In the animal kingdom, most species have both male and female sexes. However, there are some interesting exceptions where animals can reproduce without males, resulting in populations that are entirely female. In this article, we’ll explore some of these unique female-only animal species and understand how they are able to exist and thrive.
The All-Female Whiptail Lizard
One of the most well-known examples of an all-female animal species is the New Mexico whiptail lizard. This lizard, which is found in the southwestern United States and northern Mexico, is able to reproduce through a process called parthenogenesis. This means the lizards can lay eggs that develop into embryos without being fertilized by a male’s sperm.
The New Mexico whiptail lizard has two chromosome sets (it is diploid), which allows its eggs to develop normally without fertilization. All the members of the species are female and genetically identical clones of their mother. While this lack of genetic diversity would threaten many species, the whiptail lizard continues to thrive, likely thanks to the hardy nature of its habitat.
Researchers believe the New Mexico whiptail resulted from the hybridization of two related lizard species, the western whiptail and the little striped whiptail. The hybridization allowed for the possibility of parthenogenesis, leading to the evolution of the all-female species.
Amazon Molly Fish
Like the whiptail lizard, the Amazon molly is an all-female fish species that relies on parthenogenesis to reproduce. Native to the streams and rivers of Texas and Mexico, the Amazon molly resulted from the hybridization between sailfin mollies and Atlantic mollies.
The Amazon molly needs the sperm of a male from a closely related species to trigger parthenogenetic reproduction. However, the male’s genes do not contribute to the offspring. Instead, the eggs develop into clones of the mother. By using the sperm of related mollies, the Amazon molly continues to thrive, despite its lack of genetic diversity.
Komodo Dragon
In another unusual example of single-sex reproduction, female Komodo dragons at the Chester Zoo in England have been observed producing viable eggs through parthenogenesis. Unlike the whiptail lizard and Amazon molly, the Komodo dragon is not an all-female species in the wild.
After being separated from males for an extended period, the female dragons began reproducing asexually. The babies resulting from the virgin births were genetic clones of their mothers. While this phenomenon has only been observed in captivity, it suggests an evolutionary adaptation that allows the dragon species to maintain its survival.
Crayfish
Several species of crayfish, small freshwater crustaceans, have all-female populations that reproduce through parthenogenesis. These species originate from hybridization between two parent crayfish species, allowing for asexual reproduction. For example, the marbled crayfish is native to Florida and is descended from the slough crayfish and the procambarus fallax.
Like other parthenogenetic species, each marbled crayfish is genetically identical to its parent. Populations spread rapidly as females clone themselves. While having minimal genetic diversity, these crayfish thrive in freshwater lakes, streams, and swamps.
Termites
Termites are perhaps the most familiar example of an animal group with both male and female members as well as all-female reproduction. All termite colonies start with a male and female pair. But if the royal couple dies, termite workers have the ability to reproduce asexually via parthenogenesis.
Female worker termites can lay viable eggs even without mating, giving rise to more worker females. This evolutionary ability allows a termite colony to survive and thrive even after losing its founding reproductive pair. The workers cannot produce new queen termites or males, however.
Sharks
Certain species of sharks are also capable of asexual reproduction in special circumstances. For example, the bonnethead shark, which lives in the Gulf of Mexico and along the East Coast, has been observed reproducing via parthenogenesis in captivity. When females could not mate with males, they produced viable eggs and healthy pups through virgin births.
In the wild, female bonnetheads always reproduce sexually. But the ability to clone indicates the species has evolved a mechanism to maintain populations even without males, at least for a period of time. Other sharks, including zebra sharks, blacktip sharks, and angel sharks, have shown the same capability in captivity.
Honey Bees
Honey bees display one of the most complex examples of asexual reproduction in the animal kingdom. Honey bee colonies have a queen (the only fertile female), drones or males, and worker bees that are sterile females.
The queen reproduces sexually, mating with drones and laying fertilized eggs. These develop into sterile female workers. But the queen also lays unfertilized eggs that hatch into male drones through parthenogenesis. This mixed system allows the colony to function and thrive with specialized groups of bees.
Turkeys
Believe it or not, domestic turkeys are another animal that can sometimes reproduce asexually. Turkey eggs that have not been fertilized by sperm can occasionally still undergo cell division and develop into embryos. The resulting poults are genetically identical clones of their mother.
Parthenogenesis is very rare in turkeys and other birds. But the ability exists as a quirk of domestic turkeys’ reproductive systems. Other poultry like chickens and quails have also been known to have virgin births on extremely uncommon occasions.
