Behaviour relevant to control

Public Health Significance of Urban Pests 393 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- Commensal rodents 392 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

Much of the behaviour exhibited by commensal rodents allows them to exploit their envi- ronment to its fullest, which can adversely affect the lifestyles of people. Knowledge of this behaviour, as it relates to interactions with people, is a prerequisite for effective control, either by using rodenticides or by securing environmental changes to make the habitat unsuitable. Young rats can squeeze through openings of less than 25 mm Lund, 1994, and house mice can get through openings of half that size WHO Regional Office for Europe, 1998 – a factor that should be taken into account when rodent proofing buildings. These rodents can also create their own points of entry in walls and containers, gaining access to resources and contaminating food stores in the process. Rodents gnaw continually to keep their ever-growing incisors sharp. They gnaw through the insulation of electrical wires, causing fires, and occasionally puncture lead pipes and concrete dams Nowak, 1999. This continual gnawing causes physical damage to timber and other building materials. Brown rats are skilled burrowers. The complexity of their system of burrows may vary with the density of the population in the colony Nieder, Cagnin Parisi, 1982. In loose soil, these burrows can reach depths of 3m, although they do not usually exceed 0.5m. The burrow system may be short in length less than a metre, but in established colonies burrows may interconnect, creating an extensive network of interconnecting tunnels. Four factors influence the location and entrance of a brown rat burrow. The first has been termed the thigmotropic response, whereby rats, on direct contact with a solid sur- face or object, prefer to move against a vertical or under a horizontal surface Calhoun, 1962. This movement, characterized by close contact with walls and other surfaces, lea- ves a revealing sign in the form of a smudge. This smudge, in addition to other signs, is indicative of an infestation and may prove useful in identifying well-used runs for control efforts. The second factor is overhead cover, such as shrubs, low trees and floors of buildings. This appears to favour the location of a burrow, because it reduces both direct light and 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