Bone adaptation is modulated by the timing, direction, rate and magnitude of mechanical loads. To investigate whether frequent slow, or infrequent fast, gaits could dominate bone adaptation to load, we compared scaling of the limb bones from two mammalian herbivore clades that use radically different high-speed gaits, bipedal hopping (suborder Macropodiformes; kangaroos and kin) and quadrupedal galloping (order Artiodactyla; goats, deer and kin). Forelimb and hindlimb bones were collected from 20 artiodactyl and 15 macropod species (body mass M 1.05–1536 kg) and scanned in computed tomography or X-ray microtomography. Second moment of area (Imax) and bone length (l) were measured. Scaling relations (y = axb) were calculated for l versus M for each bone and for Imax versus M and Imax versus l for every 5% of length. Imax versus M scaling relationships were broadly similar between clades despite the macropod forelimb being nearly unloaded, and the hindlimb highly loaded, during bipedal hopping. Imax versus l and l versus M scaling were related to locomotor and behavioural specializations. Low-intensity loads may be sufficient to maintain bone mass across a wide range of species. Occasional high-intensity gaits might not break through the load sensitivity saturation engendered by frequent low-intensity gaits.
Bone adaptation is modulated by the timing, direction, rate, and magnitude of mechanical loads. To investigate whether frequent slow, or infrequent fast, gaits could dominate bone adaptation to load, we compared scaling of the limb bones from two mammalian herbivore clades that use radically different high-speed gaits, bipedal hopping and quadrupedal galloping. Forelimb and hindlimb bones were collected from 20 artiodactyl and 15 diprotodont species (body mass M 1.05 -1536 kg) and scanned in clinical computed tomography or X-ray microtomography. Second moment of area (I max ) and bone length (l) were measured. Scaling relations (y = ax b ) were calculated for l vs M for each bone and for I max vs M and I max vs l for every 5% of length. I max vs M scaling relationships were broadly similar between clades despite the diprotodont forelimb being nearly unloaded, and the hindlimb highly loaded, during bipedal hopping. I max vs l and l vs M scaling were related to locomotor and behavioural specialisations. Low-intensity loads may be sufficient to maintain bone mass across a wide range of species. Occasional high-intensity gaits might not break through the load sensitivity saturation engendered by frequent low-intensity gaits. IntroductionDuring daily rest and activity, bones experience a range of mechanical loading conditions that relate to each behaviour's physical intensity. Bones respond anabolically, that is, by increasing bone tissue formation and decreasing bone resorption, when they experience a small number of novel high strain and high strain rate events with a rest period between bouts of loading [1,2]. Repetitive loading has a saturation or habituation effect, in which tissue is no longer responsive to mechanical loads after a few tens of cycles [2]. Large numbers of loading cycles without sufficient rest are associated with fatigue or 'stress' fractures, typically seen in new military recruits [3] and racing animals such as greyhounds [4] and horses [5,6]. The distributions of occasional maximal loads and habitual moderate loads vary within the skeleton and depend on locomotor activity, which should appear as a morphological signal in clades that adopt very different characteristic gaits [7].
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