Biologically, rats are classified in the class Mammalia. They are mammals. This class of animals has more than 15,000 species, of which human beings are one. One may define mammals as being tetrapod vertebrates which means that they are four-limbed animals with backbones. The diagnostic feature of the group as a whole is the possession of mammary glands which produce milk, on which the young are fed during the early stages of their lives. They are homeothermic or warm blooded, having an approximately constant body temperature irrespective of the external conditions. With few exceptions, mammals have hairy skin and are viviparous, retaining the young in the body of the mother during the early stages of development, and giving birth to live young instead of laying eggs (Rowett, 1960).
As far as rat reproduction and neonatology (the study of infant animals) is concerned, a general synopsis is provided below:
After a gestation period of between 21 and 23 days, a litter of 6 – 15 pups is born (n = 8). During her gestation, the mother rat will display vigorous nest-building activity, especially in cool environmental conditions. She will carefully construct a nest, provided she is given the materials with which to do so. Mother rats readily tear up paper to build a nest (Weihe, 1989) which can be rather elaborate in structure (Meehan, 1984). Rat mothers who do not have nest materials given to them will attempt to make a nest out of whatever floor covering is available. The nest is constructed usually in the corner of the cage or housing facility (Fallon, 1996). Thus demonstrating a strong maternal instinct to want to protect and nurture the young. Nest materials serve a five-fold purpose – 1. covering to protect the young and keep them out of sight of predators, 2. a place of additional warmth where the temperature can be controlled, 3. a place of rest, 4. a place to rear the young and 5. a place of food storage (Twigg, 1975; Meehan, 1984; Fallon, 1996; McDuling, 2006, in press).
The young are born pink, naked, blind, without hearing, and unable to walk or fend for themselves, and completely helpless. They make bird-like peeps. They are thus entirely dependent on the mother for food and warmth (Warren, 1995; Daly, 2002). The head is always searching with quick response to olfactory (smell) and taste stimuli (Farris, 1950). Pups are extremely sensitive to rearing conditions. The newborn rat is essentially poikilothermic (unable to adjust their own body temperature), and must rely upon the micro-environment of the nest maintained by the mother. They rely upon the huddling of the litter to achieve temperature regulation stability (thermal homeostasis). Within two weeks, they are able to regulate their own temperature, their coat develops and their eyes open (Koolhaas, 1999). They are fed on their mother’s milk until they are about three weeks old by which time they have grown hair, can see, hear and run about and can feed on a mixed diet similar to that of the adults (Rowett,1960; Daly, 2002).
In addition, the new born rat lacks the ability to urinate and defaecate on their own. The mother stimulates the pups to do so by licking their anuses and genital regions (Daly, 2002). If one is raising baby rats by hand, simulated maternal stimulation of defaecation and urination must be provided (Harkness and Wagner, 1995). One of the first acts carried out by the mother on the newborn is licking of the anogenital region. Infants in which this licking does not occur soon die (Meehan, 1984). This clearly demonstrates the absolute necessity, without exception, for the young to remain with the mother. Further, it would suggest that pups who do not have this maternal action are subject to much physiological stress.
Rat pups do not acquire thermoregulatory abilities (the ability to keep themselves at constant temperature) until the end of the first week of life. They remain in the nest and are kept warm by each other in the nest, where they are completely covered, and by the mother. Even adult rats display huddling behaviour to keep warm. The litter has a high demand for heat whilst the young are hairless (Fallon, 1996; Weihe, 1989). Harkness and Wagner (1995) also state that young rats of less than 16 days are easily chilled (they become hypothermic) and must be kept warm if one is raising them by hand, to prevent the aspiration of foodstuffs introduced into the stomach. In studies done by McDuling (2006, in press), it was found that in addition to warm bedding, an infra-red lamp was provided as an additional source of heat, and placed at an appropriately safe distance away from the young rats so as to maintain the temperature of the internal environment at between 21є and 26є C. The actual temperature in which rats should be housed varies. Some authors advocate a temperature of between 18є and 22є C (Weihe, 1989) whilst others say that a temperature of between 18є and 27є C with an optimal room temperature of 22є C is acceptable (Harkness and Wagner, 1995).
