Dense reconstruction of elephant trunk musculature

Abstract

The elephant trunk operates as a muscular hydrostat1Kier W.M. Smith K.K. Tongues, tentacles and trunks: the biomechanics of movement in muscular-hydrostats.Zool. J. Linn. Soc. 1985; 83: 307-324Crossref Google Scholar,2Smith K.K. Kier W.M. Trunks, tongues, and tentacles: moving with skeletons of muscle.Am. Sci. 1989; 77: 28-35Google Scholar and is actuated by the most complex musculature known in animals.3Cuvier G. MacGillivray W. The Animal kingdom of the Baron Cuvier.J. Nat. Hist. 1839; 31Google Scholar,4Shoshani J. It's a nose! It's a hand! It's an elephant's trunk!.Nat. Hist. 1997; 106: 36-45Google Scholar Because the number of trunk muscles is unclear,5Hooper S. Schulz A. Proboscidea morphology.in: Encyclopedia of Animal Cognition and Behavior. Springer International Publishing, 2020: 5660-5665https://doi.org/10.1007/978-3-319-47829-6_1320-1Crossref Google Scholar we performed dense reconstructions of trunk muscle fascicles, elementary muscle units, from microCT scans of an Asian baby elephant trunk. Muscle architecture changes markedly across the trunk. Trunk tip and finger consist of about 8,000 extraordinarily filigree fascicles. The dexterous finger consists exclusively of microscopic radial fascicles pointing to a role of muscle miniaturization in elephant dexterity. Radial fascicles also predominate (at 82% volume) the remainder of the trunk tip, and we wonder if radial muscle fascicles are of particular significance for fine motor control of the dexterous trunk tip. By volume, trunk-shaft muscles6Shoshani J. On the dissection of a female Asian elephant (Elephas maximus maximus Linnaeus, 1758) and data from other elephants.Elephant. 1982; 2: 3-93https://doi.org/10.22237/elephant/1521731887Crossref Google Scholar comprise one-third of the numerous, small radial muscle fascicles; two-thirds of the three subtypes of large longitudinal fascicles (dorsal longitudinals, ventral outer obliques, and ventral inner obliques);7Wilson J.F. Mahajan U. Wainwright S.A. Croner L.J. A continuum model of elephant trunks.J. Biomech. Eng. 1991; 113: 79-84https://doi.org/10.1115/1.2894088Crossref PubMed Scopus (29) Google Scholar,8Endo H. Hayashi Y. Komiya T. Narushima E. Sasaki M. Muscle architecture of the elongated nose in the Asian elephant (Elephas maximus).J. Vet. Med. Sci. 2001; 63: 533-537Crossref PubMed Scopus (14) Google Scholar,9Wilson J.F. Compliant Robotic Structures. Department of Civil and Environmental Engineering, Duke University, 1985Crossref Google Scholar and a small fraction of transversal fascicles. Shaft musculature is laterally, but not radially, symmetric. A predominance of dorsal over ventral radial muscles and of ventral over dorsal longitudinal muscles may result in a larger ability of the shaft to extend dorsally than ventrally10Schulz A.K. Boyle M. Boyle C. Sordilla S. Rincon C. Hooper S. Aubuchon C. Reidenberg J.S. Higgins C. Hu D.L. Skin wrinkles and folds enable asymmetric stretch in the elephant trunk.Proc. Natl. Acad. Sci. USA. 2022; 119e2122563119Crossref Scopus (5) Google Scholar and to bend inward rather than outward. There are around 90,000 trunk muscle fascicles. While primate hand control is based on fine control of contraction by the convergence of many motor neurons on a small set of relatively large muscles, evolution of elephant grasping has led to thousands of microscopic fascicles, which probably outnumber facial motor neurons.