Environmental factors influence embryonic limb proportions through developmental plasticity in response to embryo movement. Incubation temperature affects motility and limb bone growth in West African Dwarf crocodiles, altering limb proportions that may influence post-hatching performance. Pharmacological immobilization of embryonic chickens showed that altered motility, independent of temperature, affects limb growth. Using chickens, researchers combined histological, immunochemical, and cell proliferation studies with unbiased array profiling to identify cellular and transcriptional targets of embryo movement. These findings revealed that movement alters limb proportions and regulates chondrocyte proliferation in specific growth plates, related to intrinsic mTOR pathway activity. Environmental factors can influence cellular activity in growing bones, generating phenotypic variation during prenatal development. Limb specializations vary widely in nature, with all tetrapods having a basic limb design. Limb proportions are tailored to fulfill roles, emerging during prenatal development and varying within species. Natural selection has led to limb proportions adapted to improve locomotor performance in specific environments. However, the extent to which these proportions are influenced by shorter-term environmental changes within an organism's lifetime is not well understood. Recent evidence shows environmental cues can cause profound changes in behavior and musculoskeletal form postnatally, similar to adaptive changes in evolution. For example, modifications in bone shape and limb function are seen in the terrestrialization of Polypterus fish. Morphological changes, including limb/extremity proportions, have also been observed in mammals raised in different climates. Phenotypic plasticity during prenatal periods may allow organisms to incorporate shorter-term environmental input into variation before birth/hatching. Morphological diversity is observed in species with divergent climates, such as extinct crocodylomorphs. Inter-specific adaptation for smaller surface area to body size ratios in cooler climates ("Allen's rule") is widespread. Intra-species variation in limb form in individuals from thermally divergent locations has also been observed in lizards but remains unexplained. Phenotypic differences in the North American lizard Sceloporus occidentalis include longer limbs in warmer locations and reduced relative limb length in cooler, northern populations. Incubation temperature influences embryonic motility and skeletal growth in chickens. Changes in embryonic limb bone growth induced by pharmacologically stimulating embryo motility suggest mechanical input, rather than temperature, regulates skeletal growth. Experiments using embryonic crocodiles and chickens tested the hypothesis that developmental plasticity during prenatal stages allows variation in limb proportions via environmentally-triggered movement. Crocodiles, which are ectothermic and not directly incubated, provide an opportunity to explore whether motility and limb proportions are modified by incubation temperature. Embryos incubated at extremes of the normal range (32°C and 28°C) exhibited altered motility and limb proportions. Measurements of body size, limb length, and limb element lengths were taken. Embryonic chickens were used to explore the hypothesis, testing whether pharmacologicalEnvironmental factors influence embryonic limb proportions through developmental plasticity in response to embryo movement. Incubation temperature affects motility and limb bone growth in West African Dwarf crocodiles, altering limb proportions that may influence post-hatching performance. Pharmacological immobilization of embryonic chickens showed that altered motility, independent of temperature, affects limb growth. Using chickens, researchers combined histological, immunochemical, and cell proliferation studies with unbiased array profiling to identify cellular and transcriptional targets of embryo movement. These findings revealed that movement alters limb proportions and regulates chondrocyte proliferation in specific growth plates, related to intrinsic mTOR pathway activity. Environmental factors can influence cellular activity in growing bones, generating phenotypic variation during prenatal development. Limb specializations vary widely in nature, with all tetrapods having a basic limb design. Limb proportions are tailored to fulfill roles, emerging during prenatal development and varying within species. Natural selection has led to limb proportions adapted to improve locomotor performance in specific environments. However, the extent to which these proportions are influenced by shorter-term environmental changes within an organism's lifetime is not well understood. Recent evidence shows environmental cues can cause profound changes in behavior and musculoskeletal form postnatally, similar to adaptive changes in evolution. For example, modifications in bone shape and limb function are seen in the terrestrialization of Polypterus fish. Morphological changes, including limb/extremity proportions, have also been observed in mammals raised in different climates. Phenotypic plasticity during prenatal periods may allow organisms to incorporate shorter-term environmental input into variation before birth/hatching. Morphological diversity is observed in species with divergent climates, such as extinct crocodylomorphs. Inter-specific adaptation for smaller surface area to body size ratios in cooler climates ("Allen's rule") is widespread. Intra-species variation in limb form in individuals from thermally divergent locations has also been observed in lizards but remains unexplained. Phenotypic differences in the North American lizard Sceloporus occidentalis include longer limbs in warmer locations and reduced relative limb length in cooler, northern populations. Incubation temperature influences embryonic motility and skeletal growth in chickens. Changes in embryonic limb bone growth induced by pharmacologically stimulating embryo motility suggest mechanical input, rather than temperature, regulates skeletal growth. Experiments using embryonic crocodiles and chickens tested the hypothesis that developmental plasticity during prenatal stages allows variation in limb proportions via environmentally-triggered movement. Crocodiles, which are ectothermic and not directly incubated, provide an opportunity to explore whether motility and limb proportions are modified by incubation temperature. Embryos incubated at extremes of the normal range (32°C and 28°C) exhibited altered motility and limb proportions. Measurements of body size, limb length, and limb element lengths were taken. Embryonic chickens were used to explore the hypothesis, testing whether pharmacological