Mothers with normal milk production of around 10 – 20 mL/day, usually feed the young for around an hour and then move off the nest for a while to rest and eat, and then return to the nest to nurture the young. A pup’s body weight doubles within five days. A fine, thin, lanugo hair occurs after day five near the end of the first week. As the insulating fur grows, temperature regulation becomes more efficient so that the young are less dependent on the mother’s body heat. The pups are fully covered with a thin fur by day nine, but this is not thickly confluent until at least day 16. Around day nine, they begin to move about and their incisors are large enough to allow them to nibble solids, but still need their mother’s milk as additional nutrition. Soon after this, their eyes open. At this time, the mother is producing her maximum milk yield. Rats younger than 16 days cannot eat solid foods as their molars have not erupted yet. The young increase their solid consumption whilst still suckling until they become fully independent of the mother’s milk supply at about three weeks or 21 days of age (Weihe, 1989). If suckling stops for whatever reason, the mother’s milk rapidly dries up (Meehan, 1984).
It is thus of extreme importance that the young are left with their mothers until at least 16 days of age, and preferably until weaning at 21 days of age. Most authors advocate leaving the pups with their mother until 28 days of age which ensures better survival rate and better socialisation later on in life.
Many studies on maternal separation have been carried out, and the results have been rather alarming. In a study by Greenberg and Ackerman (1986), rats who were prematurely weaned at day 15 were very susceptible to developing hypothermia as well as erosions in their stomachs (gastric erosions or ulcers). These ulcers are highly indicative of stress. It was found that these rats also had reduced fat stores. Fat is the tissue which, when broken down, provides heat to the body.
Prolonged hypothermia and starvation also has a profound effect on the infant rat’s physiology and metabolism. Plasma glucagon, glucose and free-fatty acid concentrations rose significantly after 20 hours of induced hypothermia and starvation (Hoo-Paris, et al, 1991). Glucagon is the hormone released when the body undergoes a stressful stimulus such as starvation, and promotes the breakdown of glycogen in the liver to glucose which is then released into the bloodstream. The fact that the concentrations of glucose and free fatty acids rose so significantly suggests that the body was breaking down its own reserves in order to maintain a steady state. The body was actually being broken down, and thus deteriorating. Such deleterious changes may limit survival time in hypothermia, and suggest that unless these infant rats were fed and warmed, they would surely die.
Blumberg, et al (1999) found that prolonged maternal separation inhibited endogenous heat production in infant mammals exposed to cold. Cold can be defined as any temperature lower than that necessary to sustain life. In work on early postnatal infant rats who were separated from their mothers for 18 hours and left in a relatively cold environment, it was found that the infant rats could no longer produce any heat of their own. Heat production or thermogenesis was inhibited and this appeared to occur many hours before energy stores such as glucose and fats have been fully depleted. These rats were, quite frankly, in a state of hypothermia, and close to death. With respect to the previous study, it would appear that the rats used by Hoo-Paris, et al were in a state of severe deprivation.
Further studies revealed that maternal separation resulted in a significant fall in heart rate (bradycardia) and respiratory rate (Hofer, 1973; Hofer and Weiner, 1975). Decreases in both heart rate and respiratory rate suggest that the animal is pre-mortem, i.e., close to death. If infant rats are taken away from their mothers before the appropriate weaning age when they are capable of surviving on their own, an enormous stress is exerted on them which could result in their ultimate death. In the study by Hofer and Weiner, 1975, this bradycardia was reversed by the intragastric infusion of milk. This further confirms that starvation and maternal separation have a huge impact on the survival of the infant rat.
