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Animals · Animal ethology · Comparative psychology · Animal models · Outline · Index

Animal psychopathology is the study of mental or behavioral disorders in non-human animals.

Historically, there has been an anthropocentric tendency to emphasize the study of animal psychopathologies as models for human mental illnesses[1]. But animal psychopathologies can, from an evolutionary point of view, be more properly regarded as non-adaptive behaviors due to some sort of a cognitive or emotional impairment or distress. This article provides a non-exhaustive list of animal psychopathologies.

Eating Disorders

Animals in the wild appear to be relatively free from eating disorders although their body composition fluctuates depending on seasonal and reproductive cycles. However, domesticated animals including farm, laboratory and pet animals are prone to disorders. Evolutionary fitness drives feeding behavior in wild animals. The expectation is that farm animals also display this behavior, but questions arise if the same principles apply to laboratory and pet animals.

Activity Anorexia

Activity Anorexia (AA) is a condition where rats begin to exercise excessively while simultaneously cutting down on their food intake, similar to human anorexia nervosa. When given free access to food and an exercise wheel rats develop a balance routine between exercise and food intake turning them into fit rats. However, if food intake is restricted and wheel access is unrestricted rats begin to exercise more and eat less resulting in excessive weight loss and ultimately death. The running cycles shift so that most of the running is done in hours before feeding is scheduled. In other conditions AA does not develop. Unrestricted food access and restricted wheel access will not cause any significant change in either feeding or exercise routine. Also if rats are restricted both in food intake and wheel access they will adjust accordingly. In fact if rats are first trained to the feeding schedule and then given unrestricted access to a running wheel they will not develop AA behavior. Results support that the running interfered with the adaptation to the new feeding schedule and is associated with the reward system in the brain[2]. One theory is that running simulates foraging, a natural behavior in wild rats. Laboratory rats therefore run (forage) more in response to food shortages. The effect of semi-starvation on activity has also been studied in primates. Rhesus monkey males become hyperactive in response to long-term chronic food restriction[3].

Thin Sow Syndrome

Thin Sow Syndrome(TSS) is a behavior observed in stalled sows that is similar to AA where some sows after early pregnancy are extremely active, eat little and waste away resulting very often in death. They suffer from emaciation, hypothermia, a depraved appetite, restlessness and over-activity[4]. The syndrome may mainly be related to social and environmental stressors. Studies on the effects of overcrowding were pioneered by John B. Calhoun in the 1940s by placing Norway pregnant rats in a room with plenty of water and food and observing the population growth. The population reached a number of individuals and did not grow after; overcrowding produced stress and psychopathologies. Even thought there was plenty of water and food the rats stop eating and stop reproducing[5]. These effects have also been observed in jam-packed beetles. When overcrowding occurs female beetles destroy their eggs and turn cannibalistic, eating each other. Male beetles lose interest in the females and although there is plenty of water and food there is no population growth. Similar effects have been observed in overcrowded situations in jack rabbits in Minnesota, and deer in the Chesapeake Bay[6]. Stress in stalled sows is often perceived as the consequence of the restraint of animals that happens in intensive production units. The sows that suffer the most restraining conditions are those lactating or pregnant as they have very little room to move around because they are kept in barred gestation crates or tethered for the 16 weeks of pregnancy which prevents natural and social behaviors[7]. However, increased movement and freedom is also stressful for adult sows, which is usually the case after weaning. When placed into groups they fight vigorously with one dominant sow emerging that eats greedily and becomes large and fat. It is also very likely that two subservient sows make up part of the group who actively avoid competitive feeding situations and are bullied around by the dominant sow. Affected sows have poor appetite but often show PICA and excessive water intake and are anemic[8].


