Water environment and social isolation affect association preference rather than kinship in toad tadpoles (Bufo formosus)

Kin-biased association is widespread among animal societies. Animals capable of kin recognition often rely on phenotype matching based on odor cues. Because individuals must learn the scent of their relatives, kin discrimination frequently depends on early social experience and environmental conditions. This study investigated how water environment and social experience influence association preferences in Bufo formosus (eastern Japanese common toad) tadpoles. Using a 2×2 factorial design, subject tadpoles were reared either in tap or pond water and under group or solitary conditions. Binary-choice tests were then conducted to examine whether tadpoles preferred siblings or non-siblings. No significant kin-biased association was detected in any treatment, suggesting that B. formosus tadpoles do not employ phenotype-matching-based kin recognition. However, both social experience and water type significantly affected general social tendencies: group-reared tadpoles interacted more with conspecifics than solitary-reared ones, and those raised in tap water spent more time near siblings than those reared in pond water. These findings indicate that social aggregation in B. formosus is shaped by social experience and environmental context rather than kinship.