Rapid morphological change in black rats (Rattus rattus) after an island introduction.

Rapid morphological change has been shown in rodent populations on islands, including endemic deer mice (Peromyscus maniculatus subspp.) on the California Channel Islands. Surprisingly, most of these changes were towards a smaller size. Black rats were introduced to Anacapa Island in the mid-1800s (probably in 1853) and eradicated in 2001-2002. To assess possible changes in these rats since their introduction, eleven cranial and four standard external measurements were taken from 59 Rattus rattus specimens collected from 1940-2000. All rat cranial traits changed 3.06-10.43% (724-2567 d, 0.06-0.42 h), and all became larger. When considered in haldanes, these changes are among the fastest on record in any organism, and far exceed changes found in other island rodents. These changes were confirmed by MANOVA (Wilk's λ < 0.0005, F d.f.15 = 2974.386, P < 0.0005), and all 11 cranial traits significantly fit linear regressions. We speculate that concurrent changes in mice may have been due in part to competition with and/or predation by rats. Future research might evaluate whether the vector of mouse evolution on Anacapa is again changing after rat eradication.