PICA is the ingestion of nonnutritive substances and has so far been poorly documented. In non-human animals in the laboratory it has been examined through the ingestion of kaolin (a clay mineral) by rats. Rats were induced to intake kaolin by administering various emetic stimuli such as copper sulfate, apomorphine, cisplatin, and motion. Rats are unable to vomit when they ingest a substance that is harmful thus PICA in rats is analogous to vomiting in other species; it is a way for rats to relieve digestive distress[9]. In some animals PICA seems to be an adaptive trait but in others it seems to be a true psychopathology like in the case of some chickens. Chickens can display a type of PICA when they are feed-deprived (Feeding restriction has been adopted by the egg industry to induce molting). They increase their non-nutritive pecking, such as pecking structural features of their environment like wood or wire on fences or the feathers of other birds. It is a typical response that occurs when feeding is restricted or is completely withdrawn. Some of the non-nutritive pecking may be due to a redirection of foraging related behavior[10]. Another animal that has displayed a more complex PICA example are cattle. Cattle eat bones when they have a phosphorus deficiency. However, in some cases they persist on eating bones even after their phosphorus levels have stabilized and they are getting adequate doses of phosphorus in their diet. In this case evidence supports both a physical and psychological adaptive response. Cattle that continue to eat bones after their phosphorus levels are adequate do it because of a psychological reinforcer. “The persistence of PICA in the seeming absence of a physiological cause might be due to the fortuitous acquisition of a conditioned illness during the period of physiological insult.”[11]

Cats also display PICA behavior in their natural environments and there is evidence to support that this behavior has a psychological aspect to it. Some breeds (such as the Siamese cat) are more predisposed to showing this type of behavior than other breeds, but several types of breeds have been documented to show PICA. Cats have been observed to start by chewing and sucking on non-nutritive substances like wool, cotton, rubber, plastic and even cardboard and then progress into ingestion of these substances. This type of behavior occurs through the first four years of a cat’s life but it is primarily observed during the first two months of life when cats are introduced into new homes is most common[12]. Theories explaining why this behavior becomes active during this time suggest that early weaning and stress as a consequence of separation from the mother and littermates and exposure to a new environment are to blame. Eating wool or other substances may be a soothing mechanism that cats develops to cope with the changes. PICA is also observed predominately during 6-8 months of a cat’s life when territorial and sexual behaviors emerge. PICA may be induced by these social stressors[13]. Other theories contemplated include PICA as a redirection of prey-catching/ ingestion behavior as a result of indoor confinement, especially common among oriental breeds due to risk of theft[14]. In natural environments PICA has been observed in Parrots, Macaws and other birds and mammals. Charles Munn has been studying Amazon Macaws lick clay from riverbeds in the Amazon to detoxify the seeds they eatCite error: Invalid <ref> tag; refs with no name must have content. Amazon Macaws spend two to three hours a day licking clay[15]. Munn has found that clay helps counter the tannin and alkaloid in the seeds the Macaws ingest, a strategy that is also used by Indians in the Andes Mountains in PeruCite error: Invalid <ref> tag; refs with no name must have content.

PICA also affects domesticated animals. While drugs like Prozac are often able to diminish troublesome behaviors in pet dogs, they don't seem to help with this eating disorder. The following story about Bumbley, a wirehaired fox terrier who appeared on 20/20 as a result of his eating disorder, is taken from a book by Dr. Nicholas Dodman.

"This dog's presenting problem was light chasing (otherwise known as shadow chasing). It chased shadows for hours on end, even excavating through plasterboard walls to pursue its will-o'-the-wisp illusions...The one thing that didn't come across clearly in the show was that Bumbley ate everything in sight and the house had to be "Bumbley-proofed" against his relentless ingestion of anything his owners left around...He had already had surgery to relieve intestinal obstructions resulting from his habit and, each day, his owners reentered their house with trepidation after work, fearing that Bumbley might have eaten something else."[16]

Dodman talks about new research relating bulimia and compulsive eating to seizural behavior in human patients. He suggests that antiepileptic medication might be a possible treatment for some cases of PICA in animals.

