Bergmann’s Rule hypothesizes that higher latitudes, with their colder environments, tend to engender the evolution of larger species and larger individuals within wide-ranging species. Simply stated: as one travels to the poles, animal species tend to get larger in order to deal with the rigors of colder climates.
Situated at about 32 degrees latitude north, our Sky Islands region clearly leans towards the tropics. Applying Bergmann’s Rule, one might then surmise that, on average, our bioregion should be largely populated with somewhat smaller species and smaller individuals within wide-ranging species. As with most biological “rules” the truth is far more fascinating than mere theory!
Arizona does host some classic examples that German biologist Carl Bergmann could use to support his hypothesis. Mexican gray wolves, for example, represent the smallest North American subspecies of this broadly distributed predator. So did the now extinct Mexican grizzly bear. Both these predators are excellent examples of species trending larger toward – in this case – the North Pole. There are other examples where I might be able to back up Bergmann’s Rule, but this exploration is about the exceptions – relative giants that inhabit our Sky Islands – all thumbing their collective noses at this biological maxim.
Starting with invertebrates, we have a good number of examples of large species in our area. The queen of our Mexican leaf cutter ant – nearly an inch long – is North America’s largest ant species. Female Sonoran carpenter bees dwarf most other bee species in the world and are the largest in North America. Look for them near stalky plants where they nest, such as sotols and yuccas.
Our tarantula hawks win the prize for largest wasps in the states, befitting a species that hunt our largest spiders – the tarantulas. I’ll never forget my first encounter with a tarantula hawk, as I eyed what I surmised to be a hummingbird showing an interest in my government issue research truck. Upon closer inspection it dawned upon me that I was in fact watching a wasp too big to be real, temporarily putting on hold my plans to reenter the truck!
Giant darner dragonflies, black witch moths, western Hercules beetles, and giant mesquite bugs – all at or near the top in terms of size within their respective taxonomic groups – only further detract from the veracity of Bergmann’s Rule.
Jumping to vertebrates, we find more exceptions to the rule. Having been a Gould’s wild turkey researcher, I can attest to the behemoth size of some of the gobblers. A large male may approach or even exceed 30 pounds. Perhaps their size helps protect them against the legion of Sky Island predators who strive to include them in their diets.
While wrens are generally diminutive, we have the rather gargantuan cactus wren in our area. They inhabit deserts where resources seem to be in short supply. So why the large size? Since they often seem to be monogamous, large body size may equate to a better chance of a pair attaining and holding a breeding territory.
Two other large birds are of note in Arizona: the California Condor, a Pleistocene relic, and the five-inch blue-throated hummingbird – a gargantuan species among its Lilliputian relatives.
Circling back to mammals, our huge antelope jackrabbit, the greater bonneted bat, and the American beaver, all are tops in their class for either body size and/or weight in North America. Our elusive jaguar is our country’s largest cat species, though these northern populations also contradict Bergmann’s Rule, averaging smaller than their tropical compatriots whose rich diet equates to a larger average size.
We are also home to North America’s largest lizard, the Gila monster, and the largest toad, the Sonoran toad. Their bulk no doubt helps to deter predators, along with their respective venomous and poisonous biochemistries. Larger size also broadens what they can consume as prey.
Finally, we end with perhaps our most famous colossus, the giant saguaro. While the cardon cactus of Mexico may average heavier, the tallest giant saguaro ever measured topped out at 78 feet! I hope one day to gawk at a saguaro, while listening to a cactus wren with a Gila monster lurking in the shade of this giant succulent and a spooked antelope jackrabbit bolting by – a congregation of relative leviathans at my disposal.
If he is able to contemplate his main contribution to science from “the other side,” clearly Mr. Bergmann is not a happy man. Our Sky Island jumbo species may represent vexing exceptions to his idea, but they are a wellspring of biological wonder for us still extant humans.
Vincent Pinto and his wife, Claudia, run RAVENS-WAY WILD JOURNEYS LLC, their Nature Adventure & Conservation organization devoted to protecting and promoting the unique biodiversity of the Sky Islands region. RWWJ offers a wide variety of private, custom-made courses, birding & biodiversity tours. Visit: www.ravensnatureschool.org