Behavioral responses to partial-gravity conditions in rats

The effects of microgravity or hypergravity on living organisms have been studied extensively; however, thus far no studies have addressed the effects of “partial-gravity”, that is, the low-gravity levels between the unit gravity (1 G) on Earth and zero gravity (0 G) in space. The purpose of the present study was to examine behavioral responses in rats under partial-gravity conditions. Rat behavior was monitored by video cameras during parabolic flights. The flight trajectory was customized in order to generate graded levels of partial gravity. Gravity-dependent behavior patterns were observed in rats. In the conditions of 0.4 G through 0.2 G, rats showed startle and crouching. Hindlimb stretching emerged at 0.15 G and was more frequently observed toward 0.01 G. Different thresholds may exist for emotional and balance/posture-related behaviors.

► Graded levels of low-gravity were used as parameter to measure biological response.
► Rat behavior showed stereotypical responses related to specific low-gravity levels.
► Rats switched two antagonistic behaviors (crouching vs. stretching) at 0.2–0.15 G.
► The lunar gravity may evoke significant behavioral change in rats.