Predation can provide both positive and negative effects on formation of social groups in rodents. On the basis of observational data of predation by the desert monitor lizard (Varanus griseus caspius) on the great gerbil (Rhombomys opimus), a social rodent of desert Central Asia, we predicted that in a year after a peak in prey density, when the abundance of terrestrial predators is still high, the group mode of life of a prey species might become disadvantageous. Social groups could advertise themselves and attract predators. We therefore hypothesized that the probability of predation would be higher for gerbils in outlying social groups than in groups closely associated with each other, in larger compared with smaller groups, and in groups with an active adult male compared with solitary females with no resident male. We also analyzed whether the survival of gerbils and the stress levels in adult and juvenile males were related to frequency of visits by monitor lizards at gerbil colonies (isolated systems of burrows used by gerbil groups of any size), distance between colonies, and size of social groups. We found that frequency of visits by the monitor lizard depended on distance to the nearest neighboring colony. The more distant the gerbil colony, the more visits by the lizard and the lower the survival of juveniles. We also found a positive correlation between concentration of fecal corticosterone in young males and frequency of monitor lizard visits at colonies. This pattern was not as pronounced in adult males. Results did not support the hypothesis that larger groups would have higher predation because frequency of predator visits, distances to the nearest occupied colonies, and survival of juveniles did not correlate significantly with the size of family groups. There was no difference in survival of juveniles in colonies occupied by single females compared with colonies in which an adult male was present. These results suggest that there is a possible trade-off between competing strategies of antipredator behavior and that factors other than predation are influencing gerbil survival.
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