Marmosets in Research

Introduction

The common marmoset (Callithrix jacchus) is a platyrrhine primate originating from Central and South America. It is the most commonly used new world monkey in scientific procedures due to its comparatively rapid breeding rate, tractability and relation to humans. As a non-human primate, marmosets have special protections under the Animals (Scientific Procedures) Act 1986 and may only be used in research where no alternative models are available.

At the Comparative Biology Centre (CBC), the majority of marmoset work is focused on basic research into the brain and nervous system.

How to sex juveniles and adults

Common marmosets have shown minimal sexual dimorphism compared to other species of non-human primate; for example, males and females have similar weights and growth curves.  The external genitalia can be used to sex adults and is easily observed in the cage, particularly as animals will readily display their genitalia.

In adult males, the scrotum can be seen as a hairless horseshoe-shaped elevation of skin, covered in small pearly nodules. The tip of the penis is visible in the midline of the scrotum and will protrude during aggressive displays and mating.

Adult females have a large vulva with a superficially similar appearance to the scrotum, but there is a considerably greater distance between vulvar entrance and anus. Females also have a pair of pectoral nipples, although these are difficult to see due to their fur.

In young marmosets, sexing is slightly harder as the genitalia are not fully developed, and restraint and manipulation is required to determine sex with confidence.  In neonatal males, the scrotum appears as an irregular fold of skin with small nodules, with a small rounded slit at the top of the skin fold showing the location of the penis – manipulation is required to reveal it. At 45 days of age, the penis is more easily exposed, and the nodules develop a pearly white appearance. The testes do not appear until 2-3 months of age and do not fully descend until 8-11 months.

The perineal skin of the neonatal female appears similar to that of the male and also has small nodules. Determination of the animal as female is decided by the greater ano-genital distance and the presence of a longer slit like opening. At 45 days, the nodules turn white and palpation will reveal the vaginal opening.

Estimating the age of a marmoset

Common marmosets almost always give birth overnight, meaning that the offspring (of which there are normally 2-3) will be seen and recorded by a technician completing the daily checks required by ASPA the next day. As non-human primates, each marmoset must have an individual history file including information on its date and place of birth therefore it is unlikely that an individual marmoset’s age could not be worked out using its file.

However, it is still important to be able to at least roughly visually identify an animal’s life stage, especially if working with unfamiliar or new animals, as this will inform the technician’s understanding of its normal appearance, behaviour and response to stressors and procedures, enabling health problems to be noticed and addressed more quickly. Knowledge of the animal’s normal development is also important so that animals that are not developing as expected can be recognised. For example, marmosets only have two nipples so when triplets are born one may struggle to feed. Depending on the colony management, this animal will either be euthanised, or a healthy sibling taken for hand rearing to give it a chance.

The age of young animals can be estimated relatively reliably (details given in the table below) as infants have different coat patterns to older animals and have not undergone the changes associated with puberty. It is important to note that there will always be individual differences in growth and appearance due to differing experience levels of parents, genetics and environmental influences such as food availability and quality. 

Once an animal has reached adulthood and sexual maturity, distinguishing between younger and older adults is much trickier as there are few obvious physical changes except some greying around the face, which again may be affected by that individual’s experience. Laboratory marmosets also appear to remain fertile up to and possibly after 8 years of age, so sexual behaviour and reproduction is not a reliable indicator of age. As non-human primates, marmosets may also have quite distinct ‘personalities’ that affect how they behave into old age, and familiarity with the animal would be required to pick up subtle indications of age-related degeneration.

Physical and behavioural indicators of age in the Common Marmoset

Physical appearance	Physical indicators	Behavioural indicators
Infant 0 – 3 months	Birth weight is 25-35g Marmosets are born precocial, with eyes open. Neonates have a fine coat of grey or brownish hairs. They do not have the white ear tufts that characterise adult C.jacchus. The face is darker than in the adult and a dark stripe runs down the centre of the head. The first milk teeth develop within 3 days of birth. At birth, the pudendal pad (used for scent-marking) is dark brown; by 45-60 days of age it has faded to a pale pink.	0-2 weeks: infants have a very strong cling instinct and will not voluntarily leave their carrier’s back. 2 weeks – 3 months: time off carrier increases and infants develop greater coordination and locomotion. At this age they are almost completely weaned and capable of self-feeding, except for gum feeding, where they still rely on adults to gouge holes for them.

