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1. Respiratory anatomy and physiology in diving penguins

   https://doi.org/10.1098/rstb.2023.0422  

   Ponganis, P J; Williams, C L; Scadeng, M (February 2025, Philosophical Transactions of the Royal Society B: Biological Sciences)

   ---- (Ed.)

   The anatomy and function of the respiratory systems of penguins are reviewed in relation to gas exchange and minimization of the risks of pulmonary barotrauma, decompression sickness and nitrogen narcosis during dives. Topics include available lung morphology and morphometry, respiratory air volumes determined with different techniques, review of possible physiological and biomechanical mechanisms of baroprotection, calculations of baroprotection limits and review of air sac and arterial partial pressure of oxygen (P_O2) profiles in relation to movement of air during breathing and during dives. Limits for baroprotection to 200, 400 and 600 m in Adélie, king and emperor penguins, respectively, would require complete transfer of air sac air and reductions in the combined tracheobronchial tree—parabronchial volume of 24% in Adélie, 53% in king penguins and 76% in emperor penguins. Air sac and arterial P_O2profiles at rest and during surface activity were consistent with unidirectional air flow through the lungs. During dives, P_O2profiles were more complex, but were consistent with compression of air sac air into the parabronchi and air capillaries with or without additional air mixing induced by potential differential air sac pressures generated by wing movements. This article is part of the theme issue ‘The biology of the avian respiratory system’. 

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2. Evolved increases in hemoglobin-oxygen affinity and the Bohr effect coincided with the aquatic specialization of penguins

   https://doi.org/https://doi.org/10.1073/pnas.2023936118  

   Signore, Anthony V; Tift, Michael S; Hoffman, Federico G; Schmitt, Todd L; Moriyama, Hideaki; Storz, Jay F. (March 2021, Proceedings of the National Academy of Sciences of the United States of America)

   Somero, George N. (Ed.)

   Dive capacities of air-breathing vertebrates are dictated by onboard O2 stores, suggesting that physiologic specialization of diving birds such as penguins may have involved adaptive changes in convective O2 transport. It has been hypothesized that increased hemoglobin (Hb)-O2 affinity improves pulmonary O2 extraction and enhances the capacity for breath-hold diving. To investigate evolved changes in Hb function associated with the aquatic specialization of penguins, we integrated comparative measurements of whole-blood and purified native Hb with protein engineering experiments based on site-directed mutagenesis. We reconstructed and resurrected ancestral Hb representing the common ancestor of penguins and the more ancient ancestor shared by penguins and their closest nondiving relatives (order Procellariiformes, which includes albatrosses, shearwaters, petrels, and storm petrels). These two ancestors bracket the phylogenetic interval in which penguin-specific changes in Hb function would have evolved. The experiments revealed that penguins evolved a derived increase in Hb-O2 affinity and a greatly augmented Bohr effect (i.e., reduced Hb-O2 affinity at low pH). Although an increased Hb-O2 affinity reduces the gradient for O2 diffusion from systemic capillaries to metabolizing cells, this can be compensated by a concomitant enhancement of the Bohr effect, thereby promoting O2 unloading in acidified tissues. We suggest that the evolved increase in Hb-O2 affinity in combination with the augmented Bohr effect maximizes both O2 extraction from the lungs and O2 unloading from the blood, allowing penguins to fully utilize their onboard O2 stores and maximize underwater foraging time. 

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3. Blood oxygen transport and depletion in diving emperor penguins

   https://doi.org/10.1242/jeb.246832  

   Ponganis, Paul J.; Williams, Cassondra L.; Kendall-Bar, Jessica M. (March 2024, Journal of Experimental Biology)

