Management of Zoo Animals

Exhibits should approximate the animal's natural environment and enhance the visual experience for zoo visitors. Naturalistic enclosures with soil and vegetation are appealing to the public and are more stimulating for the animals; however, they present challenges for both sanitation and parasite control programs and can complicate restraint procedures.

Mixed-species exhibits can increase risk of disease transmission between species and can result in interspecific aggression if species are not chosen appropriately. Healthy mammals and birds can tolerate a fairly wide temperature range if given access to shade and water in hot weather and to a dry, draft-free shelter with a warm spot and ample food to meet increased energy requirements in cold weather (see mixed-species habitat image). Ensuring that each animal has access to a protected environment and that one dominant individual does not exclude others from shelter, food, or water is essential.

Mixed-species habitat, zoo

Image

This habitat at the San Diego Zoo Safari Park demonstrates both the natural substrate and browse opportunities that can be provided in a mixed bird and mammal exhibit; standing water and soil issues must also be managed to decrease parasite transmission.

Courtesy of Dr. Meredith M. Clancy.

Feeders should be easy to clean, with designs that avoid fecal contamination and make thorough cleaning possible. Feeder design should also consider the species in the exhibit, and feeders should be adequate in number for the exhibit's population. Domestic animal nutrition guidelines can help determine the need for additional feeders, especially for hoofstock, with models according to animal size and feeder bunk space.

Additionally, the timing of feedings should be considered. Feeding throughout the day can stimulate activity; however, considerations for a species' natural feeding habits can provide additional opportunity for behavioral enrichment (eg, feeding whole carcasses or instituting fast days for large carnivores).

Browsing animals generally eat continually. The inability to replicate both the components of their diet and an environment that supports continual browsing can lead to health issues. Food for behavioral enrichment should always be part of a well-balanced nutritional plan and included in calculations when formulating diets.

Reproductive management is fundamental to maintaining healthy ex situ animal populations. For many animals in accredited institutions, studbooks are maintained, and breeding recommendations are made by population biologists based on genetic or familial relatedness.

Breeding recommendations for some species in institutions accredited by the Association of Zoos and Aquariums (AZA) or European Association of Zoos and Aquariums (EAZA) are managed by Association of Zoos and Aquariums Species Survival Programs (SSP) and EAZA Ex situ Programmes (EEP), respectively. Contraception is necessary when reproductively mature animals of different sexes cannot be housed separately and can be either surgical or medical.

Some nondomestic species exhibit low reproductive success when not bred shortly after reaching sexual maturity or for protracted periods of time. This phenomenon of asymmetric reproductive aging is well documented in some mammals under human care, exemplified by aged female rhinoceroses that develop irreversible anestrus if not bred when young (1).

Male mammals can be surgically sterilized by castration or vasectomy. Castration is irreversible sterilization by surgical removal of the testes. Males might be able to impregnate females for a short period of time after castration because of residual testosterone in the bloodstream and viable sperm in the ductus deferens. Castration can eliminate male-specific behaviors, which can change social dynamics and affect group cohesion. The loss of testosterone can also eliminate some secondary sexual characteristics, especially those that manifest after the onset of puberty. Vasectomy is potentially reversible and prohibits reproduction while maintaining testosterone production, which preserves secondary sexual characteristics and male-specific behaviors and is therefore less likely to destabilize social groups.

Female mammals can be surgically sterilized by ovariectomy, ovariohysterectomy, or tubal ligation. Ovariectomy and ovariohysterectomy allow for permanent contraception of females and decrease or eliminate risk of diseases associated with the female reproductive tract. These are performed using standard techniques from domestic animal practice, including laparotomy from ventral midline or flank approaches and laparoscopy. Ligation of the fallopian tubes (tubal ligation) is potentially reversible and preserves ovarian endocrine function. It can be used for contraception of nonhuman primates; however, it does not decrease the risk of endometriosis. It is contraindicated in carnivores because of increased risk of uterine and mammary diseases, including neoplasia.

In most birds, only the left ovary is present. However, some species have bilateral ovaries, such as the kiwi (Apteryx spp). Surgical contraception of female birds can be achieved by salpingectomy (removal of the salpinx). It can be performed when medically indicated; however, it is rarely performed for preventive care or population management alone because of the invasiveness and high-risk nature of the procedure. Ovariectomy (surgical removal of the ovaries) is rarely feasible in birds because the ovarian vasculature is too closely associated with the descending aorta and caudal vena cava. By contrast, most female reptiles have bilateral ovaries, and surgical contraception is achieved by ovariectomy alone, rather than ovariosalpingectomy.

Medical contraception of females can be achieved with synthetic progestins, gonadotropin-releasing hormone (GnRH) agonists, or porcine zona pellucida vaccines. Synthetic progestins include megestrol acetate, medroxyprogesterone acetate, and melengestrol acetate, and they are effective in all mammal species. They work by increasing the density of mucus in the cervix and by blocking uterine implantation of fertilized ova. Females might continue to ovulate and exhibit physical and behavioral signs of estrus even when effectively contracepted using progestins.

