To boost aerobic performance, fraudulent athletes engage in blood doping to increase the haemoglobin levels in their blood. Surprisingly, however, during reproduction – a period of huge metabolic investment into the next generation – birds often exhibit a reduction in the haemoglobin concentration in their blood. This is due, at least in part, to increased levels of oestrogen that suppress the formation of new red blood cells. Reproductive output is at the crux of Darwinian fitness, so the possibility that anaemia compromises breeding success has clear evolutionary relevance. However, evaluating the importance of oxygen transport by haemoglobin in supporting reproductive exertions represents no small challenge.
Undaunted, a team from Simon Fraser University, Canada, including Raime Fronstin, Julian Christians and Tony Williams, recently investigated the link between haemoglobin and reproductive success in wild starlings. Over 3 years, they monitored approximately 150 nest boxes at a local dairy farm and accurately recorded when each bird laid eggs, the number of eggs in a clutch and the size of the chicks. The authors also collected blood samples from the birds during the course of their observations. Then, through a series of elegant experiments, they related the measurements of the birds’ reproductive output to their haematology.
Fronstin and colleagues first investigated the natural variation in haematocrit (the percentage of blood volume occupied by red blood cells) in their starling population. From each nesting bird, they took a baseline blood sample, which revealed that females with haemoglobin-packed blood laid their eggs considerably earlier and raised more fledglings than those with low haemoglobin concentrations. However, these early hatchlings did not necessarily get more worms: the chicks that hatched from eggs that were laid earlier grew no larger than the later chicks.
Spurred on by the enticing correlations, the team next adopted a classical experimental approach: they rendered some of the maternal starlings anaemic by injecting them with a compound, phenylhydrazine, which bursts red blood cells. The first time they conducted this experiment, in 2010, the results could not have been clearer: the anaemic females took longer to lay eggs, had smaller chicks and, by the time the chicks left the nest, had fewer than half the number of fledglings. Haemoglobin was clearly crucial for reproductive success.
The next year, 2011, when the authors repeated their efforts, the promising results could not be replicated. The anaemic females laid just as many eggs and successfully raised equally large fledglings. For most scientists, this inconsistency would be a source of frustration. But Fronstin and his co-workers saw something deeper. They realised that in 2011, hatchlings were on average larger than in previous years. Conversely, in 2010 all of the chicks were smaller than usual. They accordingly suggest that the importance of haemoglobin varies with the availability of resources, such as food.
This study represents an impressive unification of mechanistic physiology and reproductive fitness. It further shows that the link is not necessarily simple and opens up new avenues to establish how the importance of oxygen transport by haemoglobin may vary under different conditions in the wild.