Linking performance to powerhouse: Mitochondria functions in blood cells reflect flight endurance of a songbird

Identifying the physiological mechanisms underpinning inter-individual
differences in performance and fitness remains a key challenge in
organismal biology. Variation in mitochondrial aerobic metabolism has been
suggested to underlie inter-individual variation in performance, but this
remains seldom tested, partly because of the need to use terminal sampling
for assessing mitochondrial parameters. To fill this knowledge gap, we
investigated whether inter-individual variation in mitochondrial aerobic
parameters measured from less-invasively taken samples (i.e., blood cells)
would correlate with both anaerobic and aerobic metrics of flight
performance in house sparrows (Passer domesticus). We predicted that
mitochondrial aerobic metabolism should correlate with aerobic but not
anaerobic metrics of flight performance. As expected, we found no evidence
for a relationship between mitochondrial metabolism and the energy
required to take-off (i.e., anaerobic), but flight duration to exhaustion
(i.e., aerobic) correlated positively with both cellular mitochondrial
respiration rates and oxidative phosphorylation efficiency, a proxy of
mitochondrial efficiency to convert nutrients into ATP. Our results
therefore support the idea that inter-individual variation in
mitochondrial aerobic metabolism could underlie variation in aerobic
performance, and suggest that the nucleated blood cells of birds (and
potentially other non-mammalian vertebrates) may be a relevant tissue to
test those links.