ИСТИНА

Interactions between females are one of the blind-spots in our knowledge of reptilian social life. Apparently, females more often try to avoid same-sex individuals to decrease intrasexual competition. But sometimes we see deviant behaviour scenarios when females interplay and even cooperate together. So, studying the factors that encourage females to interact with each other is the key point in explaining this phenomenon. We studied Zootoca vivipara population in Tver Region (Russia) in years 2020 and 2022. Study area was a small meadow withing the taiga forest with a flock of rotten logs in the center. Most individuals displayed solitary social behaviour in spring, when females had small individual ranges (1–3 m2) that did not overlap with each other, and males overpassed huge areas (up to 50 m2), searching for females and their ranges overlapped broadly. In June several gravid females aggregated within permanent basking sites on the logs, uniting in small groups. The composition of their groups was constant and comprised from two to four individuals. After giving birth, the females left into the grass and no longer met each other. This bonding behaviour has been observed during the season 2020, but not in 2022, when we also observed gravid females. Interestingly, solitary lizards suddenly gather together and spend part of their time in groups with stable composition. Incentive for interaction may be associated with a specific phenological stage. Rattlesnakes Crotalus horridus is a rare example of female grouping based on kinship, whereas females aggregate together to increase effectiveness of eggs heating. We suggest, that cooperation of females of viviparous lizard in 2020 was due to relatively low ambient temperatures and might be also cooperation for joint incubation.