For most animals, sex is an egg-meets-sperm affair that requires both a male and a female. Aptly, this is called sexual reproduction. However, some species have the remarkable ability to reproduce asexually without male fertilization.
Swiss naturalist Charles Bonnet discovered this process, known as parthenogenesis (a Greek term that translates to “virgin origin”), was discovered by the Swiss naturalist Charles Bonnet in the mid 1700s. It was long considered peculiar to plants, insects, and other invertebrates, where it’s relatively common. But in recent decades, biologists have found dozens of cases among fish, reptiles, birds, and even (in special circumstances) the occasional mammal.
Parthenogenesis takes a bewildering number of forms: For some it’s a back-up plan, the last resort when mates are scarce; for others it’s the only way a new generation comes into existence; and still others seem to choose whichever method suits their fancy. Read on to learn which animals have mastered the art of asexual reproduction.
1. Komodo Dragons
In 2006, workers at two British zoos were shocked to find that their female Komodo dragons had given birth despite being isolated from males. One of them went on to mate with a male, showing how flexible the world’s largest lizard can be, switching between sexual and asexual reproduction as the moment demands.
This strategy may help the dragons (an endangered species with little more than 1,000 adults in the wild) to replenish their numbers during hard times. But unfortunately it doesn’t work in the long run — thanks to the quirks of their chromosomes, parthenogenesis only produces sons, which can’t carry out the next round of virgin births.
Read More: Mates for Life? The More We Learn About Animal Sex, the Rarer True Monogamy Becomes
2. American Crocodiles
In a similar episode, published just last year in Biology Letters, a female crocodile at a Costa Rican zoo laid a surprising clutch of fertile eggs — surprising because, like the Komodo dragons, she’d been living alone for 16 years.
Since crocodiles are part of an ancient lineage (archosaurs) that also includes dinosaurs, it’s possible they, too, shared this trait. If so, it would mean Jurassic Park was more prescient than anyone could’ve guessed: The park’s scientists made all their creations female, but in the film’s crucial plot twist, those females are able to reproduce by parthenogenesis. In the immortal words of Jeff Goldblum’s Ian Malcolm, “Life, uh, finds a way.”
In the real-life case, however, it did not. After three months in an incubator, none of the eggs hatched. One held a fully formed fetus, but it was stillborn. Nevertheless, the researchers write, “this discovery offers tantalizing insights into the possible reproductive capabilities of the extinct archosaurian relatives of crocodilians.”
Read More: Isolated Female Crocodile Gives Birth Without Mating
3. Hammerhead Sharks
By the turn of the century, asexual reproduction had been documented in nearly every branch of vertebrates, with the exception of mammals (which most biologists had written off, for reasons explained below) and cartilaginous fish. A virgin birth in sharks was long overdue.
And then it happened at a zoo in Omaha, Nebraska. Following a familiar pattern, the mother hadn’t been in contact with males for several years. Sharks can store sperm for long periods of time, so at first, experts assumed it was just another case of delayed fertilization. But genetic testing didn’t turn up any sign of a male contribution in the pup’s DNA — it was yet another unexpected product of parthenogenesis.
Read More: Asexual Reproduction is Surprisingly Common in the Animal Kingdom
4. Aphids
Asexual reproduction is nothing special among insects, but aphids are one of the few that also go through periods of sexual reproduction, a scheme known as heterogony. Once a year, in autumn, they have good old-fashioned sex. The rest of the time, females handle matters on their own terms.
Not only are they capable of parthenogenesis, but they’re already partway through the process on their first day of life. “Most aphids are born pregnant and beget females without wastrel males,” writes David L. Stern, a developmental biologist at Princeton University, in Current Biology. “These developing embryos contain developing embryos of the third generation within them, like Russian dolls.”
Read More: Do Insects Have Feelings and Consciousness?
5. New Mexico Whiptails
This bizarre lizard, which has been called “the internet’s ‘gay icon’ of herpetology,” was among the first cases of parthenogenesis found in the vertebrate world. It also happens to be one of the purest examples: Sexual reproduction isn’t even an option since the entire species is female or unisexual.
That means that all offspring are clones of their mother, sharing precisely the same genes. This seems like an evolutionary dead-end — without new genetic input (except random mutations), it should be hard for whiptails to maintain enough diversity to adapt to disease or changes in their environment. Yet, somehow, they continue to thrive.
Read More: Sexual Cannibalism: Why Females Sometimes Eat Their Mates After Sex
6. Amazon Mollies
Like whiptails, Amazon mollies, a species of freshwater fish, are all female. But they rely on an even more elaborate form of parthenogenesis called gynogenesis. They need sperm (which they “steal” from closely related species) to trigger the development of an embryo, but that sperm doesn’t contribute any genetic material. All offspring are perfect clones.
Besides lack of diversity, this leads to another conundrum: Since their genes never have a chance to recombine through sexual reproduction, you’d expect to see the genome decay as harmful mutations build up, in a process known as Muller’s ratchet.
These disadvantages predict that unisexual species should go extinct within at most 100,000 generations. But in 2018, researchers determined that mollies, as a species, are much older — in the realm of 500,000 generations. What’s more, they found no sign of genome deterioration. Something about their “hardy genetic makeup” allows them to remain sexually self-reliant.
Read More: Do Animals Masturbate Too?
7. California Condors
Of all the animals that can reproduce asexually, birds do so most rarely. And typically it’s only successful in domestic turkeys and chickens that have been separated from males. But in 2021, researchers at the San Diego Zoo added California condors to the list. (Whether parthenogenesis occurs among in the wild remains to be seen.)
Oddly enough, the mother in question was housed with a fertile male, one she’d even mated with in the past. Yet despite having the option, she chose to go it alone. The only reason we know is because the researchers ran a paternity test, which points to the possibility that there are lots of sneaky asexual reproducers out there still to be discovered
Read More: From Jaws of Death to Love Darts, These 7 Animals Mate in Unusual Ways
8. Mice
Natural parthenogenesis has never been observed in mammals, and for good reason: We and our fellow warm-blooded creatures are products of genomic imprinting, a process by which genes are chemically tagged as unreadable depending on whether they come from the male or female parent. Though we have two copies of every gene (one from each parent), one just sits there silently.
That means that if a female tried to reproduce alone, she would pass on two silenced copies of many genes, and her offspring wouldn’t develop properly. Her egg needs sperm — there’s no way around it.
Not in nature, at least; the laboratory is another matter. In 2022, a team of Chinese scientists induced asexual reproduction in mice under experimental conditions by editing DNA in regions responsible for genomic imprinting. When they removed those chemical tags, essentially rendering the genes male, mothers were able to carry viable babies to term.
Read More: Instead of Dozing Off, These Marsupials Are Too Busy… Getting Busy
Article Sources:
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review them for accuracy and trustworthiness. Review the sources used below for this article:
Brenner's Encyclopedia of Genetics. Parthenogenesis
Encylopedia Britannica. Charles Bonnet
Nature. Parthenogenesis in Komodo dragons
Biology Letters. Discovery of facultative parthenogenesis in a new world crocodile
Biology Letters. Virgin birth in a hammerhead shark
Current Biology. Aphids
High Country News. How the New Mexico whiptail became a gay icon
Mutation Research/Fundamental and Mechanisms of Mutagenesis. The relation of recombination to mutational advance
Evolution. Mutation Load and the Survival of Small Populations
Nat Ecol Evol. Clonal polymorphism and high heterozygosity in the celibate genome of the Amazon molly
Cell Research. Genomic imprinting in mammals: its life cycle, molecular mechanisms and reprogramming
