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

Force plate trials

We caught five northern short-tailed shrews (Eulipotyphla: Blarina brevicauda; body mass (m_b) 22.7 ± S.D. 3.8 g) and four meadow voles (Rodentia: Microtus pennsylvanicus; m_b = 29.3 ± 7.8 g) in live traps near Ithaca, NY. Each animal was used in our experiments within 12 h of trapping and then released at its point of capture. The Institutional Animal Care and Use Committee at Cornell University approved all protocols pertaining to this project, and it was approved by the Cornell Center for Animal Resources and Education (protocol 2003-0055). We used trapping protocols in accordance with the recommendations of the American Society of Mammalogists (Choate et al., 1998).

We placed each animal in a Plexiglas enclosure (0.48 m length, 0.15 m width, 0.11 m height) that had a force-measuring platform (0.15 m length, 0.15 m width) in the center of its floor. As animals moved back and forth across the platform, we measured ground reaction forces in three dimensions at 1,000 Hz. The plate had a resonant frequency of ≥128 Hz in all directions. We calibrated plates for load response immediately prior to data collection each day, and found linear correlations of force to output voltage within the range of force magnitudes we recorded in our experiments (r² > 0.999). Drift in the baseline signal was corrected by zeroing signals in each trial with the output voltage of the unloaded plate. The signals from the force plate were post-processed using a 58–62 Hz Butterworth filter to remove AC noise and a Butterworth lowpass filter at 25 Hz to improve the signal to noise ratio overall. Details of plate construction, calibration, and performance are further described elsewhere (Riskin, Bertram & Hermanson, 2005; Riskin et al., 2006).

While an animal moved across the plate, we recorded video at 250 Hz with a MotionMeter 250 digital high-speed camera (Redlake Systems, San Diego CA, USA). The camera recorded simultaneous lateral and dorsal views of the animal from its position ca. 2 m away, with the assistance of a mirror above the enclosure. Videos were synchronized with force platform recordings to within 0.004 s using the methods of Riskin, Bertram & Hermanson (2005); Riskin et al. (2006).

From each trial, we isolated a single stride cycle, beginning and ending with the footfall of the hindlimb facing the camera, where the animal did not make contact with any part of the cage other than the force plate surface. Average speed for the trial was calculated as the horizontal distance traversed by the tip of the nose divided by the period of the stride cycle, and stride frequency was calculated as the inverse of stride period. We discarded trials where the animal stopped and then resumed locomotion mid-sequence, keeping only trials in which the animal kept moving throughout the stride cycle.

Verification of differences in posture

To visualize the posture of these animals, two other B. brevicauda (m_b 18.1 and 22.1 g) and three other M. pennsylvanicus (m_b 20.5, 26.0, and 34.9 g) were filmed in lateral view, using a Philips Easy Diagnost Eleva cineflouroscopic system (Philips N.V., Utrecht, Netherlands). Images were collected at 8 Hz while animals crawled and stood. Digitized images were analyzed to confirm that the postures of the two species differed as predicted. We noted selected bone position and joint angles at several stance phases of the step cycle (early, middle or late stance) and during standing.

Calculations and statistical analyses

We calculated acceleration, velocity, and position of the COM using numerical integration of the ground reaction forces recorded as animals crossed the force plates. Horizontal forces (fore-aft and mediolateral) and vertical force minus the product of mass and the acceleration due to gravity (g = 9.81 m s⁻¹) were divided by the animal’s body mass to arrive at instantaneous acceleration of the COM. Acceleration was integrated to calculate instantaneous velocity, and vertical velocity was integrated to find height of the COM. To obtain constants for integration of acceleration (initial velocity), we used a custom-made program in Matlab 7.2.0 (MathWorks Inc., Natick, MA, USA) to digitize the movement of the nose tip during the video sequence. To arrive at initial velocity estimates, a linear least-squares best-fit line was calculated for fore-aft and mediolateral changes in nose position over the 10 camera frames (0.04 s) prior to the stride cycle. Changes in the pitch of the body prevented using this method for initial vertical velocity, so we calculated an initial velocity that resulted in a change in height of the COM that matched the net change in height of the nose tip in the video of the stride cycle. The constant for integration of vertical velocity (initial height) was assigned a value of 0 m.

