Artificial Heterogeneity Drives Environment-Dependent Kin Discrimination in Toad Tadpoles

Genetic admixture could reinstate kin recognition ability in lineages where they appear lost. Many animals have been reported to exhibit kin recognition based on learned “odour cues,” potentially linked to the major histocompatibility complex (MHC) genes. However, genetic components vary among lineages and local populations, and admixture may further amplify genetic differences within the group. Here, I compared the kin-biased associations of anuran tadpoles from two local populations: a naturally homogeneous population consisting solely of the Eastern Japanese toad ( Bufo japonicus formosus ), and an artificially disturbed heterogeneous population containing both Eastern and Western Japanese toads ( B. j. japonicus ). Genetic analyses revealed higher MHC haplotype diversity in the heterogeneous population. To examine the effects of social experience and aquatic environment on kin discrimination, tadpoles from each population were raised under four rearing conditions (group or solitary, using tap or pond water) and subjected to binary-choice tests. Only the heterogeneous group exhibited phenotype-matching-based kin discrimination, but pond water with higher microbial loads disrupted discrimination, suggesting that microbiota can obscure “odour cues.” No kin bias emerged in the homogeneous group. These findings underscore how population structure, social experience, and environmental conditions shape kin recognition.