In the frenzied breeding scramble that is the whitetail rut, are bucks and does selective about the age of their mates, or is rut success random? The answer is: Yes. There’s scientific evidence that age plays a role, and there’s also randomness in how some deer get lucky. Consider the Auburn University Deer Lab study in which a 6-month-old buck fawn successfully mated with a 9½-year-old doe. The same study found elderly bucks that sired fawns at ages 10½ and 11½.
That adds to a collection of scientific work that body-slams the fanciful idea that one or a few whitetail bucks do all the breeding. That’s not how whitetail reproduction works. For his master’s research at Auburn, Tristan Swartout found breeding success shared across bucks and does of many ages.
Tristan’s study produced an amazing scatter chart of deer breeding success. Let’s look at it together and extract lessons about the whitetail rut. First, I’ll describe the Auburn study so you can understand how Tristan produced the chart and what it means.
184 Whitetail Hookups
Tristan conducted his masters research at Auburn’s 430-acre enclosed deer research facility in Alabama. The enclosure hosts a study population of whitetails. For 12 years from 2008 to 2019, Auburn students and faculty captured and acquired DNA samples and other data from 474 unique deer. This led them to identify both parents for 211 fawns recruited into the population, giving them 184 mated pairs to study. (These pairs recruited more than 211 fawns, but Tristan focused on 211 cases with greater than 95% confidence in DNA recognition).
Let’s address the enclosure head-on. With studies of deer movement or other population dynamics influenced by movement barriers, I’m always a little skeptical whether deer science conducted inside high fences is relatable to wild deer. However, it is very difficult to study mated pairs in a wild population because of the certain impossibility of capturing enough bucks, does and fawns in the area for a complete picture of relatedness. Texas A&M-Kingsville built impressive father/son family trees for wild deer, but these took decades to build and don’t include both parents.
The Auburn facility is not a small enclosure, and it contains a mix of habitat and cover types from pastures to mixed pine/hardwood forests that are maintained with prescribed fire. There are food plots, creeks, and feeders. While it’s not a truly wild deer population, it comes close enough to let researchers learn a lot that applies to wild deer while still having the access and control of a laboratory.
While we should take the enclosure into account when relating Auburn’s results to wild deer, we couldn’t likely get as good a look at whitetail breeding pairs without it. In a breeding study, I might have predicted a fence would make it easier for dominant bucks in the enclosure to locate estrus does and monopolize breeding, since they only had 431 acres to patrol. However, as you’ll see, that did not happen.
The Scatter Chart of Deer Ages
This scatter chart shows you the ages for the buck/doe pairs that recruited each of 211 fawns (remember that “recruited” means the fawn survived to 6 months of age). At a glance, you can see pairings all over the spectrum, with deer of many ages, and many age combinations, recruiting fawns. Consider these highlights:
- The oldest doe to recruit a fawn was 12½ (and she was bred by a 6½-year-old buck).
- The oldest buck to recruit a fawn was 11½ (and he bred a 3½-year-old doe).
- The largest observed age difference was 9 years (a 9½-year-old doe bred by a buck fawn)
- At least one of the parents was a fawn (6 months old) for 16 of the mated pairs (8%)
- In 1 of those instances, both parents were fawns (bottom left corner of the chart)
- The oldest combined age for a matched pair was 21 years, an 11½-year-old buck and a 9½-year-old doe.
- 3½ was the most frequent age for the doe in the pair (35 pairs), and 4½ was the most frequent buck age (46 pairs).
What this chart doesn’t show is the success of age classes relative to their availability. Age classes were not equally available, and the population’s age structure for bucks and does changed across 12 years of study, making older deer more available. But Tristan ran the math to answer his question: Does age matter?
“We did see a slight positive relationship for age,” said Tristan. “As doe age increased, the age of the buck in the mated pair also increased slightly. There was still some flexibility, but not to the point of being random.”
Deer tended to prefer to mate with deer close to their own age given the opportunity, but they could be lured into dating outside their generation if a first choice wasn’t on the scene in time.
