July 7, 2008

Frog Sex is a Strange Affair

I've been in the field for most of the last month and given that this is frog breeding season I've seen some pretty bizarre things related to this reproductive frenzy. But first, some background on mating in frogs. To mate, a male clasps a female with his powerful front legs that he wraps around the female just behind her front legs. To strengthen that grip, the male has nuptial pads on the thumbs of each hand that make it very difficult for the male to be dislodged when the digits from the front legs are interlocked around the female. This clasping of the female by a male is called "amplexus", and in frogs serves two purposes. First, it allows the male to "guard" the female from other males, thereby ensuring the reproductive success of the amplexing male. Second, it positions the male to allow him to fertilize the eggs as the female releases them.

As the photo above vividly shows, amplexus is not a simple matter of a male gently holding on to a female for several minutes while she looks for a place to lay her eggs. In the photo, a male-female pair is in amplexus and three additional males are doing their darnedest to pull the pair apart. If successful, this would afford one of the other males a chance to amplex the female. These battles can go on for hours, and often result in deep abrasions on the ventrum of amplexed females from the male nuptial pads. In rare cases, during extended battles the female can actually be drowned by all of the competing males.

Wrestling with other males for hours for access to females is only the beginning of what goes on in the frog world during the mating season. On my last backcountry trip, a male I was measuring had new amplexus scars, suggesting that he was recently amplexed by another male and that additional males were battling it out for access to him! My guess is that the original male was so jacked up on hormones that he failed to realize that he had amplexed another male instead of a female, and seeing an amplexing pair the other males piled on. Even more bizarre is the not uncommon observation of males amplexing dead frogs, often frogs that have been dead for weeks or even months.

Just another example of how sex on the brain causes all kinds of bizarre behaviors....

Back to The Mountain Yellow-legged Frog Site.

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