Sika deer (Cervus nippon Temminck) from Japan were introduced into Great Britain and Ireland first in 1860, and, for a time, were much sought after by owners of deer parks. Later, they were introduced or allowed to escape into various parts of the country, and feral populations have become established in Scotland, England and Ireland.

Hybridization between Sika and Red deer (Cervus elaphus L.) has occurred frequently in deer parks but has been reported only recently from feral and wild stocks.

In part of north-west England, there have been reports of hybrids from time to time ever since the 1920s, but these have become commonplace only recently. Since the Red deer inhabiting other parts of the region are all of the native race (Cervus elaphus scolicus Lönnberg) and no other population of this race exists elsewhere in England, work was begun to assess the threat to the survival of both the race and the species posed by Sika deer and, more particularly, by the hybrids on Cartmel Fell.

Using multivariate methods of analysis on a range of skull measurements (see page 556) obtained from samples of both species and their hybrids, an attempt was made to find some means of distinguishing the hybrids from the species irrespective of the extent to which introgression had occurred. This was largely successful using canonical variate analysis, but only when the sets of data used to construct the basic matrix were both homogeneous and biologically relevant, i.e. of the same age and sex, and where the samples were obtained from the races involved in the hybridization. The present classification of the subspecies of Sika deer received little support from this study. Many of the subspecies as described in the literature were found to be indistinguishable from some of the hybrids, which suggests that some revision may be necessary.