What Goes Into Antelope Breeding Decisions?

Tye Chandler :
Posted July 6, 2020

Fossil Rim Wildlife Center has a wide variety of antelope, and thus making breeding decisions requires a lot of planning that needs to be carefully weighed for each species.

For Director of Animal Care Adam Eyres, who has 31 years of experience at Fossil Rim, what are the different considerations running through his mind for herd management?

Quite a few Fossil Rim antelope species really do well in large herds, including scimitar-horned oryx (pictured) and other oryx species, as well as sable and blackbuck.

“Obviously, the various species have different gestation lengths,” Eyres said. “The reason we do our bull swapping – switching an intact bull with a vasectomized bull – is so we don’t have animals born during the bad weather of December and January. We are also trying to optimize our conditions for good grasses in regard to when babies will start to wean and be putting their mothers under the most pressure to provide milk, which will lead to better health for mother and calves alike. We also need a good breeding plan to avoid familial breeding of closely related individuals.

“Most of the species at Fossil Rim are part of an SSP (Species Survival Plan of the Association of Zoos and Aquariums), so they have specific males and females that they want us to breed at certain times. For example, this year they might want a particular bull to be a breeding bull, but next year, or in two years, they might want a different bull for genetic reasons. All of these factors play into why and how we manage our herds.”

Fossil Rim has several antelope species that thrive in large herds, such as gemsbok, Arabian oryx, scimitar-horned oryx, sable, and blackbuck.

“For those oryx species and the sable, it is relatively easy management because we only have one herd bull out there,” Eyres said, when he was asked if larger herds necessarily mean tougher breeding challenges. “Blackbuck are different, because lots of males can live together. We don’t manage the blackbuck as intensely as most of the other species.

Unlike other Fossil Rim antelope, blackbuck are managed with multiple males and they are free to battle each other for control of their respective harems of females.

“For most of our antelope, the bulls don’t get along with each other. Herd size, as far as how many females are with a bull, is a key factor in how we manage each species.”

Greater kudu are unique to Fossil Rim in that multiple breeding bulls can coexist peacefully.

“It makes it easier for them to get along, but it makes it more difficult for us to determine parentage,” Eyres said. “If we are trying to make sure we know who the father is, we have to do genetic testing. We have to have blood samples tested. The kudu are our only large antelope where multiple males are tolerated; males of most of the other species at Fossil Rim would fight.”

Greater kudu are the only larger antelope at Fossil Rim where multiple bulls can live together peacefully. However, when the parentage of offspring needs to be determined, a blood sample will need to be sent off for testing.

Blackbuck are much smaller than the other Fossil Rim antelope outside of dama gazelle. While the males don’t injure each other, they certainly compete via clashes of horns that would not qualify as “toleration.”

Species compatibility is an important factor to consider when planning the placement of animals. It is crucial to avoid hybridization, as hybrid animals have no conservation value. At Fossil Rim, for example, the three oryx species – scimitar-horned oryx, gemsbok, Arabian oryx – are each in different pastures, while the waterbuck and Nile lechwe are also in different pastures from one another.

At Fossil Rim, Nile lechwe (top) and waterbuck have to live in separate pastures to negate the risk of hybridization between the similar species.

“Basically, for any species that are similar enough you worry about hybridization and the males fighting,” Eyres said. “There is a greater likelihood a gemsbok bull and a scimitar bull are going to fight because of the species’ similarity, plus you don’t want them to hybridize.”

One of Fossil Rim’s antelope species that thrives in a large herd, Arabian oryx (pictured) have to live in a separate pasture from scimitar-horned oryx and gemsbok to avoid hybridization.

The climate at a wildlife facility affects management decisions. Interestingly enough, although scimitar-horned oryx can thrive in the very hot weather of their native Africa, they can also do quite well in the cold when given access to proper shelter.

“Even so, we try to steer the births of all species away from winter,” he said. “The ultimate goal is to control the birth windows for all of the (antelope) species here.”

Although scimitar-horned oryx can thrive in the very hot weather of their native Africa, they can also do quite well in the cold when given access to proper shelter. Even so, the Fossil Rim hoofstock staff does not want them to have calves in the winter.

An important part of antelope management is to maintain as much genetic diversity as possible. It is less disruptive to rotate the herd bull in and out than it is to add additional females. Rotating a herd bull every three years allows a facility to minimize the chance of a bull breeding its female offspring while also maximizing the amount of time it can be used effectively.

“Currently, each breeding bull either rotates out of the herd or out of Fossil Rim all together,” Eyres said. “Some might still live at Fossil Rim, but are no longer with the herd. If it’s a vasectomized bull, we’ll keep him until he dies of old age.”

When some Fossil Rim animals no longer play a breeding role, if they don’t leave the facility, they often live the remainder of their lives comfortably in the Retirement Pasture like these addax are doing.

When rotating bulls, it is important to have a new bull brought in prior to the need for rotation so that a breeding season is not missed.

