| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Feature The Selfish Dragonfly For
better or worse, animals have a single overriding, albeit subliminal,
goal in life: to pass on their genes. For a male this comes down to making
sure it is his sperm, and not another’s, which impregnates a female. There
are about as many kinds of sperm competition as there are species of animals,
but among dragonflies, the game “I will get my sperm into her first and
nobody else shall” has been played and perfected for more than a hundred
million years. Dragonflies were Earth’s
first flying animals: omnispective, aerial predators. With wings of some
species spanning more than two feet, Carboniferous dragonflies dominated
the air above vast, humid swamps. Below them giant salamanders slithered,
cockroaches scurried, and huge scorpions, spiders, millipedes, and centipedes
crawled about—all looking, except for their size, quite similar to the
way they appear today. Almost all male insects
directly inseminate their mates. And a successful insemination is often
followed by mate-protecting behaviors, which help to insure the survival
and well-being of the female, the “gene-vehicle,” and their offspring.
In the case of dragonflies, mate-protecting behaviors may precede insemination
as well. Unlike most insects and other animals, a male dragonfly does
not use his primary genitalia to deposit sperm into a vulva. Over the
ages, dragonfly males have evolved complex accessory genitalia in a swollen
pouch on the underside of the abdomen which serves as a kind of sperm
bank. Upon encountering a
lone female, the male grasps her with his legs and immediately clamps
his tong-like, true genitalia on her. If he is a typical dragonfly of
the suborder Anisoptera, he grabs the crest of her head. However, if he
is a damselfly (suborder Zygoptera), he grips the front of her thorax.
Once firmly linked in
this strange way, the pair may fly off in tandem—the male in the lead.
Sometime later, while clinging to a plant stem, or even while in flight,
the pair forms a “wheel,” as the female curls the tip of her abdomen forward
against her mate’s sperm-charged accessory genitalia. The male reacts
with vigorous, prolonged genital pumping. He appears to be forcibly injecting
sperm into her vulva. However, the male is doing exactly the opposite.
Using a rigid pseudopenis in his pouch as a tool, he is scraping out any
sperm which might have been deposited during a prior mating by another
male. Following this elaborate sperm displacement (reported in one species
to be 88 to 100 percent effective), the male releases his sperm. Why did such a strange,
complicated method evolve? Surely, over the millennia, a simpler procedure
could have developed? I think that the advantage of this complex strategy
is that it reduces the hazards of egg-laying in aquatic habitats. The
most serious hazard may be the danger of an unescorted female being tipped
over and drowned, or repeatedly harassed by overzealous, mate-seeking
males during her oviposition (egg-laying) in flight or on the water’s
surface. For a successful outcome—for both male and female and, ultimately,
for the offspring—it is better that the male use his primary genitalia
as a tool for protectively clasping his mate than as a device for direct
insemination. While maintaining a
firm grip on a female’s head or thorax, the male steadies her as she lays
eggs and also assists in lift-offs and flight as she moves from place
to place. Because some damselflies seem to anticipate the possibility
of the water level of an evaporating pond lowering before the eggs hatch,
a female may completely submerge, dragging the male down with her as she
backs down a stem to lay eggs. The male helps her return to the surface.
If the female is of
a species that habitually lays eggs in flight, a male often disconnects
earlier. But he protectively hovers over her, or perches nearby, ready
to drive off bothersome rival males. Such protection may appear to be
altruistic, but it is just one more way a male attempts to immortalize
his genes. There are perhaps human parallels in a good husband, who, influenced
by that deeply imbedded, gene-engineered trait—called love—supports, protects,
and fosters the well-being of his mate, his gene vehicle. On her part,
a female selects a potentially good provider and protector. “What is this thing called love—this funny thing called love?” Edward S. Ross is Curator Emeritus of entomology at the California Academy of Sciences |
Summer 2000
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