Freezing frog sperm might help bolster numbers (Image: David Tipling/Image Bank/Getty)

The freezer could be the future for frogs and other amphibians. Efforts announced today are currently underway around the world to boost amphibian numbers with cryopreservation and assisted reproduction.

Breeding frogs and their cousins to increase numbers could help vulnerable species survive looming extinctions. But getting amphibians to mate is not always straightforward, so researchers are developing other techniques to give them a helping hand.

One proposal resembles the doomsday seed vault which opened this year in Norway. Only instead of plant seed, the amphibian vault would store sperm, guaranteeing amphibian genetic diversity for times of dwindling populations.


Since June this year, the Amphibian Ark project has been doing the groundwork to set up regional amphibian “biobanks”.

Cheap option

“If a colony of rare frogs were wiped out by disease, then you could go to the sperm bank and recover that genetic diversity,” explains Rhiannon Lloyd of the Zoological Society of London.

Expense is another reason for freezing frog sperm. Maintaining breeding populations of every rare species is far more costly than storing cells.

Creating these sperm banks is not as easy, as getting males from endangered amphibian species to ejaculate in a test tube. Researchers generally surgically remove a frog’s testes to get sperm. This means sacrificing the male – not a desirable outcome in a rare species.

What’s more, sperm alone is not sufficient – eggs are needed too. As they tend to be too fragile to freeze, researchers in Germany are trying to solve the “female problem” by creating viable frog offspring from two male sperm.

Nabil Mansour of Salzburg University is developing a technique called androgenesis. This involves zapping a frog egg with ultraviolet radiation to destroy its genetic material, then fertilising the enucleated egg with two sperm which, when heat-shocked, combine to form a single genome. A variant of this method is carried out commercially in some fish species, including sturgeon.

Babies from sperm

In the long run, Mansour hopes it might be possible to use sperm from an endangered frog species and an egg from a non-endangered species, which would avoid having to freeze eggs from rare species.

“For now, I am working with lab amphibians,” says Mansour. “I am able to collect semen without hurting the males, freeze it, and use it to fertilise eggs.”

Mansour has also started to perform androgenesis and has obtained tadpoles in this way, but has not yet done the genetic tests to confirm that the tadpoles really are the result of the androgenesis.

“Efforts to cryopreserve various amphibian tissues and gametes are getting underway. But to date, they might be mostly characterised as research rather than creation of a frozen bank,” says Robert Lacy, a conservation biologist with the Chicago Zoological Society and chair of the World Conservation Union’s conservation breeding specialist group.

Mansour hopes he might be able to reliable perform androgenesis on common laboratory species within a couple years, at which point he could move on to rare amphibians. In the meantime, Lloyd says there are many other ways of helping frogs get it on.

Already, the dwindling populations of three rare species – the Wyoming toad, the Puerto Rico crested toad, and the boreal toad – have been boosted with the help of hormone therapy. “Ideally we want the amphibians to breed naturally, but if there are very few left and you cannot get them to breed what are you going to do?” says Lloyd. “Are you going to let them die?”

Frogs in heat

Cocktails of hormones can be used to overcome the environmental stimuli – such as temperature and rainfall – that species rely on to start breeding. Injecting them with hormones will artificially trigger mating season and synchronise male and female populations.

If hormones are not sufficient to stimulate the frogs to mate, they can be used to make them produce sperm and eggs, which can then be fertilised in the dish – a form of amphibian in-vitro fertilisation that is routinely carried out in laboratory research.

“If a species can be adequately safeguarded in its natural habitat, I don’t know of anyone who would not argue that is the best approach,” says Lacy. “However, for hundreds, even thousands, of species of amphibians, there are threats that we cannot currently address with assurance.”

Another reason for freezing frog sperm is to preserve genes that may not seem valuable now, but will become essential in future, Lacy adds. “We don’t know what genes will be needed for survival in the future, so our best option is to work to maintain as much of the original genetic variability as possible,” he says.

The proposals were announced at the Halting the Global Decline in Amphibians symposium, in London, UK.