Physical appearance	Physical indicators	Behavioural indicators
Juvenile 3 – 12 months	White ear tufts begin to appear at around 3 months., the animal is around 75% of adult weight at this point.   Milk teeth lost at 3 ½ months. Testicles in male fully descend at 8-11 months of age, but can be withdrawn into the abdomen if stressed. Full adult colouration is achieved at around 8 months and adult canines become more tusk like.	The juvenile stage is typified by a high frequency of play behaviour with siblings.   7-8 months – juveniles start to carry and play with younger siblings; an important part of developing successful breeding behaviour later in life.

Physical appearance	Physical indicators	Behavioural indicators
Sub-adult 13-18 months Adult 18+ months	Full sexual maturity at around 14 months. Pubertal body weight growth spurt in both males and females occurs at 12-14 months and adult weight (300-400g for males and females) reached at 15 months.	Full repertoire of adult behaviours displayed, including attempts at mating. Reproduction will not occur until the social conditions are appropriate for it.

Physical appearance	Physical indicators	Behavioural indicators
Older adult 8+ years	Older marmosets show signs of greying around the head and face	Ageing is not well characterised in the marmoset. It is expected that a greater proportion of the animal’s time will be spent inactive and play behaviour will rarely be seen. Domain specific cognitive impairment is seen in aged marmosets performing behavioural tasks.

Handling and restraint for procedures

Why is restraint necessary

In the laboratory environment, it is essential that those responsible for the health and husbandry of the animals are able to handle and restrain animals for sexing, health checking and procedures. This should be carried out in a manner that causes minimal stress to the animal and others in the colony and does not result in injury to the animal or the handler. The strength, agility and intelligence of non-human primates such as common marmosets presents a unique set of problems in regard to handling them and where possible, positive reinforcement training (PRT) should be used to promote cooperation and reduce stress. Although methods of restraint will be described in this document, competence can only be achieved by practice under the supervision of a more experienced staff member. 

Things to consider prior to restraint

Before catching or restraining any animal, the following factors should be considered:

• The species, age, sex, health status, reproductive status and temperament of the animal to be restrained;
• The purpose and duration of the restraint required;
• How to minimise the disruption to the social group if required.

Sick animals or those recovering from procedures may already be physically or psychologically stressed and may require modifications to normal handling methods. The stress of handling may exacerbate any pre-existing problems, and normally calm animals may become aggressive when in pain or stressed. Particular care should be taken when handling pregnant or nursing females as stress could cause litter loss; common marmosets will also aggressively defend infants so older animals should always be caught first or separated from the group prior to catching younger ones.

Physical restraint

Physical restraint methods are used for routine husbandry and short procedures, and are cheap, safe and effective if performed efficiently by experienced and well-trained staff. Handling and restraint are known to be stressful to marmosets so should be avoided where possible – for example, marmosets can be trained using PRT to sit on a weigh scale, enter a carry box or provide urine samples.

Marmosets must be caught before restraint – this can either be from a carry box or the enclosure. When approaching the animals, the handler should have a calm and confident manner, and speak in a gentle voice. In the cages at the CBC, marmosets can be asked to hold a target in an area that can be separated off from the main cage using plastic partitions, reducing the catching area. Marmosets may be held by the base (not the length) of the tail for temporary restraint, whilst the other hand is used to grasp over the shoulders and under the arms and lift them out of the cage.

Restraint can be carried out with or without the use of a leather glove, depending on the temperament of the animal. One hand should grasp around the upper body, with the thumb and forefinger controlling the head and one forearm. The thumb can be used to push the animal’s forearm in front of its mouth if it tries to bite, but the animal should still be able to rotate its head and the handler should not put too much pressure on its chest.  The other hand should hold the lower body for support or allow it to curl its hind limbs around the handler’s wrist. This hold allows the animal to be presented to a second person who may carry out any required procedures such as blood sampling or substance administration.  Restraint and procedural work should not be carried out by the same person because of the risk to the animal and the handler.

Chemical restraint

Chemical means of restraint are often used due to safety requirements, time constraints or because of the procedure being carried out.  Chemical restraint includes the use of sedatives such as alfaxalone for brief procedures or detailed clinical exams, or general anaesthetic for lengthy procedures such as brain surgery. Alfaxalone is preferred to ketamine for pre-medication and brief sedations as it is less damaging to muscle tissue.

