Diving Physiology
All diving species require adaptations to overcome the challenges of drag, buoyancy, hypothermia, and intermittent tissue hypoxia (low O2) while foraging underwater. Although all diving species share these challenges, the mechanisms evolved to mitigate negative impacts, particularly in regards to tissue hypoxia, vary among taxa. Diving birds have increased oxygen storage capacity, but whether this is due to increased hemoglobin (Hb) – oxygen affinity, red blood cell count (hematocrit), or Hb and myoglobin (Mb) concentration varies. Once oxygen-rich blood reaches the tissues, diving birds may increase their ability to offload oxygen from the blood and deliver and store it in the tissues for use during hypoxic periods. This can be accomplished by a suite of adaptations which work together to increase oxygen use efficiency and storage capacity.
While significant effort has focused on understanding diving response in marine mammals and emperor penguins, the mechanisms for hypoxia resistance in other bird species are less understood. My interests lie in understanding how other diving bird species compare. I am currently working on this question in two systems: double-crested cormorants and anhingas in South Florida and three species of crested penguins in New Zealand. In each I am assessing how these adaptations for oxygen transport and storage compare among sister families (cormorants and anhingas) and species (penguins).
Mechanisms for intermittent hypoxia tolerance in two South Florida diving birds
The capacity of blood to carry and store oxygen directly impacts the amount of oxygen that is delivered to tissues. This is particularly important during periods of hypoxia. Dives are highly active but must be fueled by a single final breath prior to submersion. To mitigate the negative consequences of limited oxygen supplies and maintain homeostasis in active muscles, many diving birds increase their ability to carry, deliver, and store oxygen in their blood and tissues.
For this project, I am conducting a systemic analysis of oxygen storage, transport, and use efficiency in the double-crested cormorant and the anhinga. I measured hemoglobin and hematocrit concentrations and muscle and lung mass in the field. Then for the primary swimming and flying muscles, I measured myoglobin concentration, quantified mitochondrial and capillary arrangement and density, muscle fiber type and arrangement, and maximal activity of 10 key enzymes. Combined, these data provide an assessment of tissue level adaptations to maximize oxygen use and storage in key muscles allowing these divers to maintain high levels of activity while foraging underwater.
Manuscript submitted following minor revisions: White, J., Schell, E.R., Dawson, N.J., & McCracken, K.G. Comparative mechanism for O2 storage and metabolism in two Florida diving birds: the Anhinga (Anhinga anhinga) and the Double-crested Cormorant (Nannopterum auritum). Journal of Comparative Physiology B
Blood O2 storage and transport capacity in New Zealand penguins
Diving physiology in penguins has been extensively studied in the larger, deep diving species such as emperor and gentoo penguins, however the blood oxygen storage capacity of the mid-depth diving crested penguins is less understood. For this project, I measured the concentrations of hemoglobin and hematocrit in three species of crested penguins (Fiordland penguin/tawaki, erect-crested penguin, and Eastern rockhopper penguin) in New Zealand to compare oxygen storage capacity and dive depth/time.
Data analysis for this project is still underway, but preliminary results suggest that the larger erect-crested penguin has the highest hemoglobin and hematocrit values of the three suggesting a higher oxygen storage capacity in the blood and more efficient transport of oxygen to the tissues allowing deeper or longer dives.
Data from this project was cited in New Zealand Geographic 2023 article The Future is Written in Penguin Blood (see Media tab for links). Publication of a short note containing these values is on deck for Notornis.