1. Hybridisation with an invasive species has the potential to alter the phenotype and hence the ecology of a native counterpart.
2. Here data from populations of native red deer Cervus elaphus and invasive sika deer Cervus nippon in Scotland is used to assess the extent to which hybridisation between them is causing phenotypic change. This is done by regression of phenotypic traits against genetic hybrid scores.
3. Hybridisation is causing increases in the body weight of sika-like deer and decreases in the body weight of red-like females. Hybridisation is causing increases in jaw length and increases in incisor arcade breadth in sika-like females. Hybridisation is also causing decreases in incisor arcade breadth in red-like females.
4. There is currently no evidence that hybridisation is causing changes in the kidney fat weight or pregnancy rates of either population.
5. Increased phenotypic similarity between the two species is likely to lead to further hybridisation. The ecological consequences of this are difficult to predict.