Pit Bulls
In rare instances, female pit bulls and other dogs have given birth to puppies without ever mating with a male. Experts think this “virgin birth” happens by the female absorbing a remnant of male chromosomes from a prior litter she carried.
When she ovulates, the male DNA triggers the egg to develop without being fertilized by actual sperm. The pups will be clones of their mother, since no male genes are involved. While unusual, it shows dogs have an inherent ability for asexual reproduction.
Boa Constrictors
Parthenogenesis has been documented in captive female boa constrictors kept alone in enclosures. In one case at a zoo in the UK, a boa constrictor gave birth to two litters of pups that she conceived asexually, having had no contact with male boas.
As with sharks, the ability for virgin births probably evolved so boa populations could expand even with a shortage of males available to reproduce. More research is needed to know if wild boas ever display natural parthenogenesis behaviors.
Komodo Dragon
In another unusual example of single-sex reproduction, female Komodo dragons at the Chester Zoo in England have been observed producing viable eggs through parthenogenesis. Unlike the whiptail lizard and Amazon molly, the Komodo dragon is not an all-female species in the wild.
After being separated from males for an extended period, the female dragons began reproducing asexually. The babies resulting from the virgin births were genetic clones of their mothers. While this phenomenon has only been observed in captivity, it suggests an evolutionary adaptation that allows the dragon species to maintain its survival.
Whiptail Lizards
We’ve already discussed the New Mexico whiptail lizard as one example of an all-female asexual species. But there are actually several other species of whiptail lizards that also reproduce through parthenogenesis. These include:
- Aspidoscelis neotesselata – Colorado checkered whiptail
- Aspidoscelis velox – Plateau striped whiptail
- Aspidoscelis tesselata – Common checkered whiptail
- Aspidoscelis uniparens – Desert grassland whiptail
In these lizard species, all individuals are female clones of their mothers. There are no males. They occupy arid, harsh environments in the southwestern U.S. where the ability to clone provides an advantage despite the lack of genetic diversity.
Ants
Most ant species have both male and female ants. But certain species have been found to have all-female populations with queen ants that reproduce asexually. These include the ant species Pristomyrmex punctatus, Mycocepurus smithii, and Cerapachys biroi.
In these species, the queen ant can produce female workers without mating. The ant colonies operate and forage normally despite the lack of genetic diversity. Researchers are studying these species to better understand the evolutionary benefits of asexuality in ants.
Shark DNA Findings
Recent genomic analysis of two asexual species of shark found that they evolved from sexual ancestors relatively recently. The study looked at DNA in the parthenogenetic white-spotted bamboo shark and asexual shortspine spurdog. It uncovered evidence that asexual reproduction emerged just a few generations ago.
This is noteworthy since most asexual animal lineages are ancient, dating back hundreds or thousands of generations. The research indicates that the transition from sexual to asexual reproduction can occur rapidly under the right genetic circumstances.
Factors That Favor Asexual Reproduction
The examples of all-female and parthenogenetic animal species demonstrate that asexual reproduction can be advantageous in certain environmental and genetic contexts. Here are some key factors that promote the success of asexuality:
- It allows populations to expand rapidly as each female can produce numerous offspring.
- It maintains populations even with an absence of males.
- It succeeds in harsh or challenging habitats where survival matters more than genetic diversity.
- It takes advantage of hybridization between sexual species which provides increased cell complexity and genetic stability.
While asexual species may seem unusual, they highlight the diverse reproductive strategies seen across the animal kingdom. Mother Nature has evolved some ingenious methods to ensure species survival!
Asexual Reproduction in Other Taxa
While this article has focused on examples of asexual reproduction in the animal kingdom, all-female and parthenogenetic species are found in other taxonomic groups as well:
Plants
- Orchids
- Dandelions
- Strawberries
- Komatsuna
Algae
- Desmarestia viridis
- Fucus spiralis
Fungi
- Candida albicans
- Saccharomyces cerevisiae
Flatworms
- Dugesia tigrina
- Schmidtea polychroa
So while female-only animals grab much of the attention, asexual reproduction strategies occur across the tree of life, often providing survival benefits in challenging conditions.
Conclusion
The existence of all-female animal species that reproduce asexually shows the remarkable flexibility of Mother Nature. Whiptail lizards, Amazon mollies, Komodo dragons, and other animals maintain breeding populations without males through unusual evolutionary adaptations like hybridization, cloning, and parthenogenesis.
While asexual reproduction leads to reduced genetic diversity, it gives certain species an advantage in extreme environments or when males are scarce. The ability to clone, even if rarely used, provides insurance for species survival. This highlights the manifold reproductive strategies found across the animal kingdom.