Figure 4.

Correlation Tests between Stroke Frequency, Speed, and Habitat Preference with Associated Energy Expenditure and Relation with Body Size

(A) A significant positive correlation is observed between stroke frequency and speed in species employing the tripod gait using both Spearman correlation test on raw data (rho raw: 0.78, adjusted p: 3.8e−2) and with phylogenetic correlation (rho PIC: 0.92, adjusted p: 3.6e−3).

(B) There is no correlation between stroke frequency and speed when we add species employing the rowing gait to the dataset using both Spearman correlation test on raw data (rho raw: −0.30, adjusted p: 3.3e−1, n.s.) and with phylogenetic correlation (rho PIC: 0.02, adjusted p: 0.97, n.s.). Sample sizes in term of number of videos are given in Table S1.

(C) Species using tripod gait (blue dots) spend less energy per stroke (p: 0.08, n.s.; Student’s t test).

(D) However, when normalized to the number of stroke employed per second, species using the rowing gait spend less energy (Student’s t test; p: 4.5e−5). Sample sizes and energy expenditure are given in the Supplemental Experimental Procedures and Table S1, respectively.

(E) High stroke frequency can be observed only in animals with small body size.

See also Figures 3, S3, and S4 and Tables S1 and S2.