Declining haematocrit with increasing age in a population of male zebra finches Taeniopygia guttata

Whole organismal declines in metabolic performance due to ageing are known to occur in virtually all higher vertebrates, the causes of such declines, however, are not fully understood. In mammals, haemoglobin concentration, haematocrit and red blood cell count begin to decrease in later life. In birds however, such knowledge on changes in haematological variables with increasing age is still scarce and existing data largely focuses on nestlings or differences between juveniles and adults. Here, forty-five male zebra finches Taeniopygia guttata, aged between 15 and 84 months were brought to their metabolic peak through exercise and then sampled for a full suite of haematological variables, including four directly measured primary haematological variables which were modelled against age as a continuous factor. We found a significant decline in haematocrit due to ageing in the birds (p = 0.029), a phenomenon observed in several other avian species but still unexplained. Haemoglobin did not change significantly (p = 0.156), while neither erythrocyte size or number showed any trends with age (p = 0.898 and p = 0.922 respectively) indicating that declining haematocrit is due to mechanisms other than decreasing cells size or number. We considered reticulocytes, being generally larger than mature erythrocytes as an explanation for declining haematocrit, however an a priori examination revealed an opposite result and increased with age (p = 0.016), leading us to believe that the decline in haematocrit with increasing age relates to differences in water homeostasis potential in older birds. The age dependence of haematological variables appears less clear in birds than in mammals. Besides oxygen carrying capacity, haematocrit influences the viscoelastic properties of blood, and it remains to be shown if this decline in haematocrit in birds can explain declining aerobic metabolism.

Automated summary

This study investigated the changes in hematological variables with age in zebra finches. The researchers found a significant decline in hematocrit due to aging, while hemoglobin concentration, erythrocyte size, and number did not show significant changes. The decline in hematocrit may be related to differences in water homeostasis potential in older birds.