A detailed description of chemical restraint in marmosets is beyond the scope of this document, but those involved with it should be familiar with the variety of drugs used to achieve sedation or unconsciousness, and their pharmacological properties, recovery times and potential side effects. When administering anaesthetic drugs by parenteral routes the correct PPE should always be worn, in case of needle stick injury, splash back of drugs or aggression from the animal. Staff must also be able to recognise the signs of the appropriate level of sedation in the animal. Monitoring and support should be provided throughout chemical immobilisation, and post recovery, behaviour, appetite, hydration status and the urinary and faecal output of the animal should be monitored daily (or more frequently as appropriate).

Health checking

ASPA requires all animals at a licensed establishment to be checked at least once daily by a competent person so that health problems that may cause unnecessary suffering or interfere with the scientific purposes of the work can be detected and addressed.  Health monitoring is a continuous process and should be carried each time animals are handled, fed, or watered. Animals that have undergone procedures should be inspected at a frequency appropriate to the severity of the procedure, the expected adverse effects and the potential rate of change in the animal’s condition. All staff working with marmosets should be aware of the signs that an animal is unwell but may need to flag these signs to staff that are more experienced with that species, or in the case of marmosets, the individual. More experienced staff can assess the animal’s condition and determine whether to contact the NACWO or NVS for further guidance.

As mentioned previously, restraint is stressful for marmosets and may also affect the health check itself e.g. stress can increase heart and respiratory rate. Where possible, as much information should be gathered when the animal is in the cage prior to catching and handling. Inspection of the cage itself can also be a useful source of information.

Observation is best carried out in the home cage with minimal disturbance, ideally by a staff member experienced with the normal behaviour and appearance of marmosets, and that of each individual as well. The animals should be familiar with the staff member so that their presence doesn’t cause undue disturbance.  In some cases, use of video monitoring may be more appropriate as prey animals like marmosets will frequently conceal signs of pain and distress.

The marmoset housing should be inspected for normal faecal and urinary output. Diarrhoea, mucus or bloody faeces on shelving may indicate digestive issues and blood stained urine is a sign of cystitis. Blood on the cage walls or furniture can signal fight wounds or traumatic injuries. The amount of food and water consumed should also be assessed – the loss of appetite is often the first subtle sign of illness, especially dental problems. If the animals are drinking and urinating excessively this could indicate kidney problems.

The posture, appearance and behaviour of the animal in the home cage should then be evaluated.  A hunched or crouched posture, with the head forward is a possible sign of pain as is stretching the body and pressing the belly into the ground. Repeated straining could be a sign of urinary tract or low gastrointestinal tract problems. If the animal will come forward, its general appearance should be evaluated – does it look over or underweight?  Is the coat clean and well-groomed or dull and scruffy? Are there any obvious injuries? Does the animal have any swellings or discharge around the face, especially the mouth and eyes, which could be a sign of dental abscesses? Can you hear any wheezing, coughing or sneezing that might indicate a respiratory issue?

In terms of behaviour, isolation from cage mates should be a source of concern. Marmosets in ill health may be either mainly immobile or restless and hyperactive. Abnormal movement e.g. limping should always be investigated. Any marmoset sitting on the cage floor should be checked more closely immediately as this is highly abnormal behaviour.  Increased self-scratching, piloerection, tooth grinding and tremors are all signs of pain in marmosets and excessive scent marking may indicate stress. Sick animals may show an increase or decrease in vocalisation. In marmosets this can be difficult to assess as they can call in ultrasound which humans cannot detect. Response to the staff’s presence should also be evaluated – if it is refusing treats, is more withdrawn or more aggressive than usual then it may be in pain.  Marmosets in physical or mental distress may also show stereotypic behaviour (e.g. repeated circling) or even self-mutilation.

Animals should be caught and examined more thoroughly on a regular basis, or whenever there is cause for concern. If it hasn’t been weighed recently, it should be at this point to look for signs of weight loss or gain, or a departure from the expected growth rates for that species.  Palpation of the hips and spine can be used to assess body condition score, see the image below for an example used at the CBC. The animal should be fully checked in a logical order using the guidance given in Table 2 below. Clinical samples such as bloods or nasal swabs for health screening or further investigation may be taken as necessary.  All information from health checks should be recorded to provide information on the animal’s previous life experience and health status, so that future problems can be detected more easily.