“Sometimes, we might acquire our new breeding bull and his eventual successor at the same time,” he said. “Usually, I like to have the successor in at least a year in advance of us parting with a current breeding bull. It depends on if there is a successor available, where it is, when we can get it, etc. But, we definitely prefer to have them here, out of quarantine, and ready to go when possible.”

What about the aforementioned use of vasectomized bulls as part of bull rotation, a concept that actually originated at Fossil Rim? Currently, four species here utilize rotation of an intact male to a vasectomized bull and vice versa: sable, gemsbok, waterbuck, and addax.

For a few months of the year, the vasectomized waterbuck bull lives away from the herd, but he is actually with the females for more months annually than the breeding bull. Although he cannot produce offspring, his dominant male presence serves an important role in the herd structure.

“For the rest of the antelope we manage, the intact bull is with them all the time,” Eyres said. “We don’t need to do bull rotation for the wildebeest because they only breed once a year. We will still rotate to a new breeding bull every three years.”

Theoretically, the other species with an intact bull in the herd year-round could have calves born in winter, but that is not happening thus far so there has not been an urgent need to implement bull rotation for them.

Since wildebeest only breed once each year, there is no need to do a bull rotation for them to dictate when calves are born.

For those species that use the rotation, the vasectomized bull is with the herd for the majority of the year.

“The timing of when the breeding bull goes in varies among species,” Eyres said. “We put the sable and gemsbok bulls in around June, the addax bull in the first week of July, and the waterbuck bull in during late July.”

Since the gestation period of these species is slightly different (7-9.5 months), this ensures that calves are all born at the best time – for weather, for the quality of grasses, and in the case of the addax, so that calves are being born during Fossil Rim’s busiest week of the year – Spring Break.

By utilizing a bull rotation for species like gemsbok to dictate what months of the year calves will be born, the challenge of being a newborn calf trying to survive in the winter can be eliminated.

Many people may understand the huge difference between a vasectomized bull and a castrated bull, but if not, it is essential to differentiate them.

“A castrated bull doesn’t show bull behavior,” Eyres said. “Its secondary characteristics like horns, musculature, dominance, and behavior towards females are gone and it may look different because it has no testosterone. A castrated male with the herd would essentially be a female socially; it wouldn’t work for what we are trying to do.

“Meanwhile, a vasectomized bull looks just like a breeding bull if they are similar in age. The only difference is he does not produce sperm. This bull can still provide the dominant male presence so the herd dynamics remain the same.”

An example of a castrated male at Fossil Rim is “Shiner” the giraffe. He was born at Fossil Rim and the decision was made to castrate him so that he could live alongside the breeding bull without fighting.

“Shiner” the giraffe is an example of a castrated male at the facility. Born at Fossil Rim, being castrated allows Shiner to live in peace with the giraffe breeding bull.

How does the staff switch out the intact and vasectomized bulls?

“The sable breeding bull lives in the lane pens (by the vet clinic) most of the year,” Eyres said. “We can usually walk him up into the wooden pens, and then open the gate and let him out (into the Main Pasture) to join the herd. Most of the other bulls involved in the bull rotation require immobilization to make the switch.”

The sable breeding bull lives here in the lane pens most of the year, but in June he is directed to walk out into the Main Pasture to join his herd.
When sable (pictured), addax, waterbuck, and gemsbok breeding or vasectomized bulls need to be removed from the herd, they have to be immobilized and transported from the Main Pasture. Adam Eyres (second from right) has many years of experience successfully implementing bull rotation at Fossil Rim.

If you are wondering about the gender of an antelope, look at its ear tag. All males have a tag in their right ear. What does the color of the tag mean?

“Addax ear tags are always green,” he said. “For all other antelope, the colors change every year. That way, from a distance we know what year (a given antelope) was born.”

Addax are one of four antelope species for which Fossil Rim currently utilizes a bull rotation, a concept that originated at the wildlife center. A vasectomized male is with the herd for most of the year, and then in early July he is switched out for the breeding bull. All male antelope have a tag in their right ear, and the tag of an addax is always green.

A neat side note on the ear tags is that, in many cases, the hoofstock staff at Fossil Rim does not even need to look at the tag to know the individual animal.

“Everyone on the hoofstock staff is pretty good at identifying individual animals within a species, at least in the pastures they are most familiar with,” Eyres said. “It’s more difficult in gemsbok; they look very similar to each other. But, some species are definitely easier to differentiate.”

For some antelope species, Fossil Rim hoofstock staff with years of experience like Becca McLachlan can identify individuals at a distance without reading their ear tag.

All of these factors are pretty fascinating to consider for the part they play in antelope breeding decisions. But, keep in mind, this article has been focused strictly on the antelope at Fossil Rim.

“Equids (horses/zebras) and cervids (deer) have different reproductive cycles,” Eyres said. “The Grevy’s zebras have a one-year gestation period, so it’s easy for us to put a stallion in during the month that we want foals to be born the next year. Almost all deer species have a ‘rut,’ which is the time of year that males fight for dominance to breed.

“Deer breeding seasons can’t be modified for our particular management style. Fortunately, they have evolved to breed at appropriate times for the Texas environment.”

-Tye Chandler, Marketing Associate 

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