On the affinity of the Dall's and True's Porpoises

Abstract

As the result of many previous spring observations off the Pacific coast of northern Japan, the two porpoises, Phocaena (Phocaenoides) dallii dallii True (half-white below) and Ph. (Ph.) dallii truei (Andrews) (all-white below), had been seen always in non-mixed groups, though the overlap of distribution of the southern truei and nothern dallii was noticed to some extent. But, on July 13, 1954, the author observed several mixed groups of the two forms south and east of Cape Erimo, Hokkaido. These facts may suggest the psychological isolation existing between the two during the breeding season, the spring, even in overlap area, but this will be lost in summer when truei intrudes north into the range of dallii.
However, this psychological isolation is by no means an absolute one as a truei-type faetus has been obtained from a dallii-type mother (May 16, 1950, off N. Honshu). This fact and that no intermediate individual occurs would suggest the monohybrid mechanism in their interbreeding, and the truei-type will be determined as complete dominant.
The truei, therefore, is regarded as a small racial population evolved in the Japanese water by a dominant mutation from dallii of wide distribution from the North Pacific to Okhotsk and Japan Seas.