Are Continuum Robots Created Looking at Elephant Trunk Physiology?

Elephants are the only terrestrial animals with a trunk weighing over 100 kg. Trunk is a boneless flexible appendage that enables breathing, grabbing items, storing water for drinking and spraying over the body, and olfaction. Recently, it has been found by Schulz et al that trunks are used for suction feeding too. Suction feeding is using airflow to pick objects, and this requires coordination of specialized body parts and pressure. Suction feeding was previously thought to be only used by aquatic organisms as it requires highly specialized morphology.

However, elephants evolved to have trunks that are capable of suction feeding without complex aquatic morphology. Trunks consist of structures called muscular hydrostats which are muscle fibers that are surrounded by sheaths of skin. Muscular hydrostats allow the trunk to have coordinated contractions of antagonist muscle which let the trunk bend, elongate, and shorten through shape changes that rely on incompressibility of trunk tissues and this is possible because composition of complex muscle arrangement as shown in drawing below (Dagenais et al, 2021).

In female African elephant, Schulz et al investigated suction feeding through mathematical calculations and measurement of duration, speed, and pressure that is applied by the trunk.

When <10 rutabaga cubes were given, the elephant used prehensile grabbing but when >10 cubes were given the elephant used suctioning. When tortilla, a flat chip was placed near elephant which is challenging to grab, elephant used suction to grab it without breaking it. This indicates that the elephant applies lung pressure based on object quantity, shape, and size as seen in figure 1b graph below. Investigators also found that elephant trunk capacity for water is 5.2L, however, the elephant was able to store 5.5L of water. Therefore, investigators hypothesized that elephants can increase their trunk volume using muscular action. After

testing this hypothesis through ultrasonographic imaging, the investigators found that when the trunk pumps water, the trunk muscles can dilate the volume of the nasal passages by 64% to carry more water.

Suction feeding and storing water is possible because of lung pressure. Elephants have specialized respiratory systems which consist of distensible network of collagen fibres that fill the pleural space and allow connections between lungs and chest wall leading to constrain movements that generate 150ms-1 of air velocity. Elephants can generate such high air velocity and have -20kPa of maximum lung pressure because they have thick endothoracic fascia.

Endothoracic fascia is the outermost membrane of thoracic cavity which separates the internal thoracic artery from pleura and the thicker it is, the more lung pressure animal can generate (Wikipedia Foundation, 2018).

Elephants are also able to generate high lung pressures because they have high vital lung capacity. Moreover, through power law best fit equation it was found that nostril size increases with body mass and helps in suction feeding because larger nostril allows for more air speeds hence, being able to quickly grab objects.

Investigating through a mathematical model Schulz et al were able to conclude that together with large nostrils, high lung pressure, and muscle hydrostats elephants can use trunks to suction feed unlike other mammals. All these functions of the trunk are an inspiration for creation of continuum robots.