Fig. 1

Placement of the additional load in the backpack. a Placement of the Velcro, used for closing at the dorsal side, on the chamois leather backpack. b dorsal view of a closed backpack. c Load, prepared from leaden strips is symmetrically piled (1 cm from each other) on a piece of Velcro, which can be attached to the corresponding velcro on the backpack, thus closing the backpack tightly. d Rat carrying a backpack without load. The additional load was attached to corresponding velcro on the backpack, therefore, the leaden strips were always located bilateral from the spine, starting above the armpit