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Why do you think they are bigger, heavier & more robust? Be interesting to hear some theories....................

More agriculture and colder temps

different subspecies(plural).

The farther away you go from the equator the larger animals are. It is just a proven fact.

I believe the midwestern deer are larger due primarily to the rotation of the high carb/high protien crops the farmers plant. If a deer has good genetics, and has quality food (primarily corn & beans) to feed on all year, they can put on and retain their body fat needed during the winter. The nutrients from these high quality food sources allow the antlers to grow to full potential. I also believe the midwestern states (Iowa/Illinois/Missouri/Kansas) do a much better job of managing their deer herd, therefore the bucks are gonna reach an older age class than what we typically see here.

There are many theories, but that is my $.02. They sure are fun to hunt. Hope I am fortunate enough to keep going back to Illinois every year. I love the rut in the midwest!

Bergmann's rule. Larger body size results in less surface area per unit of body mass so they are better able to conserve heat.

I believe it is a little of all of the above. The different subspecies of deer may play some role in it, but I think nutrition plays a big part in it. The northern deer also have to carry more mass to get through the winters. Age structure plays a big key. States with better deer management and hunters who are willing to let young deer walk equal older more mature bucks. I think pressure also play a part. Hunting in the Midwest for the first time this year, and seeing unpressured deer in the woods, was an eye opening experience. The deer were not as jumpy, they would enter very large green fields with out hesitation. Several times I had deer come in on me down wind and they would seldom even checked the winds. I think this calmness also helps in there feeding habits, and there weight retention.

I will add one other thing about pressure on the deer. I have talked to several people who live and hunt in Michigan, and I get the impression that like Alabama they have a lot of hunters who believe in the "if it's brown it's down" mentality, and they put a lot of pressure on their deer. In return they don't kill a lot of big deer from what I am told up there either. On top of that, I don't think they have the agriculture that you see in Illinois, Indiana, Kansas, etc.

Canadian Journal of Zoology, 1987, 65:(4) 1035-1038, 10.1139/z87-164


Abstract



Bergmann's rule, claiming that in homeotherms body size increases inversely with temperature so that, intraspecifically, body size increases latitudinally, is not valid, nor is the explanation of this rule. In large mammals body size at first increases with latitude, but then reverses between 53 and 65� N, so that small body sizes occur at the lowest and highest latitudes. This is predicted by the hypothesis that body size follows the duration of the annual productivity pulse, so that body size is a function of availability of nutrients and energy during periods of growth. Correlations between body size and temperature are shown to be spurious. If reduction in relative surface area is indeed an adaptation to conserve heat, then mammals should increase in size from south to north at rates two orders of magnitude greater than they do. Bergmann's rule has no basis in fact or theory.

Very interesting

I did a quick search and it looks like most reasearch that pops up quickly still supports Bergmann's rule although there is more debate about the rule itself than I realized and also more debate about the heat retention theory than I knew.

Most of the above.

I am not so sure agricultural practices are as important as you might think. There isn't much nutrition in corn. I am not really up to speed with Agriculture practices in Saskatchewan...but they have some of the largest whitetails in the world up there. They also have less abundant natural browse year around farther north.

Conversely, in the deep south, there is still plenty of Agriculture and deer can probably find natural browse as much as 10-11 months out of the year.

It is the same with Turkey subspecies...their average body weight varies by 10-20 percent or so.

One thing I haven't seen mentioned on here is the fact that the seasons are shorter allowing younger deer to have an advantage of not being seen during season therefore grows older. Also, some of the states don't allow high powered rifles so shooting a deer at 200 yards isn't an option, again allowing them to grow older.

Of course the vast majority is genetics. Also, consider that some of these states have outlawed rifle hunting altogether or have a one week season for rifles.

I believe that rainfall vs. snow, play a large part, along with ag practices associated with those climate differences. Genetics too.

Not much nutrition in corn? Corn is ALL about energy for animals. Energy is definitely associated to nutrition.

You must mean not much protein in corn, which falls into my thoughts on rainfall vs. snow accumulations.

yes protein

Much of the Midwest is not just corn, it is also beans which do have protein. Also look at the nutrients in the soil, that end up on the things that are grown. Look at where a lot our bigger deer are killed. It's in the black belt region of the state. Have you ever seen how black and rich the soil in the Midwest is? Where I was just at in Kansas, the dirt looked like ground up coal it was so black. Also as you go north and west you see less and less pines and more and more hardwoods.

North American Whitetail Deer distribution and Subspecies

Odocoileus virginianus borealis is most likely the largest
of the whitetail subspecies and occupies lands from central to
eastern Canada south through Minnesota, Wisconsin, and Illinois.
Traveling east it can be found throughout Indiana, Ohio, Pennsylvania, New York, and up the Atlantic coast to Maine, Nova Scotia and beyond.

Why do turkeys in Missouri regularly weigh between 20-25 pounds but just a few hours away in Alabama a 25-pounder is a rarity?

Bergman's rule? Different species? Less pressure?