Fish, frogs, gastropods and plants are among the species that change biological sex over the course of their lifetimes. These species do so because it ensures greater reproductive success and survival of the species. If creatures can change their sexual organs, they have a greater chance of finding another member of the species to mate with (as opposed to humans, who can only reproduce with someone of the opposite sex). Species also might change biological sex because of environmental factors or chemical triggers. The clownfish, for example, changes sex to fulfill the needs of an organized societal hierarchy.
The plant called lily of the valley (Anphiprion bicinctus) has beautiful white flowers that hide its dual sexuality. The lily variety is one example of many flowering plants that can fertilize itself [source: Tidwell]. After all, a lily is rooted in the ground, and although there are ways for plants to pollinate one another, it's not like the plant can wander into the neighbor's yard and look for a partner to mate with while you're sleeping, then replant itself before you head out to the patio with your morning cup of coffee. Reproducing without moving is one advantage of being a hermaphrodite. The ability of plant and animal species to change biological sex -- or hermaphroditism -- is all about species survival, showing the powerful adaptive abilities of species and begging the question: Why doesn't every animal species regularly change between sexes to ensure consistent reproduction?
A team of Yale researchers sought to answer this question and to determine why adaptive sex change is rare in the animal world when it clearly favors survival. The researchers found that one reason might be the time and energy involved in the sex change process. They're also investigating whether reproductive behaviors, such as caring for offspring, might affect the success of sex-change reproduction [source: Science Daily]. What's more, some species simply prefer sexual reproduction -- or having members of the species who only can mate with members of the opposite sex.
For the water flea (Daphnia), male members of the species only are necessary when their numbers are low compared with the amount of water and algae available in their habitats (ponds). As long as it's crowded, the tiny translucent crustaceans live happily as asexual female fleas in the pond's fresh water. But if a big rain comes, they hatch male fleas for mating, and soon there are plenty of fleas in the pond. The fleas turn their sex on and off to meet their population needs [source: Tidwell].
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