| Taxonomy | Genetic and Microbial Categories | Anatomic & Physiologic Characteristics | Behavior | Sexing |
Rodents are characterized by genetic and microbial status. Common genetic categories are outbred stocks, inbred strains, F1 hybrids, transgenics, knock outs, and mutants. Outbred stocks maintain genetic diversity by mating unrelated individuals. Inbred strains maintain genetic homozygosity by breeding closely related relatives. F1 hybrids are a cross between 2 inbred strains. Transgenic animals have specific genetic material introduced into their genome. Knock outs have specific genetic material removed from their genome. Mutant animals are strains that have spontaneously developed genetic mutations. Microbial flora of rodents can also be used to categorize rodents. Axenic rodents are free from of all microbial organisms, including both pathogens and normal commensal flora. Gnotobiotic rodents harbor only known microbial flora. Specific Pathogen Free (SPF) rodents are free from specific bacterial, viral, and/or parasitic pathogens. Conventional rodents have an unknown microbial status. Most rodents purchased at the Southern Illinois University at Carbondale are SPF rodents. Rodents housed in the Laboratory Animal Program vivarium are periodically tested for the presence of pathogens to determine their current microbial status. If rodents of different microbial statuses are worked with in the same day, the room with the least pathogens must always be entered first, followed by the room(s) with progressively more pathogens. If in doubt, contact personnel from the Laboratory Animal Program for recommendations.
Rats, mice and hamster incisors and all guinea pig teeth grow continuously throughout life and are worn down by mastication (chewing). If teeth are not properly aligned or fail to be worn down for any reason, dental overgrowth can occur. Overgrown teeth can inhibit eating and drinking. Glands behind the eyes of rats can produce dark red secretions that can stain the fur around the eyes and nose in response to stress or illness. The coloration is the result of porphyrin pigment and not blood, despite its appearance. Guinea pigs differ from rats and mice by having a longer gestation period, precocious offspring and a cellular membrane that closes over the vaginal orifice, except during estrus and parturition. Guinea pig's pubic symphysis separates during the latter half of gestation to allow passage of young during parturition. If guinea pigs are bred for the first time after 7-8 months of age, the symphysis separates incompletely and causes delivery difficulties. Hamsters have highly distensible cheek pouches that extend from the oral cavity to the shoulder blades. The cheek pouches may be used to transport or store food, bedding or other objects. Hamsters may also conceal young in their cheek pouches when threatened, however, the young may suffocate if not removed in a timely manner. Spontaneous, convulsive (epileptiform) seizures occur in approximately half of gerbils. The seizures range from mild to severe and are often elicited in unfamiliar surroundings or by a startle stimulus.
Most rodents are primarily nocturnal (during dark), but can also display diurnal (during light) activity. However, unlike other rodents, guinea pigs and gerbils may have peak activity levels during the dark or light cycles. Hamsters are not true hibernators but may become pseudohibernated in response to decreased environmental temperatures, decreased length of light cycle or other variables. During pseudohibernation, hamsters remain sensitive to touch and may become aggressive on awakening. Rodent littermates raised together from birth usually coexist peacefully. Rodents may fight when grouped together after reaching puberty, reunited after prolonged separation, or held in crowded conditions. Fight trauma may result in abscessation, dermatitis, septicemia or death. In most cases, males are more prone to fighting than females. However, female hamsters are dominant over males and can be aggressive. Whenever groups of rodents are assembled, or when new animals are added to a stable group, the group should be observed carefully to prevent fight injuries. When fighting occurs, the animals involved must be separated. Rodents may also establish hierarchy status via barbering. During barbering, the dominant animal chews hair from the muzzle, body or tail of subordinate animals.
The anogenital (between the anus and genitalia) length can be used in most rodents to determine sex in neonates and adults. The anogenital length is greater in males versus females. All rodents have open inguinal canals throughout life allowing the testicles to be retracted into the abdominal cavity and therefore may not be visible. Most adult male rodents are larger than adult females. Adult female hamsters are larger than adult males, with the exception of the Chinese or striped hamster in which the males are larger. Viewed from above, the rear margin of the male hamster is rounded because of the scrotal sacs, and the female posterior is pointed toward the tail. Differentiation of sex in guinea pigs is slightly different from other rodents. The guinea pig female has a vaginal closure membrane that can be exposed by gentle digital stretching of the genital ridge that extends from the anus to the vulva. The relaxed membrane will be seen as a shallow, U-shaped crease between the anal and urethral openings. The vaginal closure membrane is intact except during estrus and parturition. Male guinea pigs (boars) are distinguished from females (sows) by a penis that can be extruded from the circular prepuce. Also, the boar has no break in the ridge between the urethral-penile opening and the anus.
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