Nature’s Nontraditional Families

In nature most families start when a male and female mate and have offspring that combines genetic characteristics of the parents (sexual reproduction). But nature is full of exceptions even for this basic rule. Some animals employ asexual reproduction, also called parthenogenesis, in which only one parent, usually a mother, is needed. It occurs throughout nature in many forms among reptiles, birds, and insects. Remarkably, mammals (including people) are the only creatures in which this virgin birth cannot occur due to a special way that our genes work.

Aphids are parthenogenic which means that the mothers do not require fathers to reproduce. As a result, many aphids have only one parent, a mother. Each daughter is identical to every one of their siblings and also to their mother except for size. In effect they are clones, and they are their own mother’s identical twin. In the summer, aphids give birth to potentially billions of clone daughters. But in the spring, they give birth to both sons and daughters. The sons are genetically identical to the mother as well, except for the lack of an X chromosome. Even more interesting is that an aphid often gives birth to its own granddaughter which may already be developing in her daughter who is already pregnant when born!

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California sheepsheads are fish that are all born female. Later in life they all turn into males. The timing of the transition depends on the environmental conditions. There are no old ladies or young boys! And all romances are of the May-December variety.

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New Mexico Whiptail Lizards are exclusively female. They give birth to their own genetically identical daughters without the participation of males or male genetic material. They have no fathers. However, they still require a mating ritual in which another female plays the role of a male, mounting her to stimulate reproduction. They no longer need the males, but clearly they miss them!

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It was recently discovered that certain species of sharks such as the Blacktips and Bonnetheads can reproduce asexually (females giving birth without the need of a father) under certain conditions in which a father is not available. Asexually reproducing female sharks, unlike reptiles like the whiptail lizard (above), can only produce female offspring. Therefore, a single such shark cannot spawn a new generation on her own.

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Komodo Dragons normally reproduce sexually, but in times when there are no males present, females can reproduce asexually. This ability is thought to be useful when one female drifts to an island and populates the species there entirely on her own! This is possible because asexual reproduction in reptiles, unlike in fish and most insects, produces all males rather than all females. The male offspring can then sexually reproduce with the mother to form new generations.

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Though most geckos are ‘normal’ with both males and females who reproduce sexually, some populations are entirely female and reproduce asexually. But even in sexually reproducing populations, individuals can switch sex if needed. Geckos lay eggs which are normally fertilized by a male. But in asexual populations, the eggs do not require fertilization. The Geico gecko from the commercials, the one who appears to be a bit effeminate, perhaps he’s a she!

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And by the way, about the parthenogenic mammals. Well actually, there are none in nature. All mammals reproduce sexually and require both a mother and father. While scientists have successfully created fatherless rabbits, mice, and even monkeys by using an electrical or chemical stimulus, such mammals are deformed and are not viable. Unlike fish and reptiles, mammalian reproduction involves a process known as genomic imprinting which necessitates genetic material from both mother and father. Therefore people will always require both a mother and father to produce offspring. While we hear of cases of virgin birth among sharks, lizards, fish, plants and insects, in humans it’s simply precluded by the reproduction mechanism.

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Marmorkrebs are parthenogenic crayfish that were discovered in the pet trade in the 1990s. Offspring are genetically identical to the parent (mother).

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Little fire ants are very special because both the males and females reproduce asexually. In this sense, some of the males and females don’t even belong to the same species! Queens produce more queens asexually, giving birth to their clonal daughters. Sterile workers are usually produced from eggs fertilized by males. But in some of the eggs fertilized by males, the fertilization can cause the female genetic material to be discarded (a process called ameiotic parthenogenesis). This is the first recognized example of an animal species where both females and males can reproduce clonally resulting in a complete separation of male and female gene pools.

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Seahorses are the only animals in which the males become pregnant and give birth. Following an elaborate courting ritual, the male receives eggs from the female, fertilizes them, and incubates them in a special pouch on his chest. In about six weeks, he gives birth to as many as 200 babies. Then a month later he does it again! Seahorses are monogamous. The male doesn’t roam around very much. He stays in the same area and eats nearby food while the female travels far off each day in search of food. But the female returns every day to repeat a mating dance with the pregnant father in which they entwine their tails and spiral to the surface.

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Update: In April 2009  University of Arizona researchers announced the discovery of the first ant species that consists of only females and reproduces only when the queen clones herself. Read story.