Behavioral Disorders

Behavioral Disorders are difficult to study in animal models because it is difficult to know what animals are thinking and because animal models used to asses psychopathologies are experimental preparations developed to study a condition. Can a monkey effectively communicate that he is sad or that he feels overwhelmed? Lacking the ability to use language to study behavioral disorders like depression and stress questions the validity of those studies conducted. It can be difficult to attribute human afflictions to non-human animals.

Obsessive Compulsive Disorder (OCD)

Compulsive behavior in animals can be defined as a specific, unnecessary action or series of actions that is repeated more often than would normally be expected. Obsessive thoughts are more difficult to identify in non-human animals. They are unreasonable and intrusive, and unpleasant and often express themselves as compulsive behavior.

A wide variety of animals exhibit behaviors that can be considered compulsive. While obsessive-compulsive disorder (OCD) in humans is perhaps the most common example of such behaviors in scientific and popular literature, it is important to consider similar phenomenon in other species. In a more anthropocentrism sense, study of obsession and compulsion in other species may help us to understand the processes and explanations behind obsessive-compulsive disorder and behaviors in humans.

Ritualized and Stereotyped Behaviors

Though obsessive-compulsive behaviors are often thought of as pathological, as in the case of OCD, or maladaptive in other animals, ritualized and stereotyped behaviors are often beneficial. These behaviors merit consideration because of the characteristics they share with obsessive-compulsive behavior, including a high degree of similarity in form and use among many individuals and a repetitive dimension.

There are many observable animal behaviors with characteristic, highly-conserved patterns. One example is grooming behavior in rats. This behavior is defined by a specific sequence of actions that does not normally differ between individual rats. The rat first begins by stroking its whiskers, then expands the stroking motion to include the eyes and the ears, and finally moving on to lick both sides of its body[17]. Other behaviors may be added to the end of this chain, but these four actions themselves are fixed. Its ubiquity and high degree of stereotypy suggest that this is a beneficial behavior pattern which has been maintained throughout evolutionary history.

Pathological stereotyped behaviors also exist in animals, but they do not necessarily provide a similar model to OCD in humans. Feather picking in orange-winged Amazon parrots has both a genetic component, with the behavior being more likely in one sibling if the other does it, and more common in parrots close to a door when they were housed in groups[18]. The same study found that feather picking was more common in females and that there was no social transmission of the behavior; neighbors of feather picking birds were only more likely to show the behavior as well if they were related.

An Evolutionary Basis

Some researchers believe that disadvantageous obsessive compulsive behaviors can be thought of as a normally beneficial process gone too far. Brüne (2006) suggests that changes of various origin in striatal and frontal brain circuits, which play a role in predicting needs and threats that may arise in the future, may result in a hyperactive cognitive harm avoidance system, in which a person becomes consciously and unreasonably fearful of an unlikely or impossible event[19]. This may also be true in other animals.

Animal Models of OCD

Animals exhibiting obsessive and compulsive behaviors that resemble OCD in humans have been used as a tool for elucidating possible genetic influences on the disease, potential treatments, and to better understand the pathology of this behavior in general. While such models are useful, they are also limited; it is unclear whether the behavior is ego dystonic in animals. That is, it is difficult to evaluate whether an animal is aware that its behavior is excessive and unreasonable and whether this awareness is a source of anxiety.

Some examples of ways in which rats and mice, two of the most common animal models, have been used to represent human OCD are provided below.

Lever pressing in rats

Certain laboratory rat strains that have been created by controlled breeding for many generations show a higher tendency towards compulsive behaviors than other strains. Lewis rats show more compulsive lever pressing behavior than Sprague Dawley or Wistar rats and are less responsive to the anti-compulsive drug paroxetine[20]. In this study, rats were taught to press a lever to receive food in an operant conditioning task. Once food was no longer provided when they pressed the lever, rats were expected to stop pressing it. Lewis rats pressed the lever more often than the other two types, even though they had presumably learned that they would not receive food, and continued to press it more often even after treatment with the drug. An analysis of the genetic differences between the three rat strains might help to identify genes that might be responsible for the compulsive behavior.

