A unique case of convergency in mammals

Peter P. Gambaryan

Zoological Institute, Russian Academy of Sciences, Universitetskaya nab., 1, St. Petersburg, 199034, Russia

It has been known that the humerus in moles is very wide. For understanding the reasons of its rearrangement a scheme of evolutionary interrelations of the humerus and m. pectoralis in moles is proposed (Fig. 1, A, B). Shown in this scheme are the main directions of the m. pectoralis action in evolutionary alteration of the humerus position from the normal (a), i.e. initial position characteristic of mole ancestors and other terrestrial mammals up to the mole humerus directed forward (c). In the first case (a) m. pectoralis is an adductor of mammal shoulder. If the end of the m. pectoralis had not been displaced on the medial surface of the humerus (dashed line in Fig. 1, B) in its evolutionary rearrangement this muscle would have not been able to act as an abductor of the mole shoulder, which is necessary for sprawling soil during the mole digging.

Expansion of the mole humerus on its medial side is an adaptation to shoulder abduction, which is the major function during the digging. However the majority of researchers believe that the main function of the mole humerus is its rotation (Campbell, 1939; Reed, 1951, and others). They suggest that the humerus plane lies in the parasagittal plane at the final phase of the lateral "stroke". However in this case the main abductors of the humerus (mm. teres major and pectorales) would have been in extremely disadvantageous position because directions of their propulsive forces would have coincided with the longitudinal axis of the humerus. This situation would have resulted in a loss of abductor moment and a complete neutralization of the humerus expansion significance accordingly.

Our cinoradiographic analysis of the mole digging indicates that plane of its humerus never lies parasagittally but it is directed perpendicularly to saggital plane during the whole period of sprawling soil by hands (Fig. 1, C and D). In that case the abductor moment proves to be maximal. For limitation of the humerus rotation the unique mechanism is observed also in these animals: m. flexor digitorum profundus is converted to special tendon which connects five claw phalanxes with medial epicondylus of the humerus. The increase of the hand pressure on the soil during the mole digging involves automatically the augment of tension of the m. flex. digitorum profundus, which hinders the rotative moments of the humerus abductors.

Fig. 1. Schematic model of the evolutionary development of the humerus expancion in moles (A, B) and the positions of the humerus during the mole digging (C, D). A - humerus positions in relation to manubrium sterny of most terrestrial mammals; B - humerus positions in relation to manubrium sterny of moles. a - normal, i. e. initial humerus position characteristic of mole ancestors and other terrestrial mammals; b - intermediate humerus position in evolutionary rearrangement; c - humerus position of mole

Therefore the greater the abduction force of the shoulder, the stronger the tension of m. flex. digitorum profundus tendon. The expansion of the humerus proximal end increases the abduction moment and it is necessary for the sprawling soil by hands in moles. The dilation in the region of medial epicondylus of the mole humerus increases the tension of the m. flex. digitorum profundus tendon. This hinders humerus rotation and prevents the reduction of abduction moment of the main muscles (mm. teres major, pectoralis, subscapularis and latissimus dorsi) ensuring the propulsion of mole hands during the excavation of long tunnels for feeding.

Fig. 2. Cinoradiography of propulsive phase of the forelimb: A - "pronator" mode of locomotion in Tachyglossus aculeatus (Jenkins, 1970, with modification); B - "retractor" mode of locomotion in Varanus exanthematicus (Jenkins, Goslow, 1983, with modification)

It is necessary to note that the the humerus of both primary mammals and their close ancestors - Cynodonts and recent Monotremata is as wide as that in moles. The wide humerus of Monotremata is the special adaptation to unique locomotion of tetrapods with the sprawling posture of the limbs. The main function of such a wide humerus is its rotation, which increases the step length of the terrestrial locomotion (Jenkins, 1970). In contrast to Monotremata the main mechanism of the step lengthening in other tetrapods is retraction of the shoulder in propulsive phase of their locomotion. Since the main function of the humerus in propulsive phase in Monotremata is the pronation this mode of locomotion is denoted here as "pronator" contrary to "retractor" mode characteristic of other tetrapods with the sprawling posture of the limbs. It is shown in Fig. 2 that the longitudinal axis of the shoulder in Tachyglossus is directed nearly perpendicularly to the longitudinal axis of the body during propulsive phase (A) and this axis of the humerus in Varanus exanthematicus retracts up to more than 70 (B). In contrast to the special mechanism limiting humerus rotation in moles the different adaptation hindering humerus abduction was developed in Monotremata: their m. infraspinatus prevents the protraction and m. supraspinatus - the retraction of the humerus.

Thus the wide humerus of recent Monotremata (and of some mammals ancestors, probably) and the convergent expansion of the humerus in moles were developed owing to the diametrically opposite functions. In the first case the initially wide humerus is the adaptation to its rotation with the limitation of abduction ensuring horizontal state of the hand at the sprawling posture of the limbs. On the contrary the expansion of the humerus in Talpidae is the adaptation to its abduction with limitation of rotation ensuring parasagittal state of the hands during the digging. This main active motion of the mole limbs integrates both loosening and both throwing the earth in single process of the sprawling soil by hands.

Reference

Campbell, B. 1939. The shoulder anatomy of the moles. A study in phylogeny and adaptations. Amer. J. Anat., 64: 1-69.

Gambaryan, P.P. 1960. Prisposobitel'nie osobennosti organov dvigenia rojuschich mlekopitajuschich ( Adaptive features of locomotion organs in burrowing mammals). Yerevan, Akad. Nauk Armenian SSR Publ. Office, 195 p. (In Russian).

Jenkins, F.A. Jr. 1970. Limb movements in a Monotremes (Tachyglossus aculeatus): a cinoradiographic analysis. Science, 168: 1473-1475.

Jenkins, F.A. Jr. & G.E Goslow. 1983. The functional anatomy of the shoulder of the savannah monitor lizard (Varanus exanthematicus). J. Morphology,175: 195-216.

Reed, C.A. 1951. Locomotion and appendicular anatomy in three soricoid insectivores. Am. Midl. Nat., 45: 513-617.