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The crouching of the shrew: Mechanical consequences of limb posture in small mammals.

Riskin DK, Kendall CJ, Hermanson JW - PeerJ (2016)

Bottom Line: It is hypothesized that one consequence of these transitions was a decrease in the total mechanical power required for locomotion, because side-to-side accelerations of the body have become smaller, and thus less costly with changes in limb orientation.The most power consumed for both species was that used to accelerate the body in the direction of travel, and this was much larger for shrews than for voles (P = 0.01).We conclude that side-to-side accelerations are negligible for small mammals-whether crouching or more upright-compared to their sprawling ancestors, and that a more upright posture further decreases the cost of locomotion compared to crouching by helping to maintain the body's momentum in the direction of travel.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biomedical Sciences, College of Veterinary Medicine, Cornell University,IthacaNY, United States; Current affiliation: Department of Biology, University of Toronto Missisauga,Mississauga, Ontario, Canada.

ABSTRACT
An important trend in the early evolution of mammals was the shift from a sprawling stance, whereby the legs are held in a more abducted position, to a parasagittal one, in which the legs extend more downward. After that transition, many mammals shifted from a crouching stance to a more upright one. It is hypothesized that one consequence of these transitions was a decrease in the total mechanical power required for locomotion, because side-to-side accelerations of the body have become smaller, and thus less costly with changes in limb orientation. To test this hypothesis we compared the kinetics of locomotion in two mammals of body size close to those of early mammals (< 40 g), both with parasagittally oriented limbs: a crouching shrew (Blarina brevicauda; 5 animals, 17 trials) and a more upright vole (Microtus pennsylvanicus; 4 animals, 22 trials). As predicted, voles used less mechanical power per unit body mass to perform steady locomotion than shrews did (P = 0.03). However, while lateral forces were indeed smaller in voles (15.6 ± 2.0% body weight) than in shrews (26.4 ± 10.9%; P = 0.046), the power used to move the body from side-to-side was negligible, making up less than 5% of total power in both shrews and voles. The most power consumed for both species was that used to accelerate the body in the direction of travel, and this was much larger for shrews than for voles (P = 0.01). We conclude that side-to-side accelerations are negligible for small mammals-whether crouching or more upright-compared to their sprawling ancestors, and that a more upright posture further decreases the cost of locomotion compared to crouching by helping to maintain the body's momentum in the direction of travel.

No MeSH data available.


Ground reaction forces, COM velocities, and COM kinetics during a stride cycle for (A) a Short-tailed Shrew (mass = 27.0 g; mean forward velocity = 0.45 m s−1) and (B) a Meadow Vole (23.8 g; 0.41 m s−1).Thick black lines are experimental data. Thin lines represent zero on most plots, except Vertical Force and Fore-aft velocity, where thin lines represent body weight and mean forward velocity, respectively.
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fig-4: Ground reaction forces, COM velocities, and COM kinetics during a stride cycle for (A) a Short-tailed Shrew (mass = 27.0 g; mean forward velocity = 0.45 m s−1) and (B) a Meadow Vole (23.8 g; 0.41 m s−1).Thick black lines are experimental data. Thin lines represent zero on most plots, except Vertical Force and Fore-aft velocity, where thin lines represent body weight and mean forward velocity, respectively.

Mentions: As predicted, peak lateral forces were larger for shrews (26.4 ± 10.9% body weight, N = 5) than for voles (15.6 ± 2.0%; N = 4; one-tailed t = 2.16, DF = 4.34; P = 0.046). Total mechanical power per unit mass (PETOT∕mb) used by shrews during locomotion was greater than that used by voles (one-tailed t = 2.24; DF = 6.78; P = 0.03; Fig. 3). Power used to change kinetic energy (PEK∕mb) was greater in shrews than in voles (one-tailed t = 3.18; DF = 4.97; P = 0.01; Fig. 3). This was not simply the consequence of differences in net acceleration, as net change in speed over the stride cycle did not differ significantly between the two species (shrews: 16.0 ± 3.9%, voles: 19.8 ± 4.5%; two-tailed t = 1.34, DF = 6.14, P = 0.89). PEKF∕mb and PEKL∕mb were both significantly higher for shrews than for voles (one-tailed t = 3.50; DF = 4.33, P = 0.01; and one-tailed t = 2.23; DF = 4.22, P = 0.04, respectively; Fig. 3). PEKV∕mb was not significantly different between species (one-tailed t = − 0.99; DF = 3.60, P = 0.81; Fig. 3), and neither was PEP∕mb (one-tailed t = − 0.31; DF = 5.28, P = 0.62; Fig. 3). Representative force, velocity, and energy profiles over the course of a single stride cycle are shown in Fig. 4.