Rats have also been used to test the possibility of a problem with dopamine levels in the brains of animals that exhibit compulsive checking behavior. After treating rats with quinpirole, a chemical that specifically blocks dopamine D2/D3 receptors, compulsive checking of certain locations in an open field increased[21]. Some components of the checking behavior, such as the level of stereotypy in the path animals took to checked locations, the number of checks, and the length of the checks indicated and increase in compulsivity as doses of quinpirole increased; other components, such as the time taken to return from the checked location to the starting point and the time taken to make that trip remained constant after the initial injection throughout the experiment. This means that there might be both all-or-none and a sensitization aspects in the biology of the dopamine deficiency model of OCD. In addition, quinpirole might reduce a sense of satisfaction in the rats after they check a location, causing them to return to that location again and again.

Estrogen deficiency in male mice

Based on findings of changes in OCD symptoms in menstruating women and differences in the development of the disease between men and women, Hill and colleagues set out to research the effect of estrogen deprivation on the development of compulsive behavior in mice[22]. Male mice with an aromatase Gene knockout who were unable to produce estrogen showed excessive grooming and wheel running behaviors, but female mice did not. When treated with 17β-estradiol, which replaced estrogen in these mice, the behaviors disappeared. This study also found that COMT protein levels decreased in mice that did not produce estrogen and increased in the hypothalamus after estrogen-replacement treatment. Briefly, the COMT protein is involved in degrading some neurotransmitters, including dopamine, norepinephrine and epinephrine. This data suggests that there may be a hormonal component and a hormone-gene interaction effect that may contribute to obsessive behaviors.


Dr. Nicholas Dodman describes a wide variety of OCD-like behaviors in his book Dogs Behaving Badly[23]. Such behaviors typically appear when the dog is placed in a stressful situation, including an environment that is not very stimulating or a history of abuse. Different breeds of dog seem to display different compulsions. Lick granuloma, or licking repeatedly until ulcers form on the skin, affects more large dogs, like Labradors, golden retrievers, Great Danes, and Dobermans, while bull terriers, German shepherds, Old English sheepdogs, Rottweilers, wirehaired fox terriers, and springer spaniels are more likely to snap at imaginary flies or chase light and shadows. These associations probably have an evolutionary basis, although Dodman does not clearly explain that aspect of the behaviors. Dodman advocates the use of exercised, an enriched environment (like providing noises for dogs to listen to while owners are at work), and often Prozac (an SSRI used to treat OCD in humans) as treatments.

Dodman includes a story about Hogan, a castrated deaf male Dalmatian, and his compulsive behavior. Hogan had a history of neglect and abuse before he was adopted by Connie and Jim, who attempted to improve his behavior by teaching him to respond to American Sign Language. The following are some excerpts from Hogan's file.

"All was well for a year and a half when suddenly, one March morning, he woke up and started pawing everything in sight, and just wouldn't stop. He pawed rugs and blankets, hardwood floors and linoleum, grass and dirt surfaces...The similarity between what he was doing and prey-seeking behavior was remarkable."[24]
"I do believe...that Hogan was under some kind of psychological pressure at the time the compulsive pawing behavior developed... Connie and Jim were compelled to leave him for some eight hours a day while they went to work...The pendulum was set and ready to swing. The actual compulsion that develops under such circumstances is less relevant than the fact that one 'does' develop."[25]
"The "three R's" of rehabilitation are exercise, nutrition, and communication. First, I advised Connie to step up Hogan's exercise to a minimum of thirty minutes of aerobic activity a day. In addition, I advised that Hogan should be fed a low-protein, preservative-free diet. Completing the rehabilitation checklist, I exhorted Connie to work even harder with the sign-language and instructed her on a new sign to use when Hogan started digging. The sign was a piece of card with the letter 'H' written on it in thick black pen. Connie was to show Hogan this sign as soon as possible after he engaged in a bout of unwanted pawing and then leave the room. The idea was to let him know that the behavior was not wanted by signaling to him that Connie was about to leave the room...Call me a coward, but I didn't think that alone would cut it because of previous experiences with canine compulsive disorders so, employing a belt-and-suspenders strategy, I also advised medicating Hogan with the tricyclic antidepressant Elavil. Theoretically, Elavil wouldn't be that good in obsessive-compulsive behavior but, limited for reasons of expense, and bearing in mind the possible contribution of separation anxiety, Elavil was my best shot."[26]
"It took six months before Hogan was over the hump of treatment success...At this time Hogan only engaged in occasional pawing of significantly reduced intensity, and the pawing only occurred in moments of stress. Connie reported that stresses particularly likely to induce pawing included being unable to find her and sensing that he was about to be left alone...Hogan continued to improve and reached a point at which he was almost pawing-free - but not quite. That seems to be the way with compulsive disorders in man and beast. They can be reduced to the level of permitting affectees to lead relatively normal lives, but there are occasional relapses."[27]


