The transition from aquatic to terrestrial habitats was an event in vertebrate evolution that preceded a sudden radiation of species. Subsequently some vertebrate lineages have returned to their ancestral aquatic habitats. It is known that vertebrate bone structure can vary depending on habitat. The evolutionary explanation for this is attributable to the fact that loads on the skeleton varies depending on the environment organisms inhabit. Terrestrial vertebrates would be expected to experience greater loads on their bones versus aquatic vertebrates due to body support demands, but there are no experimental data to test this hypothesis or quantify the difference. We tested how loads differed on the appendicular skeleton between use in terrestrial and aquatic habitats by recoding in vivo femoral strains during swimming and walking in turtles. We predicted that since swimming exerts less force on the limbs, peak load magnitudes would be lower during swimming versus walking, but that load peaks would be nearly equal during the thrust and recovery phases of the swimming limb cycle. Our data support our first prediction, with average peak strain magnitudes of swimming being half those of walking. Loading regimes were similar between both swimming and walking with compressive axial strains experienced dorsally on the femur. However, our second prediction was not supported, because peak strains were much higher during the thrust phase. Our results indicate that even when environmental forces are lessened, limb muscles play a large role in the production of bone loads.
Sutton, J.; Blob, R.; and Young, V., "In vivo femoral strains in swimming turtles: Influence of locomotor medium on limb bone loading" (2014). Focus on Creative Inquiry. 30.