The jackalope is a mythical animal of North American folklore described as a jackrabbit with an antelopes horns. It's a popular mythological creature in hill folk culture and it made me ask a question, why don't animals other than Ungulates commonly have antlers? Why would animals such as rodents, fish and primates evolve antlers? What conditions would support the addition of these features?
A list of all of the Anatomically Correct questions can be found here
I'm guessing that jackalopes have pronghorn antelope horns, because both are American. Pronghorn horns get called 'antlers' because they shed part of them every year, but they are not true antlers - scroll down to illustration of the skull.
That's pretty much where my knowledge of pronghorn horn/antlers ends. However, I can tell you why hardly any creatures have deer type antlers!
Deer antlers are one of evolution's freak accidents. To make antlers, deer have hijacked the wound-healing mechanism. The deer use the same kinds of biochemistry and physiology that you or I use to form a scab after cutting ourselves, or healing a broken bone. The trouble is, this is incredibly dangerous. Antler formation involves turning soft tissues into bone. If the process doesn't stop, first the antler, then the top of the deer's head, then the brain will be turned into bone. As you might imagine, having your brain turned to bone is ever so slightly fatal.
Deer have solved this in several ways:
So evolution of horns. Dead easy. Everything from dinosaurs to rhinos have done that. It's evolved multiple times, so it can be expected to evolve again in the future or on alien planets.
Evolution of an antler which you shed annually - much more complicated, and may be a 'once only' thing.
One of the major reasons for animals growing horns is for protection or fighting for mates, dominance, or territory. Alternately, they can be used for digging purposes. There are animals which are not ungulates who do have horns, specifically Jackson's chameleon. If you had fewer air-based predators for your jackalopes to need to evade through speed, they might conceivably grow them for defense. Another non-ungulate is the horned lizard which has both true horns (contain a bony core) and spines made out of scales.
I can't weigh in on why other species don't commonly grow horns, but there are at least a couple which don't follow the ungulate-only trend of horns.
Compared to most body parts, horns seem pretty simple to evolve, being basically bone growths covered in keratin. They are often associated with sexual selection and intraspecific competition; while they can be used for defense against predators as well this does not seem to be their primary purpose (otherwise females should have the same horns as males, which is very rare).
It seems more likely for hares to evolve horns than rabbits, to fight off other males (though notably, the jackrabbit is actually a kind of hare). Hare's competition for females is fierce (since females are only receptive for a few hours every few weeks, the so-called 'March madness') and often involves multiple males pursuing a single female and fighting each other off at the same time. A hare capable of knocking other males around by swinging his head back and forth might have an advantage in these pursuits.
It seems unlikely that female rabbits or hares would prefer horned males, though, since horn-like growths are a common symptom of the papillomavirus (the disease that probably inspired the legend of the jackalope). This would decrease the reproductive fitness of horned males, which suggests that jackalopes, if they did evolve, would be be a highly 'rapey' species, prone to mating by force. In response to this, the female may develop horns of her own to fight off unwanted suitors.
It seems unlikely that rabbits would evolve horns since they would be a large hindrance for tunneling and moving through burrows which are a rabbit's primary defense mechanism.
A hare may do so but you have to consider scale here, most predators on a hare are significantly larger than it. Dogs, big cats, etc. Could a hare even one armed with horns realistically fight one off?
Larger animals do develop horns as they are useful defensively and they are actually large enough to make use of them. For an animal like a hair horns would just slow them down when running and risk getting caught on things slowing them down even more.
The only way it might work is if you had fake horns that are designed to be abandoned and act as decoys. This is similar to the tails of some lizards. The idea is that when a predator strikes, you shove the horns in its mouth and then run away while it is confused. Equally if the horns got tangle you could abandon them and again hope they distracted the predator. That's a lot of effort expended to grow the horns for very little win though.
Interesting fact, there's this common virus called Shop papilloma which causes temporary bony tumors to sprout from the skulls of rabbits. It's thought this is the origin of jackalopes.
