M.A. Chappell, E.L. Rezende, K.A. Hammond (2003) Age and aerobic performance
in deer mice, Journal of Experimental Biology 206:1221-1231.
ABSTRACT -- Age impacts the phenotype of all multicellular
animals, but lifetime changes in physiological traits are poorly understood
for all but a few species. Here we describe cross-sectional study of age
effects on body composition, aerobic performance, and ventilation in deer
mice, Peromyscus maniculatus. This species lives considerably longer
in captivity (in excess of 5 years) than most laboratory rodents, and the
adaptational biology of its aerobic physiology is well studied. Our deer
mice grew throughout life, and as is typical for mammals, their basal metabolism
(BMR) and maximal oxygen consumption in exercise (VO2max)
and thermogenesis (VO2sum) increased as
power functions of mass. Age did not affect BMR, but we found abrupt decreases
in growth rate, VO2max, and VO2sum
at about 485 days of age, and the mass-adjusted maximal aerobic performance
of old mice (5 years of age) was 20% (VO2max)
to 35% (VO2sum) less than that of young
animals. Breathing frequency (f) and oxygen extraction (EO2)
also declined with age but did not change abruptly. However, there were
no consistent age-related changes in tidal volume (Vt)
or minute volume (Vmin) after accounting
for the effects of mass and VO2sum. Age
influenced several aspects of body composition (lean and fat mass). However,
these changes were insufficient to explain the age-related declines in aerobic
performance, suggesting that mass-specific oxidative capacity of lean tissue
decreased with age. The performance changes we found could engender substantial
reductions in the mobility and thermal tolerances of old deer mice. However,
very few wild mice are likely to survive to ages where substantial performance
decreases occur, so these declines are probably not subjected to strong
selection in natural populations.