Sugar addiction has been examined in laboratory rats and it develops in the same way that drug addiction develops. Eating sugary foods causes the brain to release natural chemicals called opioids and dopamine in the limbic system. Tasty food can activate opioid receptors in the ventral tegmental area and thereby stimulate cells that release dopamine in the nucleus accumbens (NAc). The brain recognizes the intense pleasure derived from the dopamine and opioids release and learns to crave more sugar. Dependence is created through these natural rewards, the sugary treats, and the opioid and dopamine released into the synapses of the mesolimbic system. The hippocampus, the insula and the caudate activate when rats crave sugar, which are the same areas that become active when drug addicts crave the drug. Sugar is good because its provides energy but when the body becomes dependent on the sugar intake and the nervous system goes through a change somatic signs of withdrawal begin to appear like chattering teeth, forepaw tremors and head shakes when sugar is not ingested[28]. Morphine tolerance, a measure of addiction, was observed in rats and their tolerance on Morphine was attributed to environmental cues and the systemic effects of the drug. Morphine tolerance does not depend merely on the frequency of pharmacological stimulation, but rather on both the number of pairings of a drug-predictive cue with the systemic effects of the drug. Rats became significantly more tolerant to morphine when they had been exposed to a paired administration than those rats that were not administered morphine along with a drug-predictive cue[29].


Using dogs Martin Seligman and his colleagues pioneered the study of depression in the animal model of learned helplessness at the University of Pennsylvania. Dogs were separated into three groups, the control group, group A had control over when they were being shocked and group B had no control over when they were being shocked. After the shocking condition the dogs were tested in a shuttle box where they could escape shock by jumping over a partition. To eliminate an interference effect- that the dogs did not learn responses while being shocked that would interfere with their normal escape behavior- the dogs were immobilized using curare, a paralyzing drug while they were being shocked. Both the control group and group A tended to jump over the partition to escape shock while group B dogs did not jump and would passively take the shock. The dogs in group B perceived that the outcome was not related to their efforts[30]. Consequently a theory emerged that attributed the behavior of the animals to the effects of the shock as a stressor so extreme that it depleted a neurochemical needed by the animals for movement[30]. After the dogs study the effects of helplessness have been tested in species from fish to cats[30]. Most recently learned helplessness has been studied in Rhesus Macaques using inescapable shock, evoked through stress situations like forced swimming, behavioral despair tasks, tails suspension and pinch induced catalepsy; situations that render the monkey incapable of controlling the environment[31].

Depression and low mood were found to be of a communicative nature. They signal yielding in a hierarchy conflict or a need for help[32]. Low mood or extreme low mood (depression) can regulate a pattern of engagement and foster disengagement from unattainable goals. “Low mood increases an organism's ability to cope with the adaptive challenges characteristic of unpropitious situations in which effort to pursue a major goal will likely result in danger, loss, bodily damage, or wasted effort.”[33] Being apathetic can have a fitness advantage for the organism. Depression has also been studied as a behavioral strategy used by vertebrates to increase their personal or inclusive fitness in the threat of parasites and pathogens[34].