The crouching of the shrew: Mechanical consequences of limb posture in small mammals.

Riskin DK, Kendall CJ, Hermanson JW - PeerJ (2016)

Ground reaction forces, COM velocities, and COM kinetics during a stride cycle for (A) a Short-tailed Shrew (mass = 27.0 g; mean forward velocity = 0.45 m s−1) and (B) a Meadow Vole (23.8 g; 0.41 m s−1).Thick black lines are experimental data. Thin lines represent zero on most plots, except Vertical Force and Fore-aft velocity, where thin lines represent body weight and mean forward velocity, respectively.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4933088&req=5

fig-4: Ground reaction forces, COM velocities, and COM kinetics during a stride cycle for (A) a Short-tailed Shrew (mass = 27.0 g; mean forward velocity = 0.45 m s−1) and (B) a Meadow Vole (23.8 g; 0.41 m s−1).Thick black lines are experimental data. Thin lines represent zero on most plots, except Vertical Force and Fore-aft velocity, where thin lines represent body weight and mean forward velocity, respectively.
Mentions: As predicted, peak lateral forces were larger for shrews (26.4 ± 10.9% body weight, N = 5) than for voles (15.6 ± 2.0%; N = 4; one-tailed t = 2.16, DF = 4.34; P = 0.046). Total mechanical power per unit mass (PETOT∕mb) used by shrews during locomotion was greater than that used by voles (one-tailed t = 2.24; DF = 6.78; P = 0.03; Fig. 3). Power used to change kinetic energy (PEK∕mb) was greater in shrews than in voles (one-tailed t = 3.18; DF = 4.97; P = 0.01; Fig. 3). This was not simply the consequence of differences in net acceleration, as net change in speed over the stride cycle did not differ significantly between the two species (shrews: 16.0 ± 3.9%, voles: 19.8 ± 4.5%; two-tailed t = 1.34, DF = 6.14, P = 0.89). PEKF∕mb and PEKL∕mb were both significantly higher for shrews than for voles (one-tailed t = 3.50; DF = 4.33, P = 0.01; and one-tailed t = 2.23; DF = 4.22, P = 0.04, respectively; Fig. 3). PEKV∕mb was not significantly different between species (one-tailed t = − 0.99; DF = 3.60, P = 0.81; Fig. 3), and neither was PEP∕mb (one-tailed t = − 0.31; DF = 5.28, P = 0.62; Fig. 3). Representative force, velocity, and energy profiles over the course of a single stride cycle are shown in Fig. 4.

Bottom Line: It is hypothesized that one consequence of these transitions was a decrease in the total mechanical power required for locomotion, because side-to-side accelerations of the body have become smaller, and thus less costly with changes in limb orientation.The most power consumed for both species was that used to accelerate the body in the direction of travel, and this was much larger for shrews than for voles (P = 0.01).We conclude that side-to-side accelerations are negligible for small mammals-whether crouching or more upright-compared to their sprawling ancestors, and that a more upright posture further decreases the cost of locomotion compared to crouching by helping to maintain the body's momentum in the direction of travel.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biomedical Sciences, College of Veterinary Medicine, Cornell University,IthacaNY, United States; Current affiliation: Department of Biology, University of Toronto Missisauga,Mississauga, Ontario, Canada.

ABSTRACT
An important trend in the early evolution of mammals was the shift from a sprawling stance, whereby the legs are held in a more abducted position, to a parasagittal one, in which the legs extend more downward. After that transition, many mammals shifted from a crouching stance to a more upright one. It is hypothesized that one consequence of these transitions was a decrease in the total mechanical power required for locomotion, because side-to-side accelerations of the body have become smaller, and thus less costly with changes in limb orientation. To test this hypothesis we compared the kinetics of locomotion in two mammals of body size close to those of early mammals (< 40 g), both with parasagittally oriented limbs: a crouching shrew (Blarina brevicauda; 5 animals, 17 trials) and a more upright vole (Microtus pennsylvanicus; 4 animals, 22 trials). As predicted, voles used less mechanical power per unit body mass to perform steady locomotion than shrews did (P = 0.03). However, while lateral forces were indeed smaller in voles (15.6 ± 2.0% body weight) than in shrews (26.4 ± 10.9%; P = 0.046), the power used to move the body from side-to-side was negligible, making up less than 5% of total power in both shrews and voles. The most power consumed for both species was that used to accelerate the body in the direction of travel, and this was much larger for shrews than for voles (P = 0.01). We conclude that side-to-side accelerations are negligible for small mammals-whether crouching or more upright-compared to their sprawling ancestors, and that a more upright posture further decreases the cost of locomotion compared to crouching by helping to maintain the body's momentum in the direction of travel.

No MeSH data available.