The Buck Age Structure Effect
During the 12-year span of study, the number of mature bucks and does increased. At the beginning in 2008, the average age of all candidate bucks, including buck fawns, was 1.42 years. It was 4.38 years by 2018 at the end of the study. The average candidate doe matured from 2.15 years to 6.79 years in the same time span.
As age structure increased for bucks and does, mature mated pairs increased, and successful pairings by young bucks and does decreased. You can see that in the chart below. As the number of available bucks age 4½ and older grew, the number of fawns fathered by bucks age 3½ or younger decreased. By 2019, when half of the bucks in the population were 4½ or older, not a single buck 3½ or younger sired a fawn – even though bucks of those ages were still available to breed.
To be clear, this is not normal. Auburn’s buck age structure pretty much entered extreme territory about half way through the study in 2013, when 30% of all bucks were age 4½ or older. In a wild, hunted population, even a quality-managed population, you are doing really well if 15% of all bucks are this old! Yet, once the Auburn population became top-heavy with older bucks, a few younger bucks were still succeeding. In 2015, a buck fawn mated with a doe. In 2016, a yearling and a 2½-year-old were successful.
“The 9½-year-old doe mating with a buck fawn was later in the study,” said Tristan. “Even when the buck age structure was very mature, younger bucks still had opportunities.”
By 2018, the youngest successful buck was 3½, and the next year the youngest was 4½. In that final year of the study, only bucks age 4½, 5½ or 6½ were successful breeders. All younger and older bucks were finally shut out of the competition. In fact, that was the narrowest span of successful buck ages in 12 years of work – only three age classes.
No Buck Dominates the Rut
Remember, it’s a scramble. A buck must tend and defend an estrous doe about 24 hours to be the one who breeds her, which means he’s unavailable for other does in the area that are likely in estrus too, especially during the few days in the peak of breeding. This is when younger bucks, even buck fawns on occasion, can win. The Auburn study showed it took an extreme situation – half the bucks age 4½ or older, and a high fence to confine the playing field – before young and very old bucks were shut out of the action.
Auburn’s is not the only study to find this. Anna Bess Sorin earned her Ph.D. from the University of Michigan studying deer in Michigan’s 1,300-acre enclosed George Reserve, which is three times the size of the Auburn facility. Anna Bess found three out of 16 yearling bucks and two out of four 2½-year-olds produced fawns even when 41% of bucks in the population were 3½ or older. Also, 12 out of 14 bucks age 3½ or older were successful. Again, widespread success even in a developed buck age structure.
In the whitetail scramble to produce future deer, no deer rules the herd. While some species of animals operate this way, with a few males controlling large harems of females, whitetails do not. A wide range of deer ages and body sizes are successful in the rut. They appear to be biased toward mates their age and size, but only as long as circumstance allows. The end goal is to reproduce, and you can’t be as choosy at closing time.
I have to say I’m thankful whitetails must scramble for rut success. All that rubbing, scraping, seeking, chasing, grunting and fighting sure makes for exciting days in the woods!
Twins and Triplets that Aren’t
Don’t forget, there’s apparently more than one window of breeding opportunity per estrus cycle. I’m talking about what deer biologists call “multiple paternity.” That’s when twin fawns are not actual twins but have two different buck fathers. Various studies have documented approximately a quarter of all twin sets involving two bucks. Even triplets are sometimes the result of three different buck fathers!
Tristan found 12 cases of multiple paternity in his data. The average age difference between the two bucks in these cases was 2 years. This age gap seems to support one theory on how multiple paternity happens: A younger buck locates an estrous doe first, breeds her, and then is chased off when an older, dominant buck arrives on the scene. The older buck then also breeds the doe and stays with her, tending and defending her until the day-long-plus estrus is over.
The reverse is possible, with a younger buck sneaking an opportunity second, but previous research suggests most dominant bucks will tend and defend an estrous doe until she is no longer receptive.
Next: Are There Super Does?
According to Tristan, fawn recruitment success among does began to decline at 7½ years of age. However, you can see in the scatter chart that it didn’t disappear. Does beyond age 8½ continued to recruit fawns, even out to age 12½.
That takes me into the other half of Tristan’s work looking at parental success among does. Are all does equally good fawn recruiters? I’ll pick up there in another report coming soon.