SENSITIVITY OF THE TRUNK OF ASIAN ELEPHANTS FOR TEXTURE DIFFERENCES OF ACTIVELY TOUCHED OBJECTS

In the present study the capability of Asian elephants to discriminate objects by active touch with their trunk was quantified for the first time by psychophysical techniques. The experiment was conducted at 'A llwetterzoo Munster', Germany, using two female adult elephants (Rada and Tefi). In a two alternative forced choice procedure the elephants were required to discriminate textured surfaces. The stimuli were squa re plates (10x10 cm) made of smooth perspex, each with a set of linear grooves While the ridge width was held constant at 2 mm, the groove w idth varied from 10 down to 2 mm (in steps of 1.0, 0.5 and 0.1 mm). Th e stimuli were presented in a box, which was completely closed at the position of the test animal. In this way vision was excluded. Elephant s had access to the stimuli through a sealable opening at the front si de of the box. The stimuli were fitted to backwards moveable, U-shaped frames, which were in a vertical position at the beginning of a trial . The test animal responded by pushing a stimulus with its trunk from the vertical position backwards. In case of a correct trial a short wh istle supplied the animal with immediate feed back. The interpolated s ize of the absolute difference threshold at 75% correct choices was 0. 25 mm groove width difference (13%) for Rada, and 0.44 mm groove width difference (22%) for Tefi. This difference in performance between bot h test animals may not reflect a rear difference in sensitivity, but c an be explained by the animals' different tactual behaviours. While Ra da always touched the stimuli with the inner side of the bulge of the trunk, wetted by secretion. Tefi stroked the surfaces with the dry bac k of the trunk finger. There is no information whether these two skin areas are differentially innervated. However, studies of human tactual discriminations have shown that the presence of an intermediate layer , like the secretion at the inner side of the trunk, enhances performa nce by reducing shear forces acting on the skin.