In order to ascertain the involvement of endogenous prolactin (PRL) in the expression of courtship behavior by the male newt, Cynops pyrrhogaster, antiserum against homologous PRL was administered to male newts, which had been exhibiting courtship behavior in the field and had been taken to the laboratory. The effect of the antiserum on the preference of terrestrial over aquatic habitats also was investigated. Within 24 hr of the first injection of antiserum, both incidence and frequency of courtship behavior declined markedly compared with those in the preimmune serum-injected newts. The anti- PRL serum-induced decline became more conspicuous thereafter. These animals came to prefer a terrestrial habitat. Cessation of antiserum injection and administration of PRL restored the expression of courtship behavior to a certain extent and altered their habitat preference from terrestrial to aquatic. The results strongly suggest the involvement of endogenous PRL in the expression of courtship behavior as well as in the preference of aquatic habitat.
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