The lack of neurogenesis has been linked to depression. Animals with stress (isolated, cortisol levels) show a decrease in neurogenesis and antidepressants have been discovered to promote neurogenesis. Rene Hen and his colleagues at Columbia University ran a study on rats in which they blocked neurogenesis by applying radiation to the hippocampal area to test the efficacy of antidepressants. Results suggested that antidepressants failed to work when neurogenesis was inhibitedCite error: Invalid <ref> tag; refs with no name must have content.


Sapolsky has extensively studied baboons in their natural environment in the Serengeti in Africa. He noticed that baboons have very similar hierarchies in their society as do humans. They spend very few hours searching for food and fulfilling their primary needs leaving them with time to develop their social network. In primates mental stresses show up in the body. Primates experience psychological stresses that can elicit physiological responses that overtime can make them sick. Sapolsky observed the baboon’s ranks, personalities and social affiliations then collected blood samples of the baboons to control the cortisol (stress hormone) levels of the baboons then matched social position to cortisol levels. Most of the data have been collected from male baboons because at any given time 80 percent of the females where pregnant[35]. Three factors influenced a baboon’s cortisol levels; friendships, perspective and rank. Baboons that played with infants and cultivated friendships, could tell if a situation was a real threat and could tell if they were going to win or loose, and were top ranking had lower levels of Cortisol. Cortisol levels rise with age and hippocampal cells express fewer hormone receptors on their surface to protect themselves from excess, making it harder to control stress levels[36]. Cortisol levels are elevated in half of people suffering from major depression, it is the hippocampal region that is affected by both. Stress can have negative effects on gastrointestinal function causing ulcers, and it can also decrease sex drive, effect sleeping patterns and elevate blood pressure but it can also stimulate and motivate. When animals experience stress they are generally more alert than when they are not stressed. It may help them be better aware of unfamiliar environments and possible threats to their life in these environments[37]. Yerkes and Dodson developed a law that explains the empirical relationship between arousal and performance illustrated by and inverted U-shape graph[38]. According to the Yerkes-Dodson Law performance increases as does cognitive arousal but only to a certain point. The downward part of the U-shape is caused by stress and as stress increases so does efficiency and performance but to a certain point[39]. When stress becomes too great performance and efficiency decline. Sapolsky has also studied stress in rats and his results indicate that early experiences in young rats have strong, lasting effects. Rats that were exposed to human handling (stressful situation) had finely tuned stress responses that may have lowered their lifetime exposure to stress hormones compared to those that were not handled. In short stress can be adaptive. The more exposure to stressful situations the better the rat can handle that situation[40].


Stereotyped behaviour or stereotypies are repetitive, sometimes abnormal behaviors like pacing on the perch for birds. There are adaptive stereotypic behaviors such as grooming in cats and preening in birds. Captive parrots commonly perform a range of stereotypies. These behaviors are repeated identically and lack any function or goal. Captive parrots perform striking oral and locomotor stereotypies like pacing on the perch or repetitive play with a certain toy. Feather picking and loud vocalizations can be stereotypies but are not as rigid and may be reactions to confinement, stress, boredom and loneliness as studies have shown that parrots that are in cages closest to the door are the most prone to feather pick or scream. Feather picking is not a true stereotypy and is more like hair pulling in human and loud vocalizations or screaming can be a stereotypy but vocalization is part of a parrot’s natural behavior. Captive parrots lack sufficient stimulation. Presumably they suffer from lack of companionship and opportunities to forage[41]. Stereotypies can evolve from the social environment for example the presence or absence of certain social stimuli, social isolation, low feeder space and high stocking density (especially for tail biting in pigs). These behaviors can also be transmitted through social learning, bank voles, pigeons and pigs when housed next to animals that show stereotypies, pick them up as well as through stimulus enhancement which is what happens in tail biting in pigs and feather pecking by hens[42].

