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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,
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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
The potential breeding rate of brown rats is enormous. For example, a story in a weekly agribusiness newsletter in the United States suggested that the young from just one pair
of rats could be responsible for bringing forth 3.5 million more rats in three years Anonymous, 1984. The brown rat is capable of reproducing year-round in some wild
populations, but there are usually spring and autumn peaks Berry, 1970. Capable of mating within 18 hours of giving birth, the polyestrous females may bear anywhere from
1 to 12 litters a year of up to 22 pups per litter, with an average of 8–9 pups. Gestation is typically 21–26 days, and the young are born naked and blind. After two weeks, they are
fully furred and open their eyes. The pups are weaned at around three weeks and leave the nest at that time. They reach sexual maturity at 2–3 months of age Whitaker, 1980;
Lund, 1994; Nowak, 1999. While huge numbers of offspring are mathematically possi- ble, predation, limited resources and unfavourable climate, as well as behavioural cha-
racteristics, limit the realization of this rat’s reproductive potential Whitaker, 1980.
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visibility of entrances to predators. The third factor is the slope of the terrain, as sloping terrain appears to be preferred to
flat terrain. Brown rats prefer to move downward with the force of gravity, displaying a tendency for positive geotaxis Meehan, 1984.
The fourth factor that influences the location of the entrance to a burrow is the proximity of major resources, such as food and water. Calhoun 1962 reported that the shortest rou-
tes are not always those travelled to such goals. The orientation of the run is from one vertical object to another or along a continuous vertical object. Once rats have learned
the location of targets and objects in the immediate environment, runs become well esta- blished Meehan, 1984.
Unlike house mice, both species of commensal rats exhibit a behaviour called neophobia. This avoidance of the novel is especially prevalent with new food items and may have
more to do with the rat’s naturally timid demeanour than with food selection, but it may facilitate the association of adverse effects with the eating of new food MacDonald,
Mathews Berdoy, 1999. Neophobia can confound control efforts and must be com- pensated for by pre-baiting MacDonald, Mathews Berdoy, 1999. Pre-baiting allows
the rat population to become accustomed to the novel food at the bait station before it is replaced with poisoned food.
Barnett 1975 suggested that in the wild the choice of food is not influenced socially and that there is no evidence of true imitation or observational learning by young rats. The
choice of food is not learned from parents, but is a matter of habit. Rats tend to consume a small quantity of something with which they are unfamiliar and see what happens, and
then they eat a little more if all is well, eventually feeding more freely Quy, 2001. However laboratory studies have shown that rats can be influenced by the scent of ano-
ther rat that has eaten a particular food Galef Wigmore, 1983; Posadas-Andrews Roper, 1983 and that information about the food, such as palatability or toxicity, can be
transferred from mother to offspring, even through mother’s milk Galef Clark, 1972; Bond, 1984; Hepper, 1990.
Neophobia also extends to novel objects that suddenly appear in the rat’s environment. Bait boxes and traps are often avoided because they are new to the immediate surroun-
dings. Even after rats have become accustomed to the presence of the traps, it is often only juveniles or socially low ranking adults that are caught Calhoun, 1962, as these low
ranking members of the population are forced to seek food in alternative places. Thus, it seems that without a well-organized strategy, it will be largely the socially inferior rats
that are caught and killed. The more dominant and virile rats will survive to breed and eventually restore the population to its original size.
In contrast to neophobic behaviour, rats may exhibit neophilic tendencies curiosity in selecting unfamiliar objects to gnaw Quy, 2001. This is evidenced by their gnawing such
structures as pipework for no apparent reason. Barnett 2001 suggested that in an uns- table environment, such as a waste landfill site where everything is new, neophobic beha-
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and small reptiles Reid, 1997; Nowak, 1999; Myers Armitage, 2004. The roof rat, although omnivorous, relies less on animal matter and prefers a diet that
consists of fruits, grains and seeds, when available Lund, 1994. It will also feed on insects, carrion, refuse and faeces.
Like rats, commensal house mice will eat any accessible human food, as well as paste, glue, soap and other household materials Ballenger, 1999. Wild populations will
consume a variety of available food, including leaves, seeds, roots, stems, and some insects and meat, when available Ballenger, 1999; Nowak, 1999.
12.2.4. Behaviour relevant to control
