How to Select Humane Endpoints  | Body Condition Scoring  | Endpoint Scoring Systems 


Humane Endpoints in Animal Research

Humane endpoints refer to one or more predetermined physiological or behavioral signs that define the point at which an experimental animal’s pain and/or distress is terminated, minimized or reduced by taking actions such as euthanizing the animal, terminating a painful procedure or giving treatment to relieve pain and/or distress (CCAC). Humane endpoints function as an alternative to experimental endpoints and provide investigators with an effective way to refine their research. The establishment of humane endpoints prior to the start of an experiment allows the investigator to prevent unnecessary animal pain and distress while ensuring accurate and timely data collection.

To be effective, humane endpoints must be clearly defined and based on objective criteria. Non-specific signs of illness such as inactivity, hunched posture or a rough coat are an indication that an animal should be examined more closely. By themselves these signs do not often constitute an endpoint. Familiarity with the animal model in use is necessary to select endpoints that are both humane and scientifically sound. As experience with and data collected from a specific animal model accrue, endpoints can be refined or modified. Further information on humane endpoints may be found on these and other websites:

Humane Endpoints in Laboratory Animal Experimentation

UC Davis Center for Animal Alternatives

USDA National Agricultural Library

Canadian Council on Animal Care

Investigators should include a precise definition of the humane endpoint(s), including specific assessment criteria, when describing how humane endpoints will be used in their IACUC protocols. The frequency of animal observation and assessment must also be clearly stated. Note: Normal, healthy experimental animals must be observed at least once a day. Animals in studies involving pain and/or distress will often require more frequent observations to effectively determine the time at which a specific endpoint has been reached. An appropriate monitoring schedule must be specified in the IACUC protocol for each study. In addition, the IACUC protocol must describe the training provided for personnel responsible for observation and assessment and the action(s) to be taken when an animal reaches a humane endpoint.

Type of Humane Endpoints

Humane endpoints are often based on the following:

  • Clinical signs
  • Pathophysiological changes
  • Behavioral changes
  • Biochemical changes
  • Hormonal changes

The exact time of the endpoint (the point at which an animal is removed from study) will depend on the objective of the experiment but should occur before the onset of distress (i.e., unable to adapt completely to a stressor) or as soon as possible thereafter.

Moribund Animals and Death as an Endpoint

The term moribund refers to an animal that is near death or in the process of dying. Animals in this state are often comatose (unresponsive and unaware of stimuli) and so beyond awareness of suffering. However, an animal may have experienced much pain and distress prior to reaching a moribund state. Stating that animals will be euthanized when they become moribund is not an appropriate humane endpoint as this may not reduce or alleviate any suffering that the animal will experience. The purpose of identifying endpoints is to prevent or minimize animal pain and distress. While certain types of studies have historically used death of the animal as a scientific endpoint, this is now rarely accepted and investigators must present conclusive evidence to support the use of such an endpoint.

How to select and use humane endpoints:

Choose appropriate endpoints that are objective and relevant for the assessment of pain/distress in the species. This may include:

  • Body weight changes
  • External physical appearance
  • Behavioral changes
  • Physiological changes (e.g., body temperature, hormonal fluctuations, clinical pathology, etc…)

Research personnel responsible for observing and evaluating animals must be adequately trained and experienced in the recognition of these signs for the species being used. Especially when using behavioral assessment, personnel must be familiar with “normal” before they can be expected to recognize “abnormal”. Investigators are responsible for ensuring these students and employees are appropriately trained and have the skill and authority to treat or euthanize animals who have reached an endpoint.

Pilot studies (experiments) can be useful in determining endpoints, especially when the effects of an experimental treatment in animals are not well known. They may also function to refine experimental studies by allowing for the establishment of earlier endpoints and provide training for personnel in the recognition of endpoints.

Examples of humane endpoints:

Deteriorating body condition score

  • Objective and easy to use for assessing the condition of animals used in research, especially studies where animals may experience some degree of debilitation as the study progresses.
  • Scoring methods have been developed for many species including mice.

