Genetic admixture could restore kin recognition ability, even in lineages where it appears lost. Many tadpoles have been reported to recognize kin based on learned “odor cues,” potentially linked to major histocompatibility complex (MHC) genes. However, genetic components vary among lineages and local populations, and admixture may further amplify genetic differences among siblings.

Here, I compared kin-biased associations in toad tadpoles from two local populations: a naturally homogeneous population consisting solely of the Eastern Japanese toad (Bufo formosus) and an artificially disturbed, heterogeneous population containing both Eastern and Western Japanese toads (B. japonicus). Genetic analyses revealed greater MHC haplotype diversity in parent toads from the heterogeneous population.

To examine the effects of social experience and aquatic environment on kin discrimination, tadpoles from each population were reared under four conditions (group or solitary, in tap or pond water) and subjected to two binary-choice tests. Only tadpoles from the heterogeneous population exhibited phenotype-matching-based kin discrimination. However, this discrimination was lost in pond water, suggesting that microflora or chemical components in the water obscure odor cues. No kin bias was detected in the homogeneous population.

These findings highlight the interplay among population structure, social experience, and environmental conditions in shaping kin recognition.