Whether lifespan
scales to age-associated changes in health and disease is an urgent question in
societies with increasing lifespan. Body mass is associated with organismal
functioning in many species, and often changes with age. We here tested in
zebra finches whether two factors that decreased lifespan, sex and poor environmental
quality, accelerated the onset of body mass declines. We subjected 597 birds for
nine years to experimentally manipulated foraging costs (harsh = H, benign = B)
during development (small vs large brood size) and in adulthood (easy vs hard
foraging conditions) in a 2x2 design. This yielded four treatment combinations
(HH, HB, BH, BB) for each sex. Harsh environments during development and in
adulthood decreased average body mass additively. The body mass aging trajectory
showed a short steep increase in early adulthood, followed by a plateau and
then a decline after 5 years. This decline occurred in all groups except for HB
females, which gained mass until death. Surprisingly, the onset of body mass
decline was earlier in experimental groups with a longer lifespan. In contrast,
the onset of body mass decline was one year earlier in females, which lived two
months (4%) shorter than males. Thus, the onset of
body mass aging associated positively with the sex-specific differences in
lifespan, but negatively with the environmental modulation of lifespan. Thus,
body mass aging trajectories did not generally scale to lifespan, and we
discuss the possible causes and implications of this finding.