Feeding Biology of commensal rodents 1. Description and natural history

Public Health Significance of Urban Pests 391 Calhoun 1962 observed that on or near the time when female brown rats are capable of conception there is a precopulatory phase where the behaviour of both sexes changes. In this phase the female wanders more than normal beyond the limits of her home range as she actively seeks males. This increased activity may lead to more sightings and com- plaints from the public. The lifespan of wild brown rats is difficult to assess. The maximum known longevity in captivity is 3 years, but in the wild their lifespan is probably less than 18 months Meehan, 1984. In a farm population of brown rats, only about 5 of the population at the start of the year was alive at the end Meehan, 1984. Calhoun 1962 concluded that, for weaned rats, learning to cope with the environment reduces the proportion that survives. There is then a levelling off of the rate of attrition, but eventually individuals reach an age when learned behaviours fail to compensate for the debilities of ageing, which then increases the probability of death. Reproduction in the roof rat can occur throughout the year, with varying seasonal peaks. Gestation ranges from 21 to 29 days and litter size ranges from 1 to 11 pups, with an ave- rage of 8 pups. Females can produce up to five litters in a year. The pups are born naked and blind and are weaned and independent at 21–28 days. Females are capable of giving birth at 3–5 months of age Whitaker, 1980; Nowak, 1999; Myers Armitage, 2004. Annual mortality for roof rats that live in the wild is 91–97, with most living only a year. Captive roof rats have been known to live for 4 years Nowak, 1999. House mice are prolific breeders. They are capable of reproducing throughout the year, although seasonal breeding may occur in wild populations. Females may experience post- partum oestrus 12–18 hours after giving birth. The gestation period is 19–21 days, but may be longer if the female is lactating. Litter size ranges from 3 to 12 pups, with an ave- rage of 5–6 pups. Pups are born naked and blind and are fully furred at 10 days; eyes are open at 14 days, and they are weaned and independent at 21 days. T here is usually 60–70 mortality before independence is reached. Sexual maturity is reached at 5–7 weeks. On average, captive mice live 2 years, while the normal lifespan of wild mice is 12–18 months Ballenger, 1999; Nowak, 1999.

12.2.3. Feeding

Throughout their distribution, brown rats typically are associated with human settle- ments, colonizing areas with adequate resources. They are opportunistic omnivores, uti- lizing any food that becomes available. Kingdon 1974 states that brown rats will eat eve- rything people eat and much more, including soap, hides, paper and beeswax. In urban settings, the brown rat relies heavily on discarded human food Myers Armitage, 2004. As an accomplished swimmer and diver, this rodent has been known to prey on duc- klings Lund, 1994 and is quite adept at catching fish Grzimek, 1975; Lund, 1994; Nowak, 1999; Myers Armitage, 2004. The brown rat is more carnivorous than the roof rat and has been known to prey on mice, poultry, young lambs and piglets, insects, birds, Commensal rodents 390 culus . The house mouse is considerably smaller than the brown rat and roof rat. Its tail is long, with circular rows of scales, and it has very little fur. Its pelage varies from a nearly uni- form greyish-brown to a grey-brown dorsum and a pale grey to buff venter. The mea- surements of the house mouse range from 125 mm to 200 mm for the total length; 65 mm to 95 mm for the head and body; 60 mm to 105 mm for the tail; and 12 g to 30 g for the weight. The house mouse is distributed worldwide and tends to have a close association with peo- ple. Commensal house mice occupy a number of man-made structures, including hou- ses, barns and granaries Ballenger, 1999, and it prefers to nest behind rafters, in wood- piles, in storage areas or anywhere close to a source of food Nowak, 1999. Because of their association with people, house mice are able to occupy such areas as tundra and desert regions, which they would not be able to inhabit independently Ballenger, 1999. Wild house mice occupy cultivated fields, fencerows and wooded areas, but they seldom stray far from buildings, with some individuals moving from one habitat to the other with the changing seasons Berry, 1970; Ballenger, 1999. They live in cracks in rocks and walls or construct extensive burrows with many rooms and exits Berry, 1970. Wild house mice have even been reported in coal mines at depths of up to 550 m Bronson, 1979. Although house mice are good swimmers Nowak, 1999, they avoid water and damp conditions, as they have difficulty maintaining their body temperature when damp or wet WHO Regional Office for Europe, 1998. House mice are generally nocturnal; however, some may be active during the day in human dwellings Ballenger, 1999. They can live in almost any available space and can squeeze through extremely small openings, living and breeding under floors, in wall cavi- ties and ceiling voids, and behind skirting boards baseboards.

12.2.2. Reproduction and life cycle of commensal rodents