Study Shows Female Deer Prefer Bigger-Antlered Bucks As Mates
SAN ANTONIO — Some Southern engineering was used to conduct what is being described as remarkable behavioral research with deer. Specifically, the study addressed whether or not female deer play a role in choosing their mates.
The results of the study were presented at the recent Deer Associates meeting here by Dr. Steve Demarais, the Dale Arner Professor of Wildlife Management at Mississippi State University. The annual Deer Associates meeting, sponsored by the Caesar Kleberg Wildlife Research Institute, is a format in which the team of deer researchers at the Institute share their research with those managing deer on the landscape.
Demarais opened his presentation with a qualifier, telling listeners that while the study revealed some interesting results, the research has literally no management application. The ability to test their hypothesis came about because of another research project designed to develop a fetal growth curve for estimation of breeding dates in white-tailed deer. Because researchers would control when does were bred, that gave them an opportunity to test the idea of female choice.
Before getting into the details of the study, Demarais offered some additional background, pointing out that there are two mechanisms where breeding choices are important in wildlife. The first is intersexual competition, whereby a male battles with a rival male, fighting for access to females. The other is mate choice, which is normally a female choosing between males based on secondary sexual characteristics such as antlers or horns.
With respect to bucks, their antlers are most often talked about as armaments used in fighting, but could they also be an ornament for attracting does, much like that of the male prairie chicken with its brightly colored expanding throat patches or a pheasant’s brightly colored plumage?
As Demarais pointed out, there are many species for which appendages act as both ornaments and armaments. He cited again the male pheasant, whose spurs are used in that age-old male-to-male dominance battle, as well as the fiddler crab, whose big claw is also used to fight other males.
“The antlers for deer are clearly an armament used to establish dominance,” said Demarais, “but there’s never been a study successfully showing whether or not they could also be an ornament.”
Studies have suggested that antlers act as a “signal” of genetic quality to a doe, surmising that genetic quality should matter to a female because, after all, she’s investing a year of her life and all of her nutritional resources into the production of offspring.
Such preferences, Demarais said, have been shown to matter in other species of mammals. Specifically, he cited research where female mice were allowed to choose between two males. The female had equal access to both cages for each of the males, and the one she spent the most time next to was assumed to be the one she preferred. The results showed that when the female was allowed to breed with the preferred male, she actually produced offspring that had better survival rates. These offspring also built better nests and were more dominant.
So, if there is some kind of physiological mechanism involved in mate choice by a doe, what characteristic is she choosing for? Is it body size, age, antler size? Or is it a combination of the three? That was what Daniel Morina, the graduate student leading the project, set out to answer through the study. His hypothesis was that antlers would serve as a weapon and an ornament simultaneously, and that the females would choose larger-antlered, larger-bodied, and older males.
As Demarais reminded, all three of these traits are auto-correlated, meaning that antlers get bigger and body size gets bigger as a buck ages. Thus, some logistics had to be worked out so that researchers could independently control for each of those characteristics in the experiment. This is where some of that Southern engineering came in handy.
Consider two bucks of the same age and the same body size with hugely different antlers — how best to use these in the experiment? The researchers began by cutting the antlers off all bucks involved in the study. They then developed a base aluminum coupling designed and milled to their specifications. The coupling was attached to the base of the antlers on each buck with machine screws, caulked and made sturdy. Demarais said they’d experimented some the year prior and found that PVC pipe was not strong enough to withstand bucks fighting against a fence, thus the need to design a metal coupling.
Once they had the coupling attached, they then could affix different sizes of antlers to this coupling, and in that way researchers could assign whatever antler size they wanted.
“To control for all sources of variation, one buck had the large antlers for a period of time and the other a smaller set,” said Demarais. “Then, we’d sedate the bucks and switch antlers for another period of time.”
They also switched pens and tried to control for all other sources of possible variation.
