Immunosenescence in a captive semelparous marsupial, the red-tailed phascogale (Phascogale calura)

Abstract Background The red-tailed phascogale is a ‘Near Threatened’ dasyurid marsupial. Males are semelparous and die off shortly after the breeding season in the wild due to a stress-related syndrome, which has many physiological and immunological repercussions. In captivity, males survive for more than 2 years but become infertile after their first breeding season. Meanwhile, females can breed for many years. This suggests that captive males develop similar endocrine changes as their wild counterparts and undergo accelerated aging. However, this remains to be confirmed. The health status and immune function of this species in captivity have also yet to be characterized. Results Through an integrative approach combining post-mortem examinations, blood biochemical and hematological analyses, we investigated the physiological and health status of captive phascogales before, during, and after the breeding season. Adult males showed only mild lesions compatible with an endocrine disorder. Both sexes globally maintained a good body condition throughout their lives, most likely due to a high quality diet. However, biochemistry changes potentially compatible with an early onset of renal or hepatic insufficiency were detected in older individuals. Masses and possible hypocalcemia were observed anecdotally in old females. With this increased knowledge of the physiological status of captive phascogales, interpretation of their immune profile at different age stages was then attempted. During the breeding season, males developed a stress leukogram characterized by a marked lymphopenia, further aggravated by a severe leukopenia after the breeding season. To determine whether these changes were limited to the peripheral blood or had more profound implications, histopathology of the spleen was performed opportunistically. Adult males showed white pulp atrophy, at various degrees. The atrophy was mainly lymphoid and more severe in 1.5-year-old males than in 3.5-year-old females. These results suggest that captive males undergo accelerated immunosenescence. Conclusions Functional studies are now needed to characterize the underlying mechanisms leading to immunosenescence in marsupials. Semelparous dasyurids present great potential for studying the effects of sex and stress on immunity in marsupials. Characterization of these immune-endocrine interactions may help refine veterinary treatment plans, husbandry protocols and conservation programs to maintain the health of captive and wild populations.