The complex process of evolution via natural selection has led to a vast and dazzling array of physical and behavioral differences between males and females. Those individuals with traits that engender better survival are able to breed – among species that sexually reproduce – and pass their genes, and hence these traits, on to their offspring.
What this translates to amidst both local and far-flung wildlife can range from the subtle to the admittedly bizarre. Among the former are more uniform species lacking any or much sexual dimorphism – i.e. “distinct differences in size or appearance between the sexes of an animal.” The males and females of these species appear virtually indistinguishable.
For example, male and female white-crowned sparrows, common wintering birds in the Sky Islands, look alike to us. Another species that seems to lack sexual dimorphism is the familiar mourning doves that have exploded in numbers following our banner monsoon. Look at a group of, say, 50 of these doves and they may all seem carbon copies of each other. Put a courting pair together, however, and it’s quite clear who’s who. The male will puff up and strut as he sings his familiar song, advertising his significantly larger size and brighter colors.
But what about species that show marked, even extreme sexual dimorphism. How did this come to be? Take the anglerfish, which include several species where the male is dwarfed by the female. Think I’m exaggerating? In the anglerfish Ceratias holboelli females may be more than 60 times longer and half a million times heavier than males! The female looks like something out of a horror movie, given her exaggeratedly long and predatory fangs as well as her gaping maw. Evolution has “relegated” him to a parasitic existence, where he (or even “they”, as up to eight males may do this simultaneously) attaches to the female’s underside, draws nutrients from the blood of the female, and mates with her by releasing sperm into the sea as she releases her eggs. In my book this level of sexual dimorphism is unparalleled, though some male and female orb weaver spiders show similar size trends.
However, consider the well-known spotted hyena. While on safari in Africa years ago I observed several hyena clans, though the details of their genitalia eluded me. Females, it seems, sport large fake male genitalia that serve as not-so-subtle reminders that they and not the males are dominant within the clan.
What about some examples of sexual dimorphism in the Sky Islands? In many of our local mammal species males are larger than females. This is particularly true among the order Carnivora, which includes felines (cats), canines (dogs), ursines (bears), mustelids (weasels), procyonids (raccoons), and mephitids (skunks). Here, the larger size of a given male may equate to greater hunting success and the ability to physically exclude or minimize other males from his territory. So too with cervids – the deer family – where bucks (whitetail and mule deer) or bulls (elk) are often much larger than females and seasonally sport antlers to vie for harem dominance and access to females.
Female jackrabbits may be larger than males, which may confer extra size, energy, and survivability to her offspring. American beaver, too, may have females which are heavier than males, as can some species of predatory grasshopper mice.
Other Sky Island examples abound. Gila topminnow, a locally endangered fish, have females that are sometimes twice the size of males, which nonetheless are brighter colored. Local drake ducks, such as northern shoveler and ruddy duck have gaudy, iridescent plumage that serves to attract and win females, while the cryptically colored females are well-suited to sit on nests and incubate eggs.
With both diurnal (hawks) and nocturnal (owls) raptors we flip the tables again, as females often average significantly larger than males. One hypothesis has the larger, stronger female guarding the nest and catching larger prey, while the smaller, more maneuverable male is able to secure ample smaller prey with his agility.
Conversely many passerines or songbirds have males that are more brightly colored and larger than females. Male rattlesnakes average larger than females, who have to allocate more energy to reproduction than the males. By growing larger, the males might have access to more prey items, while also making them a more implausible meal to would-be predators. Gaudy male butterflies, dragonflies, damselflies, and grasshoppers secure more copulations, yet at great risk and peril as their ornamentation also serves to alert predators.
The list goes on with many examples of how sexual reproduction has led to a dazzling array of gender differences. Next time you spot local wildlife consider how evolution may have shaped their size, color, and behavior. It will add yet another dimension to enjoying the wilds of the Sky Islands.
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