Murine genomes share great similarity with the human genome, which has made both mice and rats fantastic models for a wide range human disease research. Rats are the second most used laboratory animals after mice. To the uninitiated, rats appear to be larger versions of mice, but they are actually two different species.
Mice evolved from Mus musculus, which are widely distributed all over the world. After long-term artificial breeding and selective breeding, more than 1,000 inbred strains and independent outbred stocks have been bred for research use. Rats, also known as Norway rats, belong to the class Mammalia, the order Rodentia, and the Muridae family. Laboratory rats are a selective breeding variant of Rattus norvegicus. As commonly used laboratory animals, rats and mice are widely used models in various fields of biomedical research.
In previous Technical Bulletins, we have shared information about mouse behavior. Presumably, everyone has a basic understanding of their behavior, response and the influencing factors behind it. So, how much do you know about rat behavior?
Mice are smaller in size and more docile in temperament, so they are easier to grasp and immobilize; Rats are relatively large in size, and their global muscle strength is relatively strong. When grasping and immobilizing a rat, the required strength and difficulty of operation are greater than for a mouse. However, frequent and gentle handling can increase the docility of the rat, which can reduce the possibility of injury to the operator and reduce the stress of the rat. Infrequent or rough handling can induce fear in rats and is often used as a stressor in research projects. In rat behavioral studies, adaptive feeding domestication of rats in the pre-weaning and weaning periods affects the degree of stress induced by subsequent manipulations and alters rat responses. "Fun-filled" manipulations can reduce stress in rats when performing manipulations such as injections.
Operational handling can cause the rat to emit >22 kHz calls, referred to the typical rat alarm calls. In a cage, stress-induced vocalizations by one rat can affect the behavior of other rats within hearing range, making manipulation of other rats more difficult. Another interesting fact about rat vocalization which illustrates its importance in rat behavior, is that rat pups vocalize in the ultrasonic range even before their ears are developed enough to hear, probably to signal their mother.
Rats are most active at night, but will also move and eat during the day. In addition to this, they are more active in the morning than in the afternoon, and this circadian rhythm affects behavioral tests. For example, the pain threshold is often assessed by the tail-flick test, whereas the tail-flick latency of female rats is relatively short in the mid-dark cycle, estrus, and post-estrus.
Rats can be kept individually or in groups. In general, male rats are less aggressive than male mice when housed in groups, and they also do well when housed individually, which is the norm in many toxicology and safety assessment studies. Although historical experience shows that laboratory rats appear to adapt well to individual housing, there are marked physiological and behavioral differences in rats housed individually compared to rats housed in groups. It is strongly recommended that, where possible, social animals such as rats be housed in socially compatible groups.
When given a choice, rats showed a preference for the solid floor, bedding litter, and nesting material consisting of large grains of poplar wood chips. Rats chewed inanimate objects such as wooden blocks, nylon bones, and balls when presented with items as part of an environmental enrichment program. The provision of environmental enrichment items is an important part of the husbandry management of high-quality research animals, but as in the case of group housing, it can significantly alter the baseline behavior and experimental responses of rats. Therefore, it is necessary to evaluate the environmental enrichment items provided to rats for potential experimental impacts.