Hematological changes associated with egg production: direct evidence for changes in erythropoiesis but a lack of resource dependence?

Reductions in hematological parameters among laying birds are well reported, but the cause of this anemia is not known. We tested specific predictions generated from several, non-mutually exclusive hypotheses for mechanisms underlying reproductive anemia associated with egg production (hemodilution, transient suppression of erythropoiesis, resource dependence) in relation to (1) the time-course of development and recovery from anemia, (2) changes in specific hematological traits, and (3) the effect of diet quality, in female zebra finches (Taeniopygia guttata). Female zebra finches showed marked decreases in hematocrit (approximately 6%), red blood cell counts ( approximately 8%), and plasma hemoglobin concentration (approximately 9%) during egg production, even on a high-quality ad libitum diet, consistent with an effect of hemodilution associated with yolk precursor production. However, our results provide strong support for the hypothesis that erythropoiesis is transiently suppressed during egg-laying and that the recovery from anemia is relatively long-lasting, extending through incubation and hatching periods. Decreased hematocrit, red blood cell counts, and hemoglobin concentration did not recover at clutch completion, but showed evidence of recovery to baseline pre-breeding levels at hatching. More importantly, there was significant time-dependent variation in the proportion of reticulocytes, which increased at clutch completion but peaked at hatching 10-12 days after clutch completion, and in mean red blood cell volume, which showed a significant increase at clutch completion; consistent with enhanced production and release of larger immature cells into the circulation following suppression of erythropoiesis. Finally, we found no evidence for resource dependence of anemia associated with egg production in relation to diet quality, i.e. exogenous lipid and protein resources available to the laying female. This study demonstrates that transient suppression of erythropoiesis and, subsequently, increased reticulocytosis, are key components of reproductive anemia in egg-laying females.