Ungulate Population Sustainability: Veterinary Challenges Associated with Big Herd Management
American Association of Zoo Veterinarians Conference 2015

Julie Swenson, DVM; Holly J. Haefele, DVM

Fossil Rim Wildlife Center, Glen Rose, TX, USA


According to a 2011 report by the Association of Zoos and Aquariums (AZA) Task Force on the Sustainability of Zoo-based Populations, many ungulate species managed through AZA institutions are not sustainable at their current population levels. Some of the reasons cited for this included: low numbers of animals in individual populations and a lack of exhibit and breeding space sufficient to maintain a population capable reaching long-term sustainability.1 In order to combat this, larger populations of these threatened and endangered species are needed in AZA facilities.

One of the largest challenges facing AZA institutions in trying to meet this goal is a lack of holding space for maintenance of normal herd structure. One solution is to create breeding centers away from heavily populated areas where significant acreage can be devoted to the maintenance and breeding of large herds of ungulate species. Although this method shows significant promise, it does bring several unique challenges to the institution in regards to veterinary medicine.

Fossil Rim Wildlife Center is an 1800-acre facility outside of Dallas, Texas devoted to the care and breeding of endangered species. Currently, Fossil Rim’s veterinary staff manages the health of more than 1000 animals representing 50 different species. The majority of these animals are large hoofstock species that have been managed for an extended period of time in herds located on large pastures.

Over its 30-yr history, veterinary procedures and protocols have had to be developed and/or altered in order to accommodate the unique environment of Fossil Rim. Many of these protocols have been documented on film and/or video for later review. Some of these include alterations to darting methodology, routine vaccination and deworming protocols, chute work, body condition scoring to assess herds over time, and herd management via the use of vasectomized males.

Darting is a common component of most zoo medicine programs. Typically this involves the keeper staff shifting an animal into a smaller space where darting on foot from outside the enclosure is possible. In large herd situations on open pasture, the methodology for darting changes dramatically, particularly in regards to chemical immobilizations. Not only are the distances often much further, the darting typically occurs from a vehicle. Once darted, a significant amount of vehicle work needs to be performed in order to maintain an appropriate darting situation. Having experienced drivers, vehicles capable of handling rough terrain, and excellent communication becomes essential.

Routine vaccination protocols have their own challenges when considering a large herd of animals. Added complications also occur when a large number of animals from the same herd need to be immobilized in relatively close proximity, as may happen with a shipment of multiple animals during a small window. In these situations, the herd may become hypersensitive to the darting vehicles and may scatter immediately when approached. Over the years various methods have been used to overcome these challenges including: the use of different darting vehicles, using a pole syringe from the back of a tour van, as well as posting a veterinarian in a tree with the dart gun and using the vehicles to push the herd past the tree to facilitate darting.

Chute work becomes extremely useful when dealing with large herds of hoofstock. It allows the veterinary team the ability to get hands on animals in very rapid succession and it avoids the complications associated with multiple anesthetic events. In order for chute work to go smoothly, communication and delineation of task responsibility is essential and should be confirmed among the team members prior to starting the procedures.

Body condition scoring is used frequently in zoo medicine to track an individual animal’s condition over time, but is rarely utilized on the larger scale. In large herds, it is assumed that some animals will be below adequate condition, some animals above, but the goal is to maintain the average body condition of the herd at an ideal level. At Fossil Rim, body condition scoring is done routinely on the various herds as a whole and used at a population level to manage the overall herd condition.

Contraception becomes more difficult when dealing with herds in open pasture. Many of the hormonal methods become less feasible when dealing with large herds and mixed species pastures. Vasectomy as a method of contraception has been used at Fossil Rim to control populations that are no longer desired as part of the collection. Most notably, a breeding herd of red deer (Cervus elaphus) is in the process of being phased out after successfully vasectomizing 37 males.

Even in herds that are intended to be maintained as breeding herds, there may be reasons why regulating the number of offspring would be useful. A common concern is weather, as winter is not an ideal time for calves to be on the ground. One method around this is to pull the bull from the herd in order to avoid parturition occurring during the winter months. However, the lack of a bull in the herd can affect the herd dynamics. In order to avoid this complication, Fossil Rim uses a vasectomized bull that is exchanged for the intact bull during the ideal “off” season. This maintains the influence of a bull in the herd without having the complications associated with a year-long calving season.

Thirty years of large herd management at Fossil Rim has created a unique environment for the development of solutions to the challenges routinely faced by large acreage breeding centers. These problems are often unique to institutions that house large herds, but the solutions may be applicable to other areas of zoo medicine as well. Whether it is at an institution that already has large herds in open pasture, an institution that is considering adding an offsite breeding center, or an institution that is interested in recreating some of the herd dynamics in their smaller herds, many of the alterations that have been developed at Fossil Rim are applicable and adaptable to other facilities.


The authors thank the husbandry staff at Fossil Rim Wildlife Center, for their willingness to share their expertise in animal handling and restraint.

Literature Cited

1.  Boyle P, Andrews B, Dorsey C, Fouraker M, Pate D, Reed M, Wiese B. Building sustainable zoo populations: a report by the AZA Task Force on the sustainability of zoo-based populations: phase 1. Connect. 2011:10–13.


Speaker Information
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Julie Swenson, DVM
Fossil Rim Wildlife Center
Glen Rose, TX, USA

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