Once they had the antler conundrum figured out, accounting for size and age was relatively simple. To test for age difference there had to be at least a three-year age difference between the two bucks being compared, while keeping antler size and body size the same. For the body size analysis, bucks that were about 35 percent different in body size were used, while antler size and age were kept the same.
The study, conducted at the Rusty Dawkins Memorial Deer Unit on the campus of MSU, was designed after the mouse study in that a doe was placed in a central enclosure with a buck on either side of her. Infrared video cameras were set up along each fenceline.
A large number of females was used in the study. All were adult females and most were in the three-plus age range. “Controlled Internal Drug Release” progesterone implants were used to control timing of doe estrus. Demarais noted that does tended to go into estrus about 24 to 36 hours after the CIDR was removed, but they didn’t ovulate until about 60 hours after the CIDR was removed.
“That gave us about two days to do our evaluations before we had to breed the doe for the fetal growth curve work,” he said.
“We were most interested in what a doe was doing in terms of proximity to each male,” Demarais explained. “So within 10 feet of that common fenceline we would monitor and calculate how much time she spent or what she would do in that 10 foot area associated with a buck. For example, if the doe spent 60 percent of her time with buck number one and 40 percent with number two, then we concluded that buck number one was her choice.”
More specifically, researchers assigned a preference when a doe spent 50 percent or more of her time with one buck or if she spent or exhibited specific behaviors indicating a choice. Walking or bedding along one fenceline were two such behaviors.
“We know when a doe is in estrous she spends more time walking,” Demarais explained. “So if she’s spending more time walking along one fence with one buck versus another, then that would be a choice, or if she beds next to the fenceline of a particular buck.”
Another more obvious behavioral indicator they watched for was a doe standing and allowing a buck to lick her through the fence, for example.
The first trial focused on body size, again using bucks that were 35 percent different in size while keeping antler size and age consistent. What they learned was that bigger-bodied bucks were not preferred over the smaller-bodied bucks in terms of the time spent walking or the time spent bedding next to them, said Demarais. They also saw no other behavioral indications of choice.
The second trial focused on age, in which there was at least three years’ age difference in the bucks being compared. Again, based on walking and bedding and the other characteristics, the doe did not show a preference for older bucks. What they determined was that the doe spent 45 percent of her time walking along one fenceline and about 55 percent on the other, indicating clearly no statistical difference. Perhaps more interesting, 70 percent of the time she bedded next to the younger buck.
The third trial used same-aged bucks with similar body size, roughly less than eight percent difference, with really large antlers and then really small antlers. The large antlers averaged 163 Boone and Crockett inches, while the small averaged 64 inches, Demarais said.
This is where things got interesting. Researchers found that with respect to walking, the doe walked the fenceline near the larger-antlered buck 80 percent of the time. Also, 79 percent of the time the doe bedded on the fenceline shared with the bigger-antlered buck. Additionally, researchers documented that four out of five females that had a behavioral choice clearly indicated a preference for the larger-antlered buck.
“We felt this was pretty conclusive evidence,” said Demarais. “They didn’t choose bigger-bodied bucks; they didn’t choose older bucks, but they sure did choose bigger antlers.”
In the question and answer session one participant asked that if females are in fact choosing larger-antlered bucks, and given what’s known about genetics, why generationally aren’t deer growing larger antlers?
Demarais responded that in other research they’ve “allometrically” looked at the ratio of antler size to body size.
“We think there is an optimal antler size that allows a male to be able to fight effectively, and it’s tied to the nutritional level of the region,” said Demarais.
He further pointed out that over time there is optimization of traits, and because antlers require a huge nutritional investment, bucks can’t afford to invest more than what’s needed to grow an adequate set of antlers necessary for armament.
Another participant pointed to other genetic research which shows that reproductively successful males do not have bigger than average antlers, indicating that the female choice is not the main driver in the reproductive equation. Rather, the armament aspect is what drives breeding.
Demarais responded that clearly the male-male dominance is the main driver of who does the breeding, but “We’ve shown there is a component operating on the female side that might matter over an evolutionary time scale.”