Dynamic and asynchronous ontogenetic changes in growth and metabolic rate in thirteen-lined ground squirrels (Ictidomys tridecemlineatus).

As mammals grow from developmentally immature neonates into adults, metabolic rate (V˙O2) has been hypothesized to scale isometrically with body mass until individuals reach a critical size, after which scaling becomes hypometric. This study aimed to determine when this occurs and gain insight into why this metabolic switch occurs in altricial thirteen-lined ground squirrels (Ictidomys tridecemlineatus). We hypothesized that the switch would be related to patterns of growth and development. We measured the masses and resting V˙O2s of I. tridecemlineatus pups using flow-through respirometry at postnatal days 0-30 (inclusive), 60, 90, 120 and at over one year old and found repeated and asynchronous ontogenetic changes in growth rate and metabolic scaling. Following birth, pups grew 1.18±0.02 g day-1 and metabolism scaled isometrically as predicted (scaling exponent B=1.01±0.03). Surprisingly, B more than doubled to 2.64±0.13 at P18-P23 while growth rate remained constant. At P29, growth rate more than quadrupled to 4.87±0.03 g day-1 while between P35-P43 the further increase in V˙O2 was proportionately less, and thus B fell to -0.27±0.43. Adult size was reached by P79, with final scaling and growth rate values maintained into adulthood (P465). The asynchronous changes in mass and resting V˙O2 appear to reflect metabolic trade-offs as pups allocated energy towards physiological development (P18-35) versus growth (P35-79): the period of hypermetabolic scaling (B=2.64±0.13) coincided with the time when pups became weaned and sufficiently physiologically and morphologically developed to live independently.