Blood and muscle O2 storage capacity in North American diving ducks

Breath-hold diving presents air-breathing vertebrates with the challenge
of maintaining aerobic respiration while exercising underwater. Adaptive
increases in the oxygen (O2) storage capacity in the lungs, blood, or
muscle tissues can enhance these reserves and greatly extend aerobic
foraging time underwater. Here, we report blood- and muscle-O2 storage
parameters (blood hemoglobin concentration ([Hb]), hematocrit, and
myoglobin concentration ([Mb]) in the pectoralis and gastrocnemius) for 16
species of diving and dabbling ducks found in North America, and
investigate which parameters are correlated with the diving behaviors
reported in both the sea ducks (Mergini) and the pochards (Aythini). Both
[Hb] in the blood and [Mb] in the gastrocnemius, a major leg muscle used
in propulsion for these predominantly leg-propelled divers, were
significantly higher in the sea ducks compared to the dabblers (Anatini).
The pochards also showed a significant increase in [Hb] and were
intermediate between the sea ducks and the dabblers in hematocrit and [Mb]
in the gastrocnemius. Among these four variables and total body mass, [Mb]
in the gastrocnemius was the most significant predictor of mean species
dive time, and these two variables were correlated across the phylogeny.
Our results indicate that the observed changes in O2 storage capacity in
the blood and muscles are positively correlated with diving behavior in
two clades of ducks, such that larger increases are correlated with longer
dive times.