Signs of ill health in the marmoset (by body part)

Body part	Signs of ill health	Signs of good health
Mouth	Lesions, malocclusion, inflammation, overgrown, missing or broken teeth. Sores or wetness on the chin may indicate a problem in the mouth causing drooling. Bad breath is usually a sign of tooth problems.	Lips, gums and teeth all clean, present and aligned properly.
Nose	Nasa discharge indicates infection.  It may be present in the nostrils, or evident on matted fur around the mouth, nose, and forelimbs. Abnormally fast or slow, laboured or noisy respiration indicates a problem.	Nose clean and clear. The normal respiratory rate for marmosets is 50-70 breaths/minute, although this may be slightly elevated with restraint.
Eyes	Glazed and sunken eyes are a sign of pain in the marmoset. Jagged or uneven pupils may indicate neurological problems. Swellings under the eye normally indicate abscesses of the upper canine.	Eyes should be clean, clear and bright with no discharge. There should be no redness or discolouration of the sclera or cloudiness of the cornea. Eyes should be focused and pupils even.
Ears	Accumulation of material such as wax, pus, mucus or blood is indicative of injury, infection or infestation with ear mites.	There should be no wounds or swelling on the outer ear (pinna).
Body	A variation more than 1̊C in body temperature may indicate a health problem. Hyperthermia could be infection, hypothermiacould be the animal going into shock. A rapid or slow heartbeat, or a thready pulse, is a health concern. Cold extremities are a sign of illness.	Normal rectal temperature for marmosets is 38.6̊C. The body surface should be free of injuries or swellings. The animal should have a body score of 3 (ideal).
Coat/skin	Unkempt, dull coat could be due to lack of grooming. Missing fur can be a sign of parasitic or bacterial infection, metabolic disorder or over grooming by the animal or a conspecific. Mammary glands in females should be checked for inflammation or lumps that could be a sign of mastitis or tumours.	Coat should be clean and well groomed, skin free of injuries, swelling, bruising and other abnormalities. When a healthy animal’s skin is tented, it should return to normal quickly, otherwise it may be dehydrated.
Anogenital region	Faecal accumulation around the anus may indicate digestive problems or that the animal is struggling to groom itself. Urine burns or inflammation around the hind parts of the animal may indicate it is struggling to move or groom itself.	In females the vulva should be clean with no trauma or abnormal discharge, and in males there should be two well-developed testes, an uninflamed penis and clean prepuce. There should be no inflammation, blood or prolapse.
Limbs	Injuries or infections may be indicated by swollen or enlarged joints, lesions or difficulty moving the affected limb. Sores may indicate abnormal movement (such as circling) or unsuitable substrate.	Nails should be intact and of a reasonable length.

Health and safety when handling

The main risks when handling are trips and falls when catching on uneven or slippery floors, injury due to the sharp teeth and claws of marmosets and the potential transmission of zoonotic diseases. The primary risks to the marmosets being caught are injury, excessive stress and diseases transmitted by the handler. There are several actions employers and staff can take to reduce these risks.

Ensuring that the procedural area is fully set up and all equipment is available will decrease the time that the animal must be handled, thereby reducing the risk associated with handling and the stress experienced by that animal. All staff handling a specific species should be competent to do so or supervised by a more experienced member of staff, and appropriate catching methods should be used. The animals should never be restrained or lifted by the end of their tails or limbs as doing so can result in fractures and distress. Nets should never be used as they can cause serious injury and are extremely stressful for the rest of the colony.

When catching marmosets, PPE (personal protective equipment) should always be worn. At a minimum, disposable vinyl or latex gloves, a properly fitted mask and a long-sleeved gown, lab coat or shirt should be worn. Long sleeves reduce the risk of scratches and minimise the likelihood of skin to skin contact between the handler and the marmoset. Use of gloves and masks also reduces the risk of disease transmission. Wearing protective eye shields should also be considered as marmosets frequently spray urine when handled. Non-slip steel toed shoes are important for catching on uneven or wet surfaces, or when moving around heavy metal caging.

Marmoset teeth are extremely sharp and capable of inflicting serious injuries so if the animal is likely to be aggressive, leather gloves should be used. The gloves should be good quality leather to prevent injuries to the animal’s mouth and teeth if it does bite. Facilities should have a procedure in place for if a bite or scratch does occur. At the CBC, wounds must be cleaned thoroughly with hibiscrub for at least 15 minutes, reported to a first-aider and the NACWO, and depending on the health status of the animal, may require a trip to hospital for health monitoring.

As mentioned previously, handler competence is an important aspect of reducing stress for the animal. With marmosets, PRT has great value in reducing stress and improving cooperation in stressful procedures and should always be the first option used. Because marmosets find handling so stressful, it should be avoided where possible e.g. through in cage weighing and training entry into carry boxes. The most common impact of handling induced stress in marmosets is inappetence and weight loss so weights should be monitored after handling, and supplementary food provided to any that have lost weight.

