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During the breeding season male frogs are compelled to grab moving objects and engage them in amplexus, the tight ‘breeding clasp’ that occurs either under the forelimbs (axillary amplexus) or around the waist (inguinal amplexus), depending on the species. Amplexus is assisted by roughened pads of tubercles or even small spikes on the male hand, wrist and/or forearm. If it’s obvious that the moving object is not a female of the same species (because it feels wrong or makes an objectionable noise [as male frogs do when grabbed by other males]), the male lets go.
Nevertheless, male Eurasian common frogs Rana temporaria – the anuran I’m most familiar with – have been reported to grab goldfish, people’s hands and, as you can see from this photo, members of other anuran species. Here, a male R. temporaria has engaged in amplexus with a Common toad Bufo bufo: presumably a female based on size and colour (males are generally small and grey, females big and brown). I don’t know what the outcome of this photo was, but if mating occurred, there is no possibility of successful hybridisation between these very distant relatives.
Even more remarkable is the photo below (from wikipedia) in which a R. temporaria is clasping a very dead Fire salamander Salamandra salamandra. The salamander is dead now: was it that way when the frog first became interested in it? Obviously, male anurans don’t seem fussy when it comes to selecting mates. They literally grab at passing animals and hope for the best. By selecting a big, fat female full of eggs a male would be maximising his chance of reproductive success, but I really doubt if they’re that selective. Once in amplexus a male hangs on for days or weeks, and once the female gets to a pond the male then has to fend off competitors. Toads are far more aggressive than ranid frogs when it comes to this competition, sometimes forming large balls of fighting, kicking toads with one poor female in the middle. On occasion she may be drowned as a result.
Once spawning has occurred, it’s game over for the female and she leaves and gets on with her life. The male, however, may now try and find another female [insert hilarious quip about parallel with human behaviour]. In fact males spend so much time doing nothing but searching for, and grabbing hold of, females that they gradually starve and many of them die immediately afterwards (don’t forget, they’ve only just emerged from hibernation: here in England spawning used to begin in January or February but seems to be occurring earlier and is now sometimes reported in December). It’s recently been discovered that males can still fertilize eggs even if they fail to grab a female: they simply hang out near a spawning pair and shed their sperm into the water as the adjacent pair spawns. One egg clutch may therefore include embryos that have more than one father (Laurila & Seppä 1998). Even more amazing is ‘clutch piracy’: only reported in 2004, this is where males grab hold of egg clutches and then fertilize them as if they were mating with a female. In one studied population of R. temporaria, 84% of clutches were fertilized this way! How the hell did the frogs ‘realise’ that they need to shed sperm on eggs without the involvement of a female? The mind boggles.
Anyway, this was meant to be nothing more than a cheap joke and a picture-of-the-day, so I’ll stop there. Lest we forget, the global effort to prevent amphibian extinction [Tet Zoo introduction on that subject here] is still continuing: keep an eye on Frog Matters for the latest news. In the news right now is the discovery that atrazine – the second most widely used agricultural pesticide in the US – has been linked to anuran decline.
Thanks yet again to Markus Bühler for hunting out the neat pictures!
Refs – -
Laurila, A. & Seppä, P. 1998. Multiple paternity in the common frog (Rana temporaria): genetic evidence from tadpole kin groups. Zoological Journal of the Linnean Society 63, 221-232.
Vieites, D. R., Nieto-Román, S., Barluenga, M., Palanca, A., Vences, M. & Meyer, A. 2004. Post-mating clutch piracy in an amphibian. Nature 431, 305-308.
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