An animal model is a living, non-human animal used during the research and investigation of human disease, for the purpose of better understanding the disease without the added risk of harming an actual human being during the process. The animal chosen will usually meet a determined taxonomic equivalency to humans, so as to react to disease or its treatment in a way that resembles human physiology as needed. Many drugs, treatments and cures for human diseases have been developed with the use of animal models. Animal models representing specific taxonomic groups in the research and study of developmental processes are also referred to as model organisms. There are three main types of animal models Homologous, Isomorphic and predictive. Homologous animals have the same causes, symptoms and treatment options as would humans who have the same disease. Isomorphic animals share the same symptoms and treatments. This is the principle research tool. Predictive models are when the animals strictly display only the treatment characteristics of a disease. This method is commonly used when researchers do not know the cause of a disease. It is also useful in screening.
Taxonomic human equivalence 
Disease models 
Animal models serving in research may have an existing, inbred or induced disease or injury that is similar to a human condition. These test conditions are often termed as animal models of disease. The use of animal models allows researchers to investigate disease states in ways which would be inaccessible in a human patient, performing procedures on the non-human animal that imply a level of harm that would not be considered ethical to inflict on a human.
To serve as a useful model, a modeled disease must be similar in etiology (mechanism of cause) and function to the human equivalent. Animal models are used to learn more about a disease, its diagnosis and its treatment. For instance, behavioral analogues of anxiety or pain in laboratory animals can be used to screen and test new drugs for the treatment of these conditions in humans. A 2000 study found that animal models concorded (coincided on true positives and false negatives) with human toxicity in 71% of cases, with 63% for nonrodents alone and 43% for rodents alone.
Animal models of disease can be spontaneous (naturally occurring in animals), or be induced by physical, chemical or biological means. For example,
- The use of metrazol (pentylenetetrazol) as an animal model of epilepsy
- Immunisation with an auto-antigen to induce an immune response to model autoimmune diseases such as Experimental autoimmune encephalomyelitis
- Occlusion of the middle cerebral artery as an animal model of ischemic stroke
- Injection of blood in the basal ganglia of mice as a model for hemorrhagic stroke
- Infecting animals with pathogens to reproduce human infectious diseases
- Injecting animals with agonists or antagonists of various neurotransmitters to reproduce human mental disorders
- Using ionizing radiation to cause tumors
- Implanting animals with tumors to test and develop treatments using ionizing radiation
- Genetically selected (such as in diabetic mice also known as NOD mice)
- Various animal models for screening of drugs for the treatment of glaucoma
- The use of the ovariectomized rat in osteoporosis research
- Use of Plasmodium yoelii as a model of human malaria 
The increase in knowledge of the genomes of non-human primates and other mammals that are genetically close to humans is allowing the production of genetically engineered animal tissues, organs and even animal species which express human diseases, providing a more robust model of human diseases in an animal model.
Behavioral sciences 
Animal models observed in the sciences of psychology and sociology are often termed animal models of behavior. It is difficult to build an animal model that perfectly reproduces the symptoms of depression in patients. Animals lack self-consciousness, self-reflection and consideration; moreover, hallmarks of the disorder such as depressed mood, low self-esteem or suicidality are hardly accessible in non-humans. However, depression, as other mental disorders, consists of endophenotypes  that can be reproduced independently and evaluated in animals. An ideal animal model offers an opportunity to understand molecular, genetic and epigenetic factors that may lead to depression. By using animal models, the underlying molecular alterations and the causal relationship between genetic or environmental alterations and depression can be examined, which would afford a better insight into pathology of depression. In addition, animal models of depression are indispensable for identifying novel therapies for depression.
In quantitative genetics, the term animal model usually refers to a statistical model in which phenotypic variance is compartmentalised into environmental, genetic and sometimes maternal effects. Such animal models are also known as "mixed models".
Many animal models serving as test subjects in biomedical research, such as rats and mice, may be selectively sedentary, obese and glucose intolerant. This may confound their use to model human metabolic processes and diseases as these can be affected by dietary energy intake and exercise.
Animal models of psychiatric illness give rise to other concerns. Qualitative assessments of behavior are too often subjective. This would lead the investigator to observe what they want to observe in subjects, and to render conclusions in line with their expectations. Also, the imprecise diagnostic criteria for psychiatric illnesses inevitably lead to problems modeling the condition; e.g., since a person with major depressive disorder may experience weight loss or weight gain, insomnia or hypersomnia, we cannot with any certainty say that a rat with insomnia and weight loss is depressed. Furthermore, the complex nature of psychiatric conditions makes it difficult/impossible to translate human behaviors and deficits; e.g., language deficit plays a major role in autistic spectrum disorders, but – since rodents do not have language – it is not possible to develop a language-impaired "autistic" mouse.
In addition to the myriad ethical concerns of using animals in biomedical research, animal studies of psychiatric illness raise further concerns about the pain and suffering inflicted on the test subjects. While some scientists argue that care is taken to prevent unnecessary suffering in animal experiments, suffering is an inherent aspect of modeling distressful psychiatric conditions (e.g., anxiety, depression, posttraumatic stress disorder).
See also 
- Britches (monkey)
- Animal testing
- Ensembl genome database
- In vivo
- Animal testing on invertebrates
- Animal testing on rodents
- History of animal testing
- Knockout rat
- Mouse models of colorectal and intestinal cancer
- Animal models of schizophrenia
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