Rodents of Bristol 7: Pests and their relatives

On display in Twilight world the final section is devoted to animals found in close proximity to man, of which three are rodents – House Mouse, Brown Rat, and Black or Roof Rat. All these are among the most succesful of all mammals, owing to their adaptation to life alongside humans (and their food stores). All are causes of serious conservation problems wherever they have been taken, especially on islands, where they readily abandon human dependency and prey on local wildlife, especially birds and their eggs.

The genus Rattus is enormously diverse, and besides the two species Bristol has on show there are over 60 others. Most are quite independent of humans, but one, the Polynesian Rat R.exulans, has been spread all over the Pacific, causing extinctions wherever it went.

The genus Rattus is also one of the few placental lineages to cross Wallace’s line, diversifying in New Guinea and Australia into at least 23 different species. Some of these are quite endangered, but as far as I am aware there are no current captive breeding programmes for any of the endangered species of rat. At least two species, both from Christmas island, have become extinct in the last two centuries.

The other pest rodent on show is the House Mouse, Mus musculus, also has more complicated natural history than might be supposed.. As well as the various forms of House Mouse, there are at least 8 other species in the genus, which range from Spain to Sumatra. The House Mouse itself appears to have originated in India. The House Mouse has been divided into at least four different subspecies, but these are now often treated as full species, although they will form hybrids when the ranges of the various forms overlap.

In addition, because of their rapid lifecycle, they evolve rapidly on islands, and many of the feral introduced forms are already quite distinct in appearance and even behaviour. On Gough island in the South Atlantic, in only 150 years since their first arrival, they have become two or even three times the size of their ancestors, and become highly predatory, feeding on seabird chicks which they sometimes attack in groups. When chicks are not available, they often feed on insects or worms instead.

Closer to home, when the remote Scottish island of St Kilda was evacuated in 1930 (at the36 remaining islanders own request) a unique endemic subspecies of House Mouse, the St Kilda house Mouse, rapidly became extinct. It apparently could not survive without the winter protection of the islanders homes, although the St Kilda Field Mouse, Apodemus sylvaticus hirtensis. Incidentally, St Kilda was also the last location in Britain where a Great Auk was killed, in 1840.

On islands, control of these introduced rodents is the most difficult of conservation issues, but the single action that will most benefit the endemic species, As rodents quickly evolve resistance to many forms of poison, and rats in particular are good at learning to avoid poison baits, the best method is to blitz the island from the air with rodent poison, ensuring that all rodents are exposed at more or less the same time. Follow up afterwards with chew sticks smeared with vegetable oil, which rats in particular find hard to resist, will enable monitoring of the island for any survivors. It is also vital that no subsequent arrivals are allowed to take hold, or the work will have been in vain.

(images from top: R.rattus, R.norvegicus, R.exulans all from Wikipedia)