Movement Biology of commensal rodents 1. Description and natural history

Public Health Significance of Urban Pests 395 Calhoun 1962 reported from observations in urban apartment blocks in the state of Maryland that predation by dogs and cats and sporadic attacks by people did not have any appreciable effect on the density of the rat population. Indeed, general observations indicated that city blocks with dogs and feral cats also had high-density populations of rats. The presence of free-ranging cats and rats in urban areas appears to be positively related, perhaps due to a common benefit derived from access to waste food Langton, Cowan Meyer, 2001. Also, the presence of pets may lead to the provision of food and shelter for rodents Langton, Cowan Meyer, 2001. For example, in England, infesta- tions of commensal rodents inside and outside homes are higher in those properties where pets or livestock are kept in the garden Langton, Cowan Meyer, 2001; DEFRA, 2005. Because predation by dogs and cats has no appreciable effect on rodent densities in urban areas, rodent control is likely to be the primary mechanism by which population densi- ties can be kept low.

12.2.6. Movement

Rats, particularly the brown rat, do not normally move great distances, especially in urban areas where streets act as barriers Twigg, 1975. This may not be the case in rural areas, where rats have been reported to move as many as 3.3 km at speeds of 0.5–1.1 km an hour in one night Taylor Quy, 1978. Involuntary dispersal may also result when rats are transported with goods in vehicles. Habitat destruction will also cause movement when rats are forced to seek alternate shelter. The diameter of the normal home range of the brown rat varies from 25 m to 150 m Grzimek, 1975. MacDonald, Mathews Berdoy 1999 reported that farming activity caused home ranges of brown rats around farms to fluctuate, with males having larger home ranges 679 linear metres when crops were in the field and smaller home ranges 90 linear metres after the harvest. Farming activity, however, appeared to have no effect on females who, in general, had smaller home ranges. Resource availability also had an effect on home range. When resources were plentiful, the home range of females was smaller 85 linear metres than when resources were not as plentiful 428 linear metres; similar effects were noted for males MacDonald, Mathews Berdoy, 1999. The home range of the roof rat is never more than about 100 m 2 . This species often has smaller territories that surround the food sources it defends Gillespie Myers, 2004. Commensal house mice have been recorded as travelling over 2 km, but this is exceptio- nal. Typically, they will not move more than 3–10 m in buildings WHO Regional Office for Europe, 1998. Murphy, Williams Hide 2005 used DNA analysis to assess rela- tedness among house mice that colonized terraced housing in England and found that mice colonizing adjoining buildings were related, but the mice in infested properties on other streets and terraces were genetically different. Their work indicated that each block represented individual breeding units and that migration rates between blocks were very low, because mice moved easily between adjoining properties, but moved little between non-adjacent housing blocks Murphy, Williams Hide, 2005. The focus of control should therefore be the blocks of properties, not individual houses. Commensal rodents 394 viour is replaced by neophilic behaviour, as rats explore their ever-changing environment.

12.2.5. Population growth and socialization of commensal rodents