As far as the mother is concerned, she is fiercely protective of her young, and will attack and bite anyone who attempts to intrude into the nest. She will even destroy her young if she feels that they are threatened. The mother rat should never be disturbed. If the cage bedding is to be changed, it should be done with care and preferably whilst the mother is off the nest. The young should not be touched by hands, but rather scooped up together with the nest and placed in the clean cage. If the young are scattered in the cage, the mother will carefully retrieve each pup back to the nest in her mouth. (Fallon, 1996; Harkness and Wagner, 1995).
Maternal protection of the young is a very important part of normal rat behaviour and the mother will defend her young from outside interference with aggression. If a nest is disturbed, the mother will invariably build a new nest in a safer location and move all the young to the safer location (Twigg, 1975; Meehan, 1984). Twigg in 1975 also stated that pregnant rats and rats who have just given birth will defend their young not only from other species, but from other rats as well.
The mother keeps the pups together in the nest. If an infant strays from the nest, a lactating mother will readily and rapidly retrieve it (Meehan, 1984; Koolhaas, 1999). As far back as 1933, it was noticed that mother rats rapidly retrieved their young when these young have either strayed from the nest or have been taken from it (Weisner and Sheard, 1933). These authors observed that if the young were removed from the nest for prolonged periods, the mother rat continued to search for them for “prolonged periods in her fruitless efforts…” They also observed that when retrieving the young, the mother was exceptionally gentle and deposited them in the nest with equal care. It has been subsequently found that pup retrieval is facilitated by ultrasonic vocalisations (sounds human ears cannot hear) sent out to the mother by the pups who have become separated from their mother and littermates (Koolhaas, 1999). The fact that pups call out for their mothers when they have been separated from them is clearly indicative of the distinct need for maternal warmth, protection and nourishment. Furthermore, if a mother rat tirelessly searches for her pups when they are separated from her, clearly demonstrates maternal commitment to these pups. It is not unlike a human child who has become separated from his/her mother and cries for that parent, and the mother doing everything in her power to find her child. All of this clearly demonstrates a concern for the young and for their well-being. One might argue that placing human qualities on animals is not scientific. However it is becoming increasingly evident that such qualities are universal to all animals, and that the study of animal behaviour has been long accepted as a science since 1973. In this year, Lorenz, Tinbergen and von Frisch were jointly awarded the Nobel Prize for their efforts to advance the notion that animals are indeed sentient beings, capable of a wide range of emotions and behaviours. Since then, the field of animal behaviour or ethology has progressed in leaps and bounds.
It is thus, in this author’s opinion that separation of infant rats from their mothers at an early age is highly detrimental and imposes not only a great amount of stress on the infant, but on the mother as well. Such stresses are both physiological and behavioural. Therefore, it would be deemed to be exceedingly cruel to forcibly separate infant rats from their mothers for even the briefest period of time. Such separation represents an imposition of human will on the natural behaviour and biology of rats. This author has always gone to great lengths to simulate the maternal environment as far as possible when hand-raising orphaned baby rats, and to provide a stress-free environment in which to allow these baby rats to develop. Of course, humans cannot hear the pups calling in ultrasound, and so conditions were provided to ensure that the babies were kept warm, well fed, and nurtured. All of their needs were taken care of as one would do with one’s own child, and their well-being was of the highest priority.
In conclusion, then, it may be stated that the practice of removing infant rats from their mothers for any purpose whatsoever should be prohibited and punishable by law. Especially should baby rats not be used as reptile fodder as this imposes a further stress on them. One cannot even imagine their confusion, disorientation, separation anxiety and fear. In fact no rat or other animal should be used as reptile fodder for the same reasons. It is hoped that laws will soon be put in place to improve the quality of life for animals in South Africa, and indeed throughout the world. In this 21st Century, we should strive to make this a better place for all concerned by the application of compassionate science.
Colleen McDuling, B.Sc(Med.Hons), MSc(Med.Sc.), Animal Behaviourist, Scientific Representative of the South African Rat Fan Club.
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by Paula Spagnoletti of the South African Rat Fan Club