We defined the fore-aft direction as parallel to the horizontal component of the nose’s change in position from the first to last camera frames of the stride cycle, and the perpendicular horizontal direction as mediolateral. Kinetic energy in the fore-aft direction was calculated using the equation $E_{KF}=0.5m_b{v}_F^2$, where v_F is the fore-aft velocity. Mediolateral kinetic energy (E_KL) and vertical kinetic energy (E_KV) were calculated analogously, but using mediolateral velocity and vertical velocity, respectively, instead of v_F. Total kinetic energy was the sum of those three components (E_K = E_KF + E_KL + E_KV). Gravitational potential energy was calculated as E_P = mgh, where h is the height of the COM. Total energy (E_TOT) was calculated as the sum of E_K and E_P.

Power used to increase any component of energy was calculated as the sum of positive increments in that component of energy over the stride cycle divided by stride period. Power used to increase E_P is denoted by P_EP, and power used to increase E_K, E_KF, E_KL, E_KV, and E_TOT are denoted by P_EK, P_EKF, P_EKL, P_EKV, and P_ETOT, respectively. Body mass-specific power values, were calculated by dividing power by m_b.

We used two-tailed t-tests of all trials to verify that the speeds of shrew and vole trials were not significantly different. For all other comparisons between species, however, we pooled all the trials of a given individual, and used individual means for statistical testing. Using one-tailed Student’s t-tests (Zar, 1999), we tested the hypothesis that shrews had larger peak-magnitude forces in the mediolateral axis than voles did. We also used one-tailed t-tests to test the hypotheses that each component of body mass-specific power (P_EKF, P_EKL, P_EKV, P_EK, and P_EP) would be higher for shrews than for voles.

The influence of speed on stride frequency in each species was assessed using a Standard Least Squares model of all trials with the individual treated as a random effect (Riskin et al., 2010).

Posture

The postures of shrews were more crouched than those of voles. The shrew humerus was often held such that the elbow was dorsal relative to the shoulder joint (Fig. 1) as revealed by an angle of 30° above horizontal (range from −39° at the beginning of stance phase to +39° at middle to late stance phase). This humeral orientation accounted in large part for the crouching posture seen in standing or crawling shrews. The humerus of the voles tended to be held close to or below horizontal throughout the stance phase of locomotion or during quiet standing. There did not appear to be large differences between the shrew and vole in terms of the orientation of the radius/ulna during locomotion.

The shrew femur tended to remain near horizontal (range from −2 to −27°) during locomotion. In contrast the vole’s femur spanned a larger range (from 23° above horizontal during early stance to −70°, or well below horizontal during high stance or late in the step cycle).

Force plate recordings

We recorded 17 trials with shrews traveling at speeds of 0.23 to 0.84 m s⁻¹ and 22 trials with voles at speeds of 0.18 to 0.51 m s⁻¹ (Table 1). The speeds of shrew trials (mean ± S.D. = 0.42 ± 0.15 m s⁻¹, N = 17) and vole trials (0.37 ± 0.09 m s⁻¹, N = 22) did not differ significantly (two-tailed t = 1.33; DF = 23.84; P = 0.19). Both species demonstrated a linear increase of stride frequency with speed (shrews: F(5, 11) = 13.89; P = 0.0002; r² = 0.86; voles: F(4, 17) = 17.41; P < 0.0001; r² = 0.80; Fig. 2).

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 (P_ETOT∕m_b) 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 (P_EK∕m_b) 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). P_EKF∕m_b and P_EKL∕m_b 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). P_EKV∕m_b was not significantly different between species (one-tailed t = − 0.99; DF = 3.60, P = 0.81; Fig. 3), and neither was P_EP∕m_b (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.