Restraint for common procedures

Blood sampling

Blood sampling of marmosets can be carried out under physical or chemical restraint. For physical restraint, the animal should be caught and restrained as described previously, with the thumb and forefinger controlling the head and upper body. The forearm may be included in this grip to prevent the animal biting the handler or sampler. The other hand should support and extend the legs as the most common site for blood sampling in the marmoset is the femoral vein.

Blood sampling in the conscious marmoset always requires two people: the handler to restrain the animal and compress the vein, and the sampler. The grip should be secure enough that the sampling site can be clipped and disinfected prior to sampling but not so much that the animal panics. The femoral vein must be detected using percussion as it is not usually visible and care should be taken to avoid the femoral artery, which runs alongside it. Blood should be withdrawn with a 25-27G needle, into a syringe no bigger than 1ml to avoid the vein collapsing. After withdrawing the needle, the sampling site should be compressed for 30-60 seconds to prevent uncontrolled bleeding.

For animals under chemical restraint, the saphenous vein and lateral tail veins may also be used as these are easier to visualise. Blood flow from each tends to be poor particularly under sedation as marmosets lose body heat rapidly when anaesthetised (personal observation).

Oral dosing

If the substance is not strongly flavoured, it can be incorporated into a favourite treat e.g. banana purée, to avoid the use of restraint altogether. In other cases, the animal must be caught and restrained as usual, with the head firmly controlled so that another person can insert a syringe into the marmoset’s mouth. In the case of oral dosing the animal’s forearm should not be included in this grip so that access to the mouth is unimpaired. Care must also be taken not to tip the animal back or give it too much of the substance too fast to avoid choking or aspiration pneumonia.

Orogastric tubes may be appropriate in some cases. They require an experienced handler to keep the animal in a still and upright position with the head prevented from moving using the thumb and forefinger method described previously. The tube should be lubricated and passed into the mouth, with a gag to prevent chewing. The person positioning the tube should feel little resistance and be able to palpate it in the neck of the animal. Once the tube is in position in the stomach, they should also be able to smell gastric acid. Sterile saline should be administered before the substance to be dosed to ensure the correct placement of the tube – coughing indicates incorrect placement. Once the substance has been administered, the tube should be withdrawn gently to prevent vomiting or regurgitation.

IM injection

Intramuscular injection of marmosets can be carried out under physical or chemical restraint. Normally, this is done under physical restraint. For physical restraint, the animal should be caught and restrained as described previously, with the thumb and forefinger controlling the head and upper body. Animals appear less stressed and resistant to injection when allowed to curl their hind limbs around the handler’s wrist and support themselves. From here, the leg can be gently extended by a second person to access the quadriceps muscle and a 25-27g needle inserted at about 45 degrees to the muscle. The shallower angle is advised because of the small size of marmoset muscles (personal observation).

SC injection

Subcutaneous injection of marmosets can be carried out under physical or chemical restraint. Normally, this is done under physical restraint. For physical restraint, the animal should be caught and restrained as described previously, with the thumb and forefinger controlling the head and upper body. Animals appear less stressed and resistant to injection when allowed to curl their hind limbs around the handler’s wrist and support themselves. From here, a tent of skin can be pinched on the scruff or flank of the animal and a 25-27g needle is inserted into the bottom part of the tent to administer the injection.

Animals that are difficult to handle

The temperament, age, experience, sex and health status of an animal all affect how it will respond to handling, as does the experience and confidence of the handler. Where animals are known to be more difficult to handle, a handler experienced with the species and ideally that individual should be used. Having a handler that is familiar to the animal is also good practice as it can reduce the stress of the procedure. A gentle, but firm grip from an experienced handler prevents the animal from feeling insecure or too constrained, both of which can result in a struggle to escape and injury to animal or handler. Animals may also struggle violently when being returned to the home cage, so care must be taken at this point to prevent them injuring themselves on doors and cage furniture.

Measures can be taken during catching to reduce the risk of aggression. As mentioned previously, marmosets will defend younger animals, so the oldest animals should be caught first, or separated from the others to reduce the risk of aggression. Additional PPE can be used, such as thick leather gauntlets and eye protection where animals are known to be aggressive. Animals should be handled away from conspecifics to avoid stressing them as well.  Where possible, marmosets should be rewarded with favoured foods for calm behaviour during handling and restraint. If the procedure is expected to have adverse effects or the animal is particularly stressed, extra monitoring should be put in place after it is returned to the home cage in case any problems emerge. In some cases, where the project licence permits, chemical restraint may be more appropriate.