Stereotypies may be coping mechanisms as results suggest from study on tethered and stalled sows. Sows that are tethered and stalled exhibited more stereotypies like licking and rubbing than sows that are in groups outdoors. This abnormal behavior seems to be related to opioid (related to the reward system) receptor density[43]. In sows, prolonged confinement, being tethered or being in gestations crates, results in abnormal behaviors and stereotypies. Mu and Kappa receptors are associated with aversion behaviors and Mu receptor density is greater in tethered sows than sows that are in groups outdoors. However, sows with stereotypy behaviors experienced a decrease both in Mu and Kappa receptor density in the brain suggesting that inactivity increases Mu receptor density and stereotypy development decrease both kappa and Mu receptor density.


Rhesus Monkeys have been observed to display self-aggression (SA) including self-biting, self-clasping, self-slapping, self-rubbing and threatening of body parts. The rhesus monkeys observed were individually caged and free of disease. Their self-aggression level rose in stressful and stimulating conditions like moving from one cage to another[44]. Stumptailed Macaques were studied to examine the source of their SA. SA increased in an impoverished environment and results support that SA may increase sensory input in poor environments. Captive Macaques do not socialize the way wild Macaques do which may affect SA. When allowed to socialize by putting another macaque in the cage or not putting them in a cage, SA levels in macaques decrease. Results indicate that SA is a form of redirected Social Aggression[45]. SA is related to frustration and social status especially in Macaques that have an intermediate dominance rank[46].

