Dynamic and asynchronous ontogenetic changes in growth and metabolic rate in thirteen-lined ground squirrels (<i>Ictidomys tridecemlineatus</i>)

As mammals grow from developmentally immature neonates into adults, metabolic rate (V̇O2) 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 mass and resting V̇O2 of I. tridecemlineatus pups using flow-through respirometry at postnatal days (P)0-P30 (inclusive), P60, P90, P120 and at over 1 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 and P43 the further increase in V̇O2 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 V̇O2 appear to reflect metabolic trade-offs as pups allocated energy towards physiological development (P18-P35) versus growth (P35-P79): 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.