Weight loss

  • Rapid weight loss of 15-20 percent within a few days. This requires frequent monitoring of body weight.
  • Gradual weight loss - over an extended period of time leading to emaciation. The degree of weight loss should be specified in terms of % or quantity (grams, pounds, kg).
  • Note: Certain debilitating conditions such as tumor growth and ascites may mask true weight loss.

The inability to rise or ambulate

  • Correlates with inability to access food or water.
  • Visually obvious, objective and easy to assess.

Tumor size

  • Usually measured as mean diameter of the mass or tumor volume as a percentage of body weight (i.e. greater than 1.5 cm diameter in mice or greater than 10% of body weight)

The presence of labored respiration

  • The animal shows increased respiratory rate and/or effort. Labored respiration is often accompanied by a strong abdominal component to breathing.


  • The skin looses its elasticity. In a hydrated animal, skin pinched over the back quickly returns to its normal position after it is released. In a dehydrated animal the skin is slow to return to normal or remains tented. Please note, dehydration must be moderate to severe before skin tenting will be noticable. Elderly, very young, obese or thin animals may show altered responses.

Ulcerated, necrotic or infected tumors.

  • The presence of open wounds.


Suggestions for the use of humane endpoints in selected studies:

Chronic studies where some degree of debilitation is expected.

  • Body Condition Scoring
  • Weight Loss
  • Loss of ability to ambulate (inability to access food or water).
  • Labored respiration may be associated with lung pathology or abdominal enlargement placing pressure on the diaphragm.


Experimental Neoplasia

  • Tumor size - tumor volume or mean diameter.
  • Tumor ulceration, infection, or necrosis.
  • For internal tumors body condition scoring may be more useful than weight loss.
  • Loss of ability to ambulate (inability to access food or water).
  • Labored respiration may be associated with lung pathology or abdominal enlargement placing pressure on the diaphragm.


Acute Studies

  • Weight loss
  • Dehydration
  • Loss of ability to ambulate (inability to access food or water).
  • Labored respiration may be associated with lung pathology or abdominal enlargement placing pressure on the diaphragm.


Experimental Surgery

Many of the same signs as listed above in addition to conditions specific to post-surgical infections or other complications including:

  • Pain, swelling, redness or discharge from surgical incisions.
  • Dehiscense (splitting apart) of surgical incisions.

These signs may not by themselves be endpoints but are medical issues that suggest there are deficiencies in surgical techniques or care that require attention.

Body Condition Scoring

Body condition scoring is a health assessment method useful for many species and can be used as a humane endpoint. Body condition scoring systems were originally designed to evaluate fat and muscle development in livestock. Scoring techniques for many species, including rodents, have since been published and are widely used to assess health and fitness in animals. Body condition scoring offers an objective and easy to use assessment method that can be incorporated into humane endpoints. Use of body condition scoring can provide a more accurate determination of health and fitness than body weight measurements, especially in studies where animals may develop tumor masses or fluid accumulation that obscures true weight loss. In addition, body condition scoring is useful in chronic studies where animals may lose muscle mass and fat over time.

A body condition scoring method for mice was developed by Ullman-Cullere, MH and CJ Foltz. Body condition scoring: A rapid and accurate method for assessing health status in mice. Laboratory Animal Science 1999, 49(3):319-323.

Endpoint Scoring Systems

An objective scoring system is often used to assist research personnel in monitoring, evaluation and documentation of humane endpoints in experimental animals. A form called a score sheet is used to record the health status of each individual animal at regular, predetermined intervals. The parameters monitored on a score sheet must be objective, easy to evaluate and relevant to the health issues expected for that procedure or experiment. Outside observers such as veterinary or IACUC personnel should be able to easily read and understand how scores are determined and decisions made based on the scores received. Analysis of an animal's score sheet must clearly show the effect of the procedure/experiment on the animal's health including a pattern of either recovery or deterioration over time.