The behavior of Thick-toed Geckos during the flotation in weightlessness.

The purpose of the research was to study the behavior of 15 female Turner's Thick-toed Geckos (Chondrodactylus turneri GRAY 1864) during a 30-day orbital experiment on the unmanned spacecraft "BION-M" No. 1. In weightlessness, geckos maintained their ability to attach to the surfaces using the subdigital pads of their toes. On average, geckos spent in the attached position 99.91% of the video recorded flight time, and only 0.09% accounted for the flotations. In quiescent geckos, spontaneous detachments of some limbs (hanging) were also observed. The detachments of hind limbs occurred in 52.3% of cases, more often than all other possible detachments taken together. Upon detachment from the surface of all four limbs without the immediate recovery of attachment, the geckos started to float, while continuing to be motionless, perhaps being asleep or dormant. That is why all flotations were divided into 2 main groups: 1 - active (flotation begins in moving and exactly awake gecko); 2 - passive (flotation begins in quiescent, possibly sleeping gecko - PF). The PF group in turn was subdivided into: 1 - short flotation (SF), when the gecko drifted from the surface to a distance of no more than 2.5 cm, and 2 - long flotations (LF, the distance was more, than 2.5 cm). SFs were more frequent and accounted for 262 cases (58.2%) of the total number of flotations (450 cases).The active geckos, when starting to float, and the floating quiescent geckos, when resuming their active condition, immediately restored the attachment by a number of behavioral responses. The discovered geckos' responses to PF were similar to the behavioral reflexes triggered by a fall under terrestrial conditions: 1 - the ventral extension of the limbs, 2 - the skydiving posture, and 3 - postural righting reflex. Ventral extension of limbs was described in weightlessness for the first time. In SF it was enough to restore the attachment, but in LF we observed all 3 reactions with high frequency (91.5, 95.7 and 82.4% of all cases of LF of quiescent geckos correspondingly), although they did not lead to quick restoration of attachment. It is known from literature, that the skydiving posture in ground-based conditions creates useful aerodynamic forces, making gliding or directed aerial descent possible and reducing the impact force when landing. In weightlessness the dilated limbs increase the likelihood of contact of the flotating gecko with any surface and restore the attachment. This reflex actuated regularly after the activation of geckos in long flotations during the entire orbital flight.The righting reflex in thick-toed geckos in the flight experiment was manifested in incomplete (by 90°) or exaggerated (up to 270-360°) forms. The duration of turns calculated for 31 instances of rotation by 180° in geckos varied from 0.08 seconds to 0.64 seconds; with an average of 0.325±0.099 seconds. It is almost 3 times more slowly, than it was described in Earth conditions for flat-tailed house geckos by Jusufi et al. (2008). These differences can possibly be caused by both the species-specific features of animals and the effect of weightlessness on thick-toed geckos. It is known that, in terrestrial conditions, the postural righting reflex is stimulated by the signals from the vestibular apparatus about the incorrect position of the body relative to the gravity vector. The obtained data suggest that in animals, as in humans, in zero gravity spatial orientation illusions are possible.The number and frequency of individual flotations during the flight did not depend on the size and weight of the geckos. There were no animals without a single flotation episode, but the frequency of flotations varied both individually and depending on the condition of the geckos. The number of flotations in the quiescent geckos was 4.5 times higher than that of the same animals in an active state. Consequently, an active control of the attachment could become weaker or inexistent in animals in quiescent states or in a sleeping state. In active geckos the number of flotations during the flight was decreased compared to the first week of the flight. Hence, it is possible, that in weightlessness, geckos adapt to changes in environmental conditions by learning from their own experiences, and strengthening the active control of the attachment. In quiescent geckos the number of flotations did not change significantly during the flight. Individual variability in the frequency of flotations was found in the active and quiescent geckos during the flight. The findings show that the ability to attach to the surfaces is an important factor in the geckos' adaptation to the conditions of orbital flight. The behavioral responses that originated under terrestrial conditions can also be adapted to weightlessness and remain partially effective.