Temperature regulates limb length in homeotherms by directly modulating cartilage growth

Abstract

Allen's Rule documents a century-old biological observation that strong positive correlations exist among latitude, ambient temperature, and limb length in mammals. Although genetic selection for thermoregulatory adaptation is frequently presumed to be the primary basis of this phenomenon, important but frequently overlooked research has shown that appendage outgrowth is also markedly influenced by environmental temperature. Alteration of limb blood flow via vasoconstriction/vasodilation is the current default hypothesis for this growth plasticity, but here we show that tissue perfusion does not fully account for differences in extremity elongation in mice. We show that peripheral tissue temperature closely reflects housing temperature in vivo, and we demonstrate that chondrocyte proliferation and extracellular matrix volume strongly correlate with tissue temperature in metatarsals cultured without vasculature in vitro. Taken together, these data suggest that vasomotor changes likely modulate extremity growth indirectly, via their effects on appendage temperature, rather than vascular nutrient delivery. When combined with classic evolutionary theory, especially genetic assimilation, these results provide a potentially comprehensive explanation of Allen's Rule, and may substantially impact our understanding of phenotypic variation in living and extinct mammals, including humans.Allen's Rule ͉ bone growth ͉ bone tissue culture ͉ cartilage biology ͉ thermoregulation E cogeographical rules relating climate to extremity length and body mass have long been central tenets of biology, and are among the best supported observations in natural-dwelling species (1). Allen's Rule codifies the observation that appendages (ears, limbs, and tails) of animals living in cold geographical regions are consistently shorter than those of closely related counterparts occupying warmer climes (2). Shortened extremities minimize heat loss by reducing surface area relative to volume and have long been viewed as genetically determined thermoregulatory adaptations (1). However, the heritability of extremity length is largely unknown, because similar phenotypes can be reproduced in laboratory mammals by modifying their ambient rearing temperature (3-6) (Fig. 1). The mechanism by which environmental temperature modulates extremity growth has remained elusive (7,8). Understanding growth plasticity is critical to basic evolutionary analyses, because many characters that have long been hypothesized to be adaptations may instead be partial or even entirely effects of ambient temperature (9, 10). Moreover, knowledge of this phenotypic plasticity will be a key factor in ecological conservation strategies for anticipated changes in global climate that may have direct impacts on human economics and sustainability (11).The traditional explanation for temperature-growth effects in skeletal extremities is an altered supply of essential nutrients and growth factors via increased or decreased blood flow that results from changes in vasomotor tone (i.e., t...

Allen's Rule Revisited: Temperature Influences Bone Elongation During a Critical Period of Postnatal Development

“…Results confirmed that the ears, limbs, and tails of warm-reared mice were significantly longer than those of siblings raised in the cold, without a change in total body mass (Serrat et al, 2008). The data did not fully support the vascular perfusion hypothesis when total bone blood flow was measured by microsphere deposition methods.…”

“…Although traditionally viewed as naturally selected adaptations for thermoregulation, at least some of the appendage differences described by Allen can be replicated experimentally by raising young animals at warm and cold temperatures throughout ontogeny. These findings suggest that Allen's "rule" might at least partially reflect phenotypic plasticity during development (Serrat et al, 2008). Such growth plasticity (which may or may not carry a thermal advantage: see Scholander, 1955;Steegmann 2007) could thus be a physiological response to an environmental stressor rather than an explicit genotypic adaptation for temperature control.…”

“…Procedures were submitted to and approved by the Institutional Animal Care and Use Committee at the Northeast Ohio Medical University in Rootstown, Ohio (IACUC Protocol 05-008). Male outbred CF-1 mice (N 5 28) were obtained from Charles River Laboratories (Wilmington, MA) at three weeks of age as previously described (Serrat et al, 2008). After a brief acclimation period, 3.5 week-old mice were randomly assorted into three sized-matched groups based on body mass, caged individually, and housed at cold 7 C (N 5 10), control 21 C (N 5 9), or warm 27 C (N 5 9) ambient temperatures for the duration of the experiments.…”

“…However, ambient and appendage temperatures were strongly correlated with outgrowth of the limbs, tails, and ears. Based on proliferation and morphology changes in cultured metatarsals, results suggested that the environmental growth response was mediated through direct effects of temperature on cartilage, and that the effect occurred soon after the initial temperature exposure (Serrat et al, 2008).…”

“…Tail length measurements are optimal for a longitudinal study because tails can be repeatedly measured noninvasively, whereas long bone measurements are difficult to obtain without subjecting animals to potentially growth-altering anesthesia and X-ray exposure (Gargiulo et al, 2012). Limb bone lengths are reported in another study (Serrat, 2007;Serrat et al, 2008).…”