[Illustration]
I used to think that the battle of the sexes so ably portrayed by James
Thurber was artificial, a man- and/or woman-made thing. But recently
I've come to see it as old--probably as old as sex itself in the animal
world.
Under the severe tide, "Secondary Sexual Characters and Ecological
Competition," in a paper from the Bird Division of the Chicago Museum,
I've outlined the possibility of competition for food, between
the sexes, being a factor in evolution, responsible in part for
characteristics of structure and traits that distinguish them.
In circles that discuss evolution the idea is current that food
competition is important between species. It may even be stated as a
rule: two species with the same food habits cannot live in the same
place. Competition drives one out, unless they have different food
habits. These differences seem especially evident when you look at
closely related species, and they are accomplished in a variety of
ways. A habitat difference is very common. The long-eared owl hunts in
the woods--its cousin, the short-eared owl, hunts the meadows; the song
sparrow favors the drier shrubbery while its cousin, the swamp sparrow,
lives in wetter shrubbery.
THE SIZE FACTOR Sometimes the difference is accomplished by size; take
the downy and hairy woodpeckers of our wood lots, very similar except
that one is larger and is adapted for larger prey, the other smaller
and adapted for smaller food items. Sometimes they feed differently,
as the Baltimore oriole, which picks flowers and pecks through their
sides, while the orchard oriole probes into flowers as they hang on the
branches. Thus more individuals of several species live in an area.
When a pair of birds "sets up housekeeping" and starts "raising a
family" they can no longer drift about, looking for easy living and
places where food is plentiful. Their wanderings are restricted by
having a fixed point, the nest, as their center of interest. Two
individuals must draw on the food supply from an area about the nest.
Competition would be extreme, and, if there were a scarcity, perhaps
critical.
We know how different the sexes may be; how different the rooster is
from the hen in our domestic fowl, or the drake and the duck in the
mallard, or the red male and the green female of the scarlet tanager.
These sexual differences have mostly correlated with display and
mating. But logically there should be differences in feeding behavior
and adaptations between the sexes.
The basic idea is contained in the old nursery rhyme:
Jack Sprat could eat no fat,
His wife could eat no lean;
And so between them both,
They licked the platter clean.
The two birds of a mated pair, limited to a single area, could be
expected to have different food preferences or adaptations for getting
it. And we find that there are cases of this. The most striking is
that of the huia from New Zealand, of which I've written in a Chicago
Museum bulletin. Both sexes have similar food preferences, especially
wood-inhabiting insects, but they get them in different ways. The male
has a short, straight, stout bill for digging out the wood-boring
grubs, woodpecker fashion; the female has a much longer, slender, and
curved bill for probing into holes for them, creeper fashion. The
female may get grubs in wood too hard for the male to chisel. They
supplement each other.
DIET VARIATION BY SEX It is possible that further study may show more
sexual differences to have a feeding advantage; the larger size of
female hawks fitting them to take larger prey; the smaller size of
certain female songbirds fitting them for smaller prey, the smaller
bills of female hornbills, the straight bill of the male western grebe,
and the upturned bill of the female. Perhaps all are of advantage to
the species in giving each sex slightly different advantages in getting
food.
Selection could have its effect in the populations with most sexual
difference in feeding habits being most successful in raising and
leaving progeny. Thus, slowly, differences between the sexes would
accumulate. However, it must be kept in mind that this sort of
evolution would be limited. The drifting apart of the sexes would be
checked by the necessity for their coming together periodically for at
least a short period, at nesting time.