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Categorical identity signatures can reduce host error rates during brood parasitism
Biological recognition is often modeled as involving discrimination of continuously-distributed (and continuously-perceived) traits according to decision thresholds. However, traits such as animal signals can be categorically distributed. Here, Dixit et al. test how such categorical distributions may influence fundamental trade-offs in signal recognition, using a brood parasite–host system involving identity recognition. The African cuckoo finch Anomalospiza imberbis parasitizes several host species, each of which has evolved inter-individual variation in egg appearance (“egg signatures”) that facilitates recognition and rejection of mimetic cuckoo finch eggs. The authors demonstrate that egg signature traits in one host species, the zitting cisticola Cisticola juncidis, are categorically distributed. Field experiments reveal that zitting cisticolas make fewer Type II errors (accepting parasitic eggs) and Type I errors (rejecting their own eggs) than hosts exhibiting continuous variation. This challenges the long-standing expectation (from classification models, statistics, and signal detection theory) of a strict trade-off between these two error types. The image shows zitting cisticola eggs, which to the human eye vary categorically in background color (blue or white), marking size (small speckles or large blotches), and marking luminance (pale or dark).
Image Credit: Tanmay Dixit