Kinematics of chisel-tooth digging by African mole-rats

Abstract

Mole-rats are known to use their protruding, chisel-like incisors to dig underground networks of tunnels, but it remains unknown how these incisors are used to break and displace the soil. Theoretically, different excavation strategies can be used. Mole-rats could either use their head depressor muscles to power scooping motions of the upper incisors (by nose-down head rotations) or the lower incisors (by nose-up head rotations), or their jaw adductors to grab and break the soil after penetrating both sets of incisors into the ground, or a combination of these mechanisms. To identify how chisel-tooth digging works, a kinematic analysis of this behaviour was performed based on high-speed videos of 19 individuals from the African mole-rat species placed inside transparent tubes in a laboratory setting. Our analysis showed that the soil is penetrated by both the upper and lower incisors at a relatively high gape angle, generally with the head rotated nose-up. Initially, the upper incisors remain approximately stationary to function as an anchor to allow an upward movement of the lower incisors to grab the soil. Next, a quick, nose-down rotation of the head further detaches the soil and drops the soil below the head. Consequently, both jaw adduction and head depression are jointly used to power tooth-digging in The same mechanism, but with longer digging cycles, and soil being thrown down at smaller gape sizes, was used when digging in harder soil.

The impact of digging on the evolution of the rodent mandible

“…Chisel‐tooth diggers have clearly evolved enlarged upper and lower incisors. This result adds to the evidence that chisel‐tooth digging rodents use both their upper and lower incisors, operated by their enlarged masticatory, neck and back muscles, to dig (Van Wassenbergh et al, ).…”

“…Lower incisor procumbency, unlike that of the upper incisors, does not correlate with digging method within subterranean rodents (Landry, ; Stein, ), which could lead to the assumption that chisel‐tooth digging rodents dig with their more procumbent upper incisors. However, a recent in vivo kinematic study of the chisel‐tooth digging mole‐rat Fukomys micklemi showed that, in fact, both the upper and lower incisors are used in concert with both jaw adductor and head depressor muscles (Van Wassenbergh, Heindryckx, & Adriaens, ). Assuming all chisel‐tooth diggers use both their upper and lower incisors to dig, we can hypothesise that the lower incisors of chisel‐tooth diggers have adapted to dig in a similar manner to the upper incisors.…”

Evomimetics: the biomimetic design thinking 2.0

“…Frequently, a single apparatus performs multiple functions. In some rodents, such as subterranean mole-rats, the biological role of the jaw apparatus is then to feed, to fight and to dig protective tunnels [38,39].…”

Mandible strength and geometry in relation to bite force: a study in three caviomorph rodents

“…; Becerra, ; Van Wassenbergh et al. ) and during territory defense and aggressive encounters between males (Vassallo & Busch, ; Zenuto et al. ; Becerra et al.…”

“…Cranial anatomy and bite forces vary greatly among species due to differences in diet (e.g. Samuels, 2009) and the engagement of incisors in other behaviors and functions, such as digging (Van Daele et al 2009;Becerra et al 2011;Becerra, 2015;Van Wassenbergh et al 2017) and during territory defense and aggressive encounters between males (Vassallo & Busch, 1992;Zenuto et al 2002;Becerra et al 2012a).…”