Can Komodo Dragons Reproduce Without a Male? Virgin Births

The Chester and London Zoo cases

The first clear evidence came from two captive females in the United Kingdom. Sungai, at London Zoo, laid a clutch of eggs in 2005 after more than two years without contact with a male. Keepers first assumed she had stored sperm from an earlier mating, which some reptiles can do for long periods.

Then in 2006 Flora, a female at Chester Zoo who had never been kept with a male, laid a clutch of eggs. Several eggs developed and hatched in early 2007, and all of the young were male. Geneticists at the University of Liverpool tested eggs and tissue from both females and found that the offspring carried only genetic material from their mothers. The results were published by Phillip Watts and colleagues in Nature in December 2006 under the title “Parthenogenesis in Komodo dragons.”

The tests overturned the sperm-storage idea for Sungai: her eggs, too, had developed without a father. Since then, zoos in the United States and elsewhere have reported further cases, starting with Sedgwick County Zoo in Kansas, where two male hatchlings from an unmated female emerged in early 2008.

How a virgin birth works

Normally, an egg cell carries one set of chromosomes from the mother and is fertilized by a sperm carrying one set from the father. In Komodo dragon parthenogenesis there is no sperm. Instead, the egg’s single set of chromosomes is doubled, either by fusing with a small cell left over from egg formation or by copying its own chromosomes. The result is an embryo with two sets, both from the mother.

Because the two sets are copies of the same half of the mother’s genes, the offspring are much more inbred than normal young. They are not clones of the mother, and not identical to each other, since each egg receives a different mix of her genes.

Why the babies are always male

The answer lies in how sex is decided. Komodo dragons, like birds and many other reptiles, use a ZW system of sex chromosomes rather than the XY system of mammals. Males have two Z chromosomes (ZZ). Females have one Z and one W (ZW).

A female passes only one of her two sex chromosomes into each egg. When that set is doubled in parthenogenesis, an egg that received a Z becomes ZZ and develops into a male. An egg that received a W becomes WW, and such embryos cannot survive. So every surviving young dragon from a virgin birth is male.

This is different from sea turtles, whose sex is set by the temperature of the nest rather than by chromosomes; see does temperature decide a turtle’s sex.

What it might mean for island colonization

Komodo dragons can swim between islands, and in the distant past the species spread from Australia into the islands of Indonesia. Scientists have suggested that parthenogenesis could help a lone female that reaches a new island by sea. She could produce sons on her own, then mate with them to produce both male and female young, starting a new breeding population.

This idea is a reasonable hypothesis, not something that has been observed. There is no record of a wild population founded this way, and in the wild most females have access to males. Still, the ability to reproduce alone would be a useful backup for an animal living on small, isolated islands.

Why zoos still want males

For zoos, parthenogenesis is a curiosity but also a concern. Offspring from virgin births have very low genetic diversity, which can make a small captive population less healthy over generations. The Komodo dragon is Endangered, and zoo breeding programs aim to keep as much genetic variety as possible. Keepers therefore manage pairings carefully and use normal sexual reproduction wherever they can.

Normal Komodo dragon breeding

In the wild, Komodo dragons mate mainly between May and August. Males wrestle on their hind legs for access to females, and females lay clutches of about 20 eggs, usually in the mound nest of a megapode bird called the orange-footed scrubfowl or in a burrow. The eggs incubate for about seven to eight months before the hatchlings dig out and climb into trees. For how big the young eventually get, see how big do Komodo dragons get.

Virgin births in other animals

Komodo dragons are far from the only animals with this ability. Some sharks in aquariums, several species of snakes including boas and pythons, domestic turkeys and even the California condor have produced young without a father. A few North American whiptail lizards take it further: their populations consist entirely of females, which reproduce only by parthenogenesis. Fathers are optional in more of the animal world than once thought, and the male seahorse shows that even pregnancy is not always a female job.