia and neutrophilia (Cheal et al. 1976). These changes are consistent with a stress response and suggest that hyperadrenocorticism may be causally related to the mortality.