References and further reading

1. University of Cambridge Training Centre, 2016. Training course for applicants for licences: new world primate. Cambridge.
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3. Whitham, J.C. and Wielebnowski, N., 2009. Animal‐based welfare monitoring: using keeper ratings as an assessment tool. Zoo Biology: Published in affiliation with the American Zoo and Aquarium Association, 28(6), pp.545-560.
4. University of Stirling, Primate Society of Great Britain, NC3Rs 2020. Common Marmoset Care. http://www.marmosetcare.com Accessed 28.01.20.
5. Cummins, K. 2019. Marmoset Husbandry and Enrichment Manual. DSTL.
6. NC3Rs. Non-surgical blood sampling in the marmoset: femoral vein. https://www.nc3rs.org.uk/marmoset-femoral-vein-non-surgical. Accessed 26.01.20.
7. Joint Working Group on Refinement, 1993. Removal of blood from laboratory mammals and birds. First Report of the BVA/FRAME/RSPCA/UFAW Joint Working Group on Refinement. Laboratory Animals, 27, pp.1-22.
8. Schultz-Darken, N.J., 2003. Sample collection and restraint techniques used for common marmosets (Callithrix jacchus). Comparative medicine, 53(4), pp.360-363.
9. Hearn, J.P., Lunn, S.F., Burden, F.J. and Pilcher, M.M., 1975. Management of marmosets for biomedical research. Laboratory animals, 9(2), pp.125-134.
10. Klein, H.J. and Murray, K.A., 1995. Restraint. In Nonhuman Primates in Biomedical Research: Volume 1 Biology and Management (pp. 286-297). Academic Press.
11. Turnquist, J, E. and Hong, N., 1995. Functional Morphology. In Nonhuman Primates in Biomedical Research: Volume 1 Biology and Management. Academic Press.
12. Southers, J.L. and Ford, E. W., 1995. Preventative Medicine. In Nonhuman Primates in Biomedical Research: Volume 1 Biology and Management. Academic Press.
13. Barnett S.W. 2016. Introduction to laboratory animal science and technology, 3rd Edition. Institute of Animal Technology.
14. Layne, D.G. and Power, R.A., 2003. Husbandry, handling, and nutrition for marmosets. Comparative medicine, 53(4), pp.351-359.
15. Ludlage, E. and Mansfield, K., 2003. Clinical care and diseases of the common marmoset (Callithrix jacchus). Comparative medicine, 53(4), pp.369-382.
16. Ruivo, E.B., Zoo, B., Stevenson, M.F. and Gardens, B.Z., EAZA BEST PRACTICE GUIDELINES. http://www.zoocentral.dk/uploads/4/9/7/5/49755431/callithridae_-_eaza_-_2015.pdf Accessed 31.01.20.
17. Cawthon Lang KA. 2005 May 18. Primate Factsheets: Common marmoset (Callithrix jacchus) Behavior. http://pin.primate.wisc.edu/factsheets/entry/common_marmoset/behav. Accessed 30.01.20.
18. Abbott, D.H. and Hearn, J.P., 1978. Physical, hormonal and behavioural aspects of sexual development in the marmoset monkey, Callithrix jacchus. Reproduction, 53(1), pp.155-166.
19. Casteleyn, C. and Bakker, J., 2019. The Anatomy of the Common Marmoset. In The Common Marmoset in Captivity and Biomedical Research (pp. 17-41). Academic Press.
20. Abbott, D.H., Barnett, D.K., Colman, R.J., Yamamoto, M.E. and Schultz-Darken, N.J., 2003. Aspects of common marmoset basic biology and life history important for biomedical research. Comparative medicine, 53(4), pp.339-350.
21. Hershkovitz, P., 1977. Living New World monkeys (Platyrrhini) : with an introduction to Primates, Chicago: University of Chicago Press.
22. Britain, G., 2000. Guidance on the Operation of the Animals (Scientific Procedures) Act 1986. Proquest LLC.
23. Home Office, 2014. Code of practice for the housing and care of animals used in scientific procedures. (24) Vanderlip, Glavis-Bloom and Reynolds, 2022. Aged common marmosets show domain-specific cognitive impairment. Poster at SFN 22.