Croak it: Female frogs fake death to avoid sex with unappealing males
In the wild world of nature, where an organism’s sole aim is to succeed at passing on their genes at any cost, it’s not surprising many species have aggressive, and even deadly, mating behaviors.
But now the tables turn within the sexual dynamics of one such species, the European common frog (Ranat emporaria), with researchers led by Carolin Dittritch from the Konrad Lorenz Institute of Ethology, Vienna, discovering by accident how females will fake their own deaths to avoid mating with an undesirable male. It counters assumptions that the randy male pile-on, which frequently occurs during these free-for-all breeding sprees, is something hapless female frogs have no autonomy over.
This act of tonic immobility, or thanatosis, is an adapted act of deception that prey species are known to employ in the presence of a predator. Recently, scientists were surprised to observe thanatosis initiated by an entire colony of Polyrhachis femorata ants on Kangaroo Island, Australia, the first time this collective ‘play dead’ had been seen. However, it's only been observed a handful of times in intraspecific sexual behaviors.
During the intense, fierce, frenzied group mating sessions known as explosive breeding – a term that leaves little to the imagination – male frogs are known to harass, intimidate and attempt to physically overpower females, usually while in water, which can lead to the death of one or both animals.
The females, however, have devised a cunning strategy to avoid the harassment from undesirably mates, by convincing their annoying suitors that they’re dead. After all, a dead female is no place for a male to waste energy attempting to pass on his precious parcel of genes. This style of mating-related tonic immobility has only been observed in one other amphibian, the sharp-ribbed newt (Pleurodeles waltl).
“We observed three female avoidance behaviors, namely ‘rotating’, ‘release call(s)’ and tonic immobility,” noted the researchers. “We defined ‘rotating’ as a female starting to rotate around her own body axis when amplexed by a male, while the male tries counteracting the rotation with its hind feet.”
The scientists observed two calls made by the female – a grunt and a squeak – and in some 33% of animals, saw the frog play the role of her life, stiffening her limbs, and playing dead, sending her unlucky-in-love mate a convincing message of fatal noncompliance. Fortunately, thanks to the high cost of energy involved in sperm production and mating, this Oscar-worthy act is enough to lose his formerly undivided attention. (After all, as the saying goes, there are plenty more frogs in the pond.)
While a majority of the females observed (83%) enlisted the shake-it-off technique of rotating their bodies while clasped by the male in water, which potentially tests both the strength of her suitor as well as making him more vulnerable to drowning, a third – particularly smaller-bodied frogs – still went all-out with thanatosis, with considerable success.
Meanwhile, scientists aren’t entirely sure why the females have adapted this behavior; male frogs provide no parental care or help defend resources, so being selective has little advantage for the frog. As for the 'test of strength' argument, larger males do not have better mating success than smaller frogs.
This behavior is particularly curious, given that more commonplace adaptation is physiological evolution to counter aggressive male copulation. Many waterfowl, for example, have co-evolved features to defend themselves against males that have selectively evolved more effective reproductive organs. These include vaginal dead-end ‘pockets’ to filter out sperm from undesirable mates, as well as elaborate designs such as spirals that counter the corkscrew-like pattern of the male’s penis.
And while the frogs' tactics were not foolproof, with just a 46% success rate of escape for those in the firm grip of aggressive males, playing dead is arguably a little quicker to master than evolution.
The study was published in the journal Royal Society Open Science.