Seasonal changes in haematocrit in captive koalas (Phascolarctos cinereus)
Cleva, GM, Stone, GM & Dickens, RK 1994, Australian Journal of Zoology, vol. 42, no. 2, pp. 233-236.
Distinct changes in microhaematocrit (ratio of red blood cell volume to the total blood volume) with season were observed in captive koalas, with raised levels in the winter and reduced values during the summer. Additionally, microhaematocrit was negatively correlated with the lowest and highest temperatures in the day. No associations of the differences in haematocrit with variations in body weight, plasma osmolarity or the concentration of plasma protein were discovered.
Nine female and six male koalas at Featherdale Wildlife Park, New South Wales underwent weekly monitoring for 11 months and 22 months, respectively. Blood samples were collected through venipuncture. In addition to haematocrit values, plasma protein concentration and plasma osmolarity values were measured as was each koala’s body weight. Other findings of the study include overall higher microhaematocrit values in males compared to females throughout the year, although this was not a significant difference. Plasma protein levels and body weight also did not change seasonally.
Haematocrit fluctuations have previously been reported in a variety of diverse species, including small mammals. Such changes have been correlated with extreme cold weather conditions or metabolic conditions such as hibernation. Past literature has extensively documented the critical role of red blood cells in metabolic adaptations to drastic changes in temperature; however, nutritional influences ought not be ignored. In the case of the present study, the relative body weight stability seems to indicate that changes in haematocrit values are a product of differences in atmospheric temperature and not nutritional status. In the summer, the studied koalas appeared to acquire a splayed posture, postulated to assist loss of heat as a behavioral mechanism to deal with heat stress. Conversely, behavioral adaptations to raise body temperatures were not apparent. It is speculated that increases in haematocrit observed in the study are related to a metabolic reaction to the higher energy demands from low temperature exposure.
The present findings provide evidence that koalas manifest significant changes in haematocrit levels due to fluctuations in temperature throughout the day as well as seasonally. Future studies should investigate the role of erythropoietin in increasing red blood cell formation in koalas, and the following secretions of the thyroid across seasonal differences in temperature.
Summarised by Alexander Murdoch
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