At higher doses, progestins suppress ovulation through a negative-feedback mechanism on luteinizing hormone. Because progestins do not completely suppress follicular activity, estradiol production continues. When administered early in pregnancy, progestins can help maintain pregnancy or cause embryo reabsorption. These drugs do not affect parturition in primates and are considered safe for lactating females and their offspring. In some cases, the chances of a successful pregnancy can decrease after withdrawal of progestin-based contraceptives.

Megestrol acetate must be administered orally daily. Missing a single dose can allow for successful impregnation. Medroxyprogesterone acetate (MPA) is a relatively long-acting injectable contraceptive. It can induce diabetes mellitus in some primates and can act as an androgen, inducing male secondary sexual characteristics in female individuals (2). Melengestrol acetate (MGA) is administered orally daily or as a long-acting implant. It is commonly used for contraception of nonhuman primates; however, it is considered ineffective in equids. It has been associated with increased risk of endometrial hyperplasia and pyometra in carnivores, in which it is often contraindicated (3, 4).

GnRH agonists (leuprolide acetate, deslorelin) act on the hypothalamus–pituitary–gonad axis. They initially elevate luteinizing hormone (LH) and follicle-stimulating hormone (FSH), causing a spike in testosterone or estrogen. This can last up to three weeks in females and six weeks in males, after which there is a decrease in endogenous GnRH from the hypothalamus and its downstream receptors. Administration of megestrol acetate for a week before and after placement of a GnRH implant can decrease the initial spike of sex hormones.

Leuprolide acetate is administered as an intramuscular injection every 3-4 weeks, whereas deslorelin is administered as a relatively long-acting subcutaneous implant. Suppressive doses and duration of action are variable among species. Generally they are more successful at contraception of females than they are of males. Treatment failures are not uncommon and are attributed to inappropriate dosing or administration.

Reproductive success is generally good once treatment with a GnRH agonist is terminated in most taxa, with the exception of felids. In birds, GnRH agonists can be used to suppress reproduction in populations and individuals. Duration of action can be dose dependent and species specific. They are sometimes used to prevent or to treat excessive egg laying with variable success.

Porcine zona pellucida (PZP) vaccines are initially administered to mammals as two injections four weeks apart. Seasonal breeders are then revaccinated immediately prior to the onset of breeding season, while nonseasonal breeders can require more frequent vaccinations. PZP vaccination is safe for short-term use in herbivores; however, it can cause permanent or irreversible reproductive failure in carnivores or in chronically treated herbivores.

Veterinarians are often as involved in planning for reproduction as they are in contraception. Monitoring reproductive cycles can include recording serial hormone concentrations in blood or in noninvasively collected samples, such as urine or feces. It is recommended to plan ahead and proactively introduce (or separate) males to (or from) females to optimize chances of successful reproduction.

Assisted reproductive techniques, including artificial insemination, in vitro fertilization, and embryo transfer, have been successfully used in many species to improve reproductive success. After successful impregnation, birth plans should include husbandry and nutritional considerations for gravid and lactating dams; indications for intervention during labor, birth, and nursing; and protocols for hand-rearing, when appropriate.

The AZA's Reproductive Management Center at the St Louis Zoo and EAZA's Reproductive Management Group and Group on Zoo Animal Contraception are excellent resources for up-to-date guidelines, recommendations, and research.

• Reproductive Management Center at the Saint Louis Zoo. Association of Zoos and Aquariums.

• Reproductive Management Group. European Association of Zoos and Aquariums.

• Group on Zoo Animal Contraception. European Association of Zoos and Aquariums.

1. Hermes R, Hildebrandt TB, Göritz F. Reproductive problems directly attributable to long-term captivity–asymmetric reproductive aging. Anim Reprod Sci. 2004;82-83:49-60. doi:10.1016/j.anireprosci.2004.05.015

2. Nederlof RA, Bruins-van Sonsbeek LGR, Stumpel JBG, Bakker J. Update on current hormonal and nonhormonal contraceptive options in nonhuman primates. J Zool Bot Gard. 2024;5(4):606-629. doi:10.3390/jzbg5040041

3. Chittick E, Rotstein D, Brown T, Wolfe B. Pyometra and uterine adenocarcinoma in a melengestrol acetate-implanted captive coati (Nasua nasua). J Zoo Wildl Med. 2001;32(2):245-251. doi:10.1638/1042-7260(2001)032[0245:pauaia]2.0.co;2

4. Munson L, Gardner IA, Mason RJ, Chassy LM, Seal US. Endometrial hyperplasia and mineralization in zoo felids treated with melengestrol acetate contraceptives. Vet Pathol. 2002;39(4):394-419. doi:10.1354/vp.39-4-419