Abstract Body: Introduction. The Anna's hummingbird has the highest recorded heart rate among endotherms, reaching up to 1200 bpm. This makes it a unique model of extreme cardiac function. Studying the hummingbird heart offers an opportunity to uncover new mechanisms that govern heart function under extreme conditions.
Question/Hypothesis. Despite its extreme physiology, the detailed morphology of the hummingbird’s heart remains uncharacterized. We hypothesize that the hummingbird exhibits unique ventricular features that support its exceptional cardiovascular demands.
Aims. We aim to achieve the first anatomical characterization of the hummingbird heart.
Method/Approach. We used high-resolution micro-computed tomography to generate volumetric reconstructions of the Anna’s hummingbird hearts (N = 3). Using 3D Slicer, we segmented and quantified the heart compartments. We also used lectin fluorescent labeling in ventricular slices to study cardiomyocyte morphology and blood vessel density (N = 6).
Results/Data. Hummingbirds display a heart-weight-to-body-weight ratio of 2.6%, four-fold higher than that observed in mice and humans. The average total heart volume was 65.4 ± 7.5 cubic millimeters. The left ventricular (LV) wall occupied 45.3 ± 3.3% and the right ventricular (RV) wall occupied 19.1 ± 0.6% of the heart’s volume. The LV:RV ratio was 2:1, a small ratio compared to 3-4:1 observed in mice and in other avian species. The hummingbird ventricle is composed of thin cardiomyocytes, with a cross-sectional cell area six-fold smaller than mouse cardiomyocytes. Cell density was 128.5 ± 5.5 cells per 10,000 square micrometers, three-fold higher than the mouse. Capillary density was even higher, with values of 310.7 ± 21.8 per 10,000 square micrometers, seven-fold more abundant than in mice.
Conclusion. We found specific adaptations in the hummingbird heart including a large heart, and engrossed RV wall. Both adaptations could be essential for sustaining the large cardiac outputs expected at this extreme cardiac function. Cellular architecture and vascularization are in line with the expected requirements for the extreme metabolic demands of the tissue.