See also


  1. Owen, J. B., Treasure, J.L. & Collier, D.A. 2001. Animal Models- Disorders of Eating Behaviour and Body Composition. Kluwer Academic Plublishers, Norwell; Massachusetts.
  2. Hampstead, B. M., LaBounty, & L. P., Hurd, C., 2003. Multiple exposures to activity anorexia in rats: Effects on eating, weight loss, and wheel running. Behavioural Processes, 61(3),159-166.
  3. Hebebrand, J., Exner, C., Hebebrand, K., Holtkamp, C., Casper, R. C., Remschmidt, H., Herpertz-Dahlmann, B., Klingenspor, M. 2003. Hyperactivity in patients with anorexia nervosa and in semistarved rats: evidence for a pivotal role of hypoleptinemia. Physiology & Behavior, 79(1). 25-37.
  4. Hebebrand, J., Exner, C., Hebebrand, K., Holtkamp, C., Casper, R. C., Remschmidt, H., Herpertz-Dahlmann, B., Klingenspor, M. 2003. Hyperactivity in patients with anorexia nervosa and in semistarved rats: evidence for a pivotal role of hypoleptinemia. Physiology & Behavior, 79(1). 25-37.
  5. Siegel, S., Hinson, R.E., & Krank, M.D. 1978. The role of predrug signals in morphine analgesic tolerance: supports for a pavlovian conditioning model of tolerance. Journal of Experimental Psychology: Animal Behavior Processes, 4, pp 188-196.
  6. Time Magazine website in the Health and Science section. A Self-Corrective for The Population Explosion? Published Feb. 28, 1964.,9171,873834-1,00.html
  7. Radostits, O.M. 2000. Veterinary Medicine: A Textbook of the Diseases of Cattle, Sheep, Pigs, Goats and Horses. Saunders Ltd.; 9th edition, pp. 1767.
  8. Owen, J. B., Treasure, J.L. & Collier, D.A. 2001. Animal Models- Disorders of Eating Behaviour and Body Composition. Kluwer Academic Plublishers, Norwell; Massachusetts.
  9. Saeki, M., Sakai, M., Saito, R., Kubota, H., Ariumi, H., Takano, Y., Yamatodani, A., & Kamiya, H., 2001. Effects of HSP-117, a Novel Tachykinin NK1-Receptor Antagonist, on Cisplatin-Induced Pica as a new evaluation of delayed emesis in rats. The Japanese Journal of Pharmacology, 86(3). 359-362.
  10. Webster, A. B., 2003. Physiology and behavior of the hen during induced molt. Poultry Science, 82, 992-1002.
  11. Mitchell, D., Winter, W., & Morisaki, C. M. 1977 Conditioned taste aversions accompanied by geophagia: Evidence for the occurrence of “psychological” factors in the etiology. Psychosomatic Medicine. 39(6), 402-412.
  12. Bradshaw, J.W.S., Neville, P.F. & Sawyer, D. 1997. Factors affecting pica in the domestic cat. Applied Animal Bahviour Science, 52(3-4), pp. 373-379.
  13. Bradshaw, J.W.S., Neville, P.F. & Sawyer, D. 1997. Factors affecting pica in the domestic cat. Applied Animal Bahviour Science, 52(3-4), pp. 373-379.
  14. Bradshaw, J.W.S., Neville, P.F. & Sawyer, D. 1997. Factors affecting pica in the domestic cat. Applied Animal Bahviour Science, 52(3-4), pp. 373-379.
  15. Alcock, J.2005. Animal Behavior: An Evolutionary Approach. Sinaur Associates, Inc; 8th Edition.
  16. Dodman, Nicholas. 1999. Dogs Behaving Badly: An A-to-Z Guide to Understanding & Curing Behavioral Problems in Dogs. Bantam Books: New York. pp. 54-55
  17. Kalueff, A. V., et al. 2007. Analyzing grooming microstructure in neurobehavioral experiments. Nature Protocols 2, 2538-2544. Available:
  18. Garner, J. P., et al. 2006. Genetic, environmental and neighbor effects on severity of stereotypies and feather picking in Orange-winged Amazon parrots (Amazona amazonica): An epidemiological study. Applied Animal Behavior Science 96, 153-168.
  19. Brüne, M. 2006. The evolutionary psychology of obsessive-compulsive disorder: The role of cognitive metarepresentation. Perspectives in Biology and Medicine 49 (3), 317-329.
  20. Brimberg, L. et al. 2006. Strain differences in ‘compulsive’ lever pressing. Behavioral Brain Research 179, 141-151.
  21. Dvorkin, A., Perreault, M. L., and Szechtman, H. 2006. Developmental and temporal organization of compulsive checking induced by repeated injections of the dopamine agonist quinpirole in an animal model of obsessive-compulsive disorder. Behavioral Brain Research 169, 303-311.
  22. Hill, R. A. et al. 2007. Estrogen deficient mice develop compulsive behavior. Biol Psychiatry 61, 359-366.
  23. Dodman, Nicholas. 1999. Dogs Behaving Badly: An A-to-Z Guide to Understanding & Curing Behavioral Problems in Dogs. Bantam Books: New York.
  24. Dodman, Nicholas. 1999. Dogs Behaving Badly: An A-to-Z Guide to Understanding & Curing Behavioral Problems in Dogs. Bantam Books: New York. pp. 33.
  25. Dodman, Nicholas. 1999. Dogs Behaving Badly: An A-to-Z Guide to Understanding & Curing Behavioral Problems in Dogs. Bantam Books: New York. pp. 34.
  26. Dodman, Nicholas. 1999. Dogs Behaving Badly: An A-to-Z Guide to Understanding & Curing Behavioral Problems in Dogs. Bantam Books: New York. pp. 34-35.
  27. Dodman, Nicholas. 1999. Dogs Behaving Badly: An A-to-Z Guide to Understanding & Curing Behavioral Problems in Dogs. Bantam Books: New York. pp. 35-36.
  28. Colantuoni, C., Rada, P., McCarthy, J., Pattern, C., Avena, N.M., Chadeayne, A., Hoebel, B.G. 2002. Evidence that intermittent, excessive sugar intake causes endogenous opioid dependence. Obesity Research, 10(6), pp. 478-488.
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