   ABSTRACT Oxygen store management underlies dive performance and is dependent on the slow heart rate and peripheral vasoconstriction of the dive response to control tissue blood flow and oxygen uptake. Prior research has revealed two major patterns of muscle myoglobin saturation profiles during dives of emperor penguins. In Type A profiles, myoglobin desaturated rapidly, consistent with minimal muscle blood flow and low tissue oxygen uptake. Type B profiles, with fluctuating and slower declines in myoglobin saturation, were consistent with variable tissue blood flow patterns and tissue oxygen uptake during dives. We examined arterial and venous blood oxygen profiles to evaluate blood oxygen extraction and found two primary patterns of venous hemoglobin desaturation that complemented corresponding myoglobin saturation profiles. Type A venous profiles had a hemoglobin saturation that (a) increased/plateaued for most of a dive's duration, (b) only declined during the latter stages of ascent, and (c) often became arterialized [arterio-venous (a-v) shunting]. In Type B venous profiles, variable but progressive hemoglobin desaturation profiles were interrupted by inflections in the profile that were consistent with fluctuating tissue blood flow and oxygen uptake. End-of-dive saturation of arterial and Type A venous hemoglobin saturation profiles were not significantly different, but did differ from those of Type B venous profiles. These findings provide further support that the dive response of emperor penguins is a spectrum of cardiac and vascular components (including a-v shunting) that are dependent on the nature and demands of a given dive and even of a given segment of a dive. 

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4. Geometric analysis of airway trees shows that lung anatomy evolved to enable explosive ventilation and prevent barotrauma in cetaceans

   https://doi.org/10.1101/2024.10.16.618729  

   Cieri, Robert L; Tawhai, Merryn H; Piscitelli-Doshkov, Marina; Vogl, A Wayne; Shadwick, Robert E (October 2024, bioRxiv)

   Two new biomechanical challenges faced cetacean lungs compared to their terrestrial ancestors. First, hydrostatic pressures encountered during deep dives are sufficient to cause nearly full lung collapse, risking substantial barotrauma during surfacing if air is trapped in the fragile smaller airways. Second, rapid ventilation in large cetaceans requires correspondingly high ventilatory flow rates. In order to investigate how airway geometry evolved in response to these challenges, we characterized airway geometry from 12 species of cetaceans that vary in common dive depth and ventilatory behavior and a domestic pig using computed tomography. After segmenting the major airways, we generated centerline networks models for the larger airways and computed geometric parameters for each tree including mean branching angle, percent volume fraction, and Strahler branching, diameter, and length ratios. When airway geometry was regressed against ventilatory and diving parameters with phylogenetic least squares, neither average branching angle, percent volume fraction, Strahler length ratio or Strahler branching ratio significantly varied with common ventilatory mode or common diving depth. Higher Strahler diameter ratios were associated with slower ventilation and deeper diving depth, suggesting that cetacean lungs have responded to biomechanical pressures primarily with changes in airway diameter. High Strahler diameter ratios lungs in deeper diving species may help to facilitate more complete collapse of the delicate terminal airways by providing for a greater incompressible volume for air storage at depth. On the other hand, lungs with low Strahler diameter ratios would be better for fast ventilation because the gradual decrease in diameter moving distally should keep peripheral flow resistance low, maximizing ventilatory flow rates. 

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5. Iron mobilization during lactation reduces oxygen stores in a diving mammal

   https://doi.org/10.1038/s41467-022-31863-7  

   Shero, Michelle R.; Kirkham, Amy L.; Costa, Daniel P.; Burns, Jennifer M. (August 2022, Nature Communications)

   Abstract The profound impacts that maternal provisioning of finite energy resources has on offspring survival have been extensively studied across mammals. This study shows that in addition to calories, high hemoprotein concentrations in diving mammals necessitates exceptional female-to-pup iron transfer. Numerous indices of iron mobilization (ferritin, serum iron, total-iron-binding-capacity, transferrin saturation) were significantly elevated during lactation in adult female Weddell seals (Leptonychotes weddellii), but not in skip-breeders. Iron was mobilized from endogenous stores for incorporation into the Weddell seal’s milk at concentrations up to 100× higher than terrestrial mammals. Such high rates of iron offload to offspring drew from the female’s own heme stores and led to compromised physiologic dive capacities (hemoglobin, myoglobin, and total body oxygen stores) after weaning their pups, which was further reflected in shorter dive durations. We demonstrate that lactational iron transfer shapes physiologic dive thresholds, identifying a cost of reproduction to a marine mammal. 

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