Do toads have a jump on how far they hop? Pre-landing activity timing and intensity in forelimb muscles of hopping Bufo marinus

Abstract

During jumping or falling in humans and various other mammals, limb muscles are activated before landing, and the intensity and timing of this pre-landing activity are scaled to the expected impact. In this study, we test whether similarly tuned anticipatory muscle activity is present in hopping cane toads. Toads use their forelimbs for landing, and we analysed pre-landing electromyographic (EMG) timing and intensity in relation to hop distance for the m. coracoradialis and m. anconeus, which act antagonistically at the elbow, and are presumably important in stabilizing the forelimb during landing. In most cases, a significant, positive relationship between hop distance and pre-landing EMG intensity was found. Moreover, pre-landing activation timing of m. anconeus was tightly linked to when the forelimbs touched down at landing. Thus, like mammals, toads appear to gauge the timing and magnitude of their impending impact and activate elbow muscles accordingly. To our knowledge these data represent the first demonstration of tuned pre-landing muscle recruitment in anurans and raise questions about how important the visual, vestibular and/or proprioceptive systems are in mediating this response.

Figure 1.

Representative EMG traces for m. coracoradialis (CR) and m. anconeus (ANC) during a long, 44 cm hop (a) and short, 20 cm hop (b) in the same animal. The vertical dashed lines represent takeoff (when the hindlimbs leave the ground). The grey boxes highlight the 50 ms intervals used for assessing pre-landing EMG intensity (i.e. the right edge of the boxes represents forelimb touch-down). Note the higher average pre-landing signal intensities during the long hop.

Figure 2.

Plots of onset timing and relative EMG intensity in m. coracoradialis (a,c) and m. anconeus (b,d) as a function of hop distance. In (a,b) black symbols represent the onset timing of muscle activity and grey lines represent the best-fit line for the time of forelimb impact in these hops for comparison. In (c,d) intensities are scaled within each animal relative to the largest value observed. Different symbols represent different individuals and lines are drawn for any significant relationships (p < 0.05). Note that the onset of m. coracoradialis activity is not related to hop distance, but its pre-landing intensity increases significantly with hop distance. The onset of m. anconeus activity gets delayed with increasing hop distance paralleling the time of forelimb impact; its pre-landing intensity increases significantly with hop distance in two of four animals.