Certainly the whitetail deer is in very, very good shape as a species right now. It has existed in the same form for 3-4 million years or so in North America, thriving as a generalist in times of ecological change and upheaval. This right here is a time of enormous ecological change and upheaval and the whitetail can be expected to take full advantage of it.
In the course of the changes that are expected to continue in North America, I often wonder whether this will result in the diversification of the whitetail into new species. The process can happen rapidly. Mule deer probably split off from whitetails no more than 5,000-7,000 years ago, having very rapidly combined some remarkable bio-mechanical adaptations with corresponding behavioral changes that set them apart from other cervids.
Central and South American brocket deer evolved from whitetails at two separate times (the grey brockets and red brockets are actually the result of convergent evolution, having whitetails as their last common ancestor). This diversification of the whitetail appears to have happened very quickly after the parent species radiated into South America, possibly due to the lack of any other ruminants in competition there at the time.
So on the one hand we have the fact that whitetails have remained intact and unchanged as a species for over 3 million years, suggesting that this is a really good plan for an animal and that maybe there is nothing that need to be changed in order to respond to this changing environment. On the other hand, there are these examples of some groups of whitetails rapidly evolving into new species in some areas.
The whitetail could potentially effect some degree of evolution without physically evolving at all. As non-native species of plants continue to be introduced to North America by humans, the whitetail's rumen-based digestive system may gradually manage to digest many of those plants that it cannot presently feed on. This could be accomplished through the evolution of the bacteria in the rumen, or perhaps by the introduction of existing species of bacteria that do not exist in the rumen at present.
Global warming may have an impact of whitetails relatively soon. The milder a winter in terms of temperature, the less fat reserves and the lower body size that will be required to ensure survival through the winter. This would allow late-dropped fawns a higher chance of survival through their first winter despite their smaller size at the onset of winter. Such a development would tend to favor does going into estrus later than usual, resulting in a less focused rut and potentially having a major impact on whitetail society.
Grasslands represent a major environmental opportunity as well as a challenge to whitetails. The species that once dominated grasslands in North America largely disappeared a scant 140 years or so ago. The bison have cleared out and there seems to be a lot of room for whitetails expand in that area. But to do so will require changes to the bodies and social behavior of those deer which would amount to the emergence of a whole new species.
All non-tropical deer with antlers shed them on an annual basis. Unlike horns, which have an internal base of living bone with an active blood supply, antlers are dead bone once the velvet is shed. Lacking a blood supply, antlers cannot be maintained by the body over time, thus they need to be shed and regrown. To do this on an annual basis requires a high amount of calcium and other minerals in the diet every year.
The tricky thing about a grasslands diet is that grasses are not high in calcium. This is probably the reason why deer have never dominated grasslands. The food is nutritionally inadequate to support the growth of large antlers annually among all adult males.
With the habitat available, perhaps some whitetails will make a go of it. But they will have to sacrifice their antlers, or at the very least they will need to develop a genetic predisposition towards very small and simple ones. A body that genetically 'wants' to grow large antlers will attempt to do so even when nutrition is inadequate. Most of the minerals that a deer uses to grow antlers are robbed by the process of resorption from the rest of the skeleton (the ribs in particular), so merely stunting the antlers by a restricted diet will result in weak overall condition and a tendency towards broken bones rather than just tiny antlers. The change has to be genetic.
Without antlers, bucks would need to develop other types of sparring behavior in order to establish a pecking order without killing each other. I have no idea what this would consist of. Also consider the fact that a pair of antlers is the major visual cue to sex among deer during the mating season. Take that away and you've arguably got a need for some other visual cue of sexual dimorphism to develop.
I envision a new plains species of whitetail 1,000 years from now, having no antlers and a distinct pattern of color among males versus females. Perhaps the does might retain their spots from fawn-hood while the bucks would lose them? These plains whitetails would rarely have cover to hide in, so they might have to reduce their use of scent markings to communicate (with some scent glands disappearing?) and they would need to become faster runners over longer distances in order to avoid the packs of coyotes that appear to be establishing themselves as new apex predators.
The big unknown is the future of non-native megafauna that have been introduced to North America by humans during the last 150 years. We have wild, breeding populations of emu, aoudad, sika deer (which is more properly a sort of tiny elk), fallow and axis deer from Europe, various swine, oryx antelope, etc. None of these species are well adapted to North America and they have hardly been here for the evolutionary bat of an eyelash. The idea of these animals, which were just dropped into a situation that the whitetail has been thriving in for over 3 million years, beating out the whitetail on its home turf seems unlikely. But we just don't know.
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