Greg Laden is guest-blogging this week while Sarah is on vacation. You can find his regular blog at Scienceblogs.com and Quiche Moraine.
We are talking mainly about bovids (cattle and antelope), which grow horns over their lifetime, and deer, which grow antlers every year. In most well known bovids and cervids, only the males grow the horns or antlers, but there are a few species where the females do as well.
For example, male and female cattle (including the many wild versions such as the African Cape Buffalo) and wildebeest (a kind of antelope) have horns, while in most other bovids only the males have horns. Both male and female caribou (a kind of deer) grow antlers each year, while in most other deer only the males do so.
This is actually a very complicated issue, and a new study of this question offers a new possible answer. But first, what did we think before this study?
There is one factor that explains most instances of female horns or antlers. The tiny monogamous deer and antelope tend to be much more "monomorphic" (that is, males and females look similar) than larger deer and antelope. These are small, pair-bonded, forest-dwelling species, and their horns or antlers are effective tools for defending territory or defending the young against small forest predators such as cats. Both the males and females have the horn or antler because they both use them, and for similar purposes. That is not particularly enigmatic.
It is also not hard to explain why in the vast majority of large cattle, antelope and deer species males and females are dimorphic (that is, males and females look different) in this trait, with only the males having the big appendages on their heads. In most of these species, males compete with each other, either in direct male-male competition or using a more show-off strategy to impress the females, in which the horns or antlers play an important role.
What's harder to explain is this: In a small number of these large species, where the males compete over females, why do females also grow horns or antlers?
One early theory suggested that females in larger species could use these appendages for anti-predator defense. In other, smaller, species the females are better off hiding or running away. In my personal experience with wild Cape buffalo, this makes sense. On many occasions while working in the Semliki Valley in the Congo, I encountered small herds of female buffalo with their young. As I would draw nearer in my vehicle, they would gather more closely and form a circle with the young in the center, watching me suspiciously and looking rather formidable, and the horns were very much part of that look. However, this does not seem to hold true for deer. In the largest deer species, females do not have antlers.
Another previous hypothesis, proposed by Richard Estes, who works with wildebeest in East Africa, suggests that horned or antlered females benefit by confusing adult males as to who the young males in the group are. This is a strategy to keep the young males in the group longer, so they can grow bigger before heading out on their own. Essentially, this is a trait that benefits mom (it makes her son more successful) but is manifest in her daughters. According to this idea, female horns or antlers should be found in species where competitive males are forced to hang around with each other more than in other species because they live in large herds that consist of "family" groups. This is, in fact, what is found in caribou and wildebeest, two of the prime example of antlered or horned females.
The new theory, proposed by Ted Stankowich of the University of Massachusetts and Tim Caro of the University of California at Davis, is that females benefit from having horns or antlers if they are of a body size or live in a habitat that makes it hard for them to hide. The more conspicuous the female, the more benefit they gain from horns or antlers, which would be needed for defense against predators. (They may also benefit from competition with members of their own species for grazing spots.) This would explain caribou and wildebeest nicely, as they both live in very open country, as well as a lot of other species. This study was done by looking at a large sample of animals for traits related to body size and vegetation cover in the habitats they live in. The sample included 82 species with female horns or antlers, of which 80 were “very conspicuous.” According to the authors, who feel the two species that did not fit for reasons that can probably be explained, that is a nearly perfect match between theory and data.
The more conspicuous the female, the more benefit they gain from horns or antlers, which would be needed for defense against predators. (They may also benefit from competition with members of their own species for grazing spots.)
Horns are an extension of the cow's sinus cavity, assisting them in smelling, identifying and digesting food. Following the internal spiraling of the sinuses, their horns spin out into unique forms for each animal. Both male and female cattle grow horns. The horns are part of how they individuate.
While it's true that female caribou have antlers, there is another animal where the female has unique “head gear.” The North American Pronghorn Antelope does not have antlers, it has horns; but unique among mammals, the female drops her horns every year! Rangifer tarandus — the Caribou or Reindeer.
Typically, only male deer grow antlers. Female deer have been documented to grow antlers when experiencing issues with regulation of the hormone testosterone, which happens very rarely. Caribou are the only deer in which females regularly grow antlers.
Animals have a variety of uses for horns and antlers, including defending themselves from predators and fighting members of their own species (horn fighting) for territory, dominance or mating priority. Horns are usually present only in males but in some species, females too may possess horns.
The more conspicuous the female, the more benefit they gain from horns or antlers, which would be needed for defense against predators. (They may also benefit from competition with members of their own species for grazing spots.)
Horns are removed because they can pose a risk to humans, other animals and to the bearers of the horns themselves (horns are sometimes caught in fences or prevent feeding).
Biologist Craig Roberts has the answers. Males of almost all deer species grow antlers, used to battle for females. But reindeer are the only species in which the females also grow antlers, and an explanation can be found by looking at bovids, a closely related family including antelopes, goats and sheep.
Animals use their horns to defend against predators. If an animal's horn is broken or damaged, it will remain that way forever. It does not grow back. Horns are permanent; they are not shed, but grow with the animal throughout its lifespan.
Horns are usually found on both males and (in a diminutive form) females. Antlers are shed and regrown yearly while horns are never shed and continue to grow throughout an animal's life. One exception is the pronghorn, which sheds and regrows its horn sheath each year.
He also pointed out that female deer with high testosterone can also successfully and healthily reproduce. It isn't unheard of to see an antlered deer nursing a fawn, a dead giveaway that the deer is definitely a doe.
Wildlife officials have reported an overpopulation of deer, making it rare to see an antlered doe of any size with only a 1-in-10,000 chance of occurrence.
You know Dasher and Dancer and Prancer and Vixen; Comet and Cupid and Donner and Blitzen? They're Santa's reindeers, by the way. And as new research suggests, they're probably female.
Heifers share much of their anatomy with other cattle. For example, as with male calves, heifer calves are born with the capability to grow horns, except in hornless breeds.
Bulls (males) average around 2 ½ feet long while cows (females) average horns 3 feet long. Steers (castrated males) grow horns around 5 to 7 feet long. At the Struthoffs' farm, 6-year-old cow Spicey has the longest horns of the herd with 98 inches from tip to tip.
Heifers share much of their anatomy with other cattle. For example, as with male calves, heifer calves are born with the capability to grow horns, except in hornless breeds.
Ankole-Watusi cattle are the show-stoppers of the bovine kingdom. Medium-sized animals, with long, large-diameter horns, they attract attention wherever they appear.
Introduction: My name is Neely Ledner, I am a bright, determined, beautiful, adventurous, adventurous, spotless, calm person who loves writing and wants to share my knowledge and understanding with you.
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