Zoologger: The spiders hit by fatal courtship muddles

Zoologger is our weekly column highlighting extraordinary animals ? and occasionally other organisms ? from around the world

Species: Paratrechalea azul and Paratrechalea ornata
Habitat: Southern Brazil, Uruguay and northern Argentina, with a tricky population overlap in Rio Grande do Sul, Brazil

Mistake another lady for your mate and you're liable to get slapped. For males of two closely related species of spider, the mistake can be their last. Court the wrong female, and they risk being devoured.

The Paratrechalea azul spider lives on boulders on the edges of streams in South America. Trouble is, so does its close relation, Paratrechalea ornata. Though they occupy many of the same habitats and look nearly identical to the human eye, the two species cannot mate successfully with each another.

Luiz Ernesto Costa-Schmidt at the National University of C?rdoba in Argentina has spent years studying these spiders in the wild. Even so, he is only sometimes able to tell them apart just by looking ? P. azul has an average body length of 4.7 mm, which is roughly half a millimetre longer than P. ornata.

And scientists are not the only ones who struggle to tell the two species apart. The spiders themselves don't do so well either.

During courtship, males of both species offer their larger female counterparts a gift: a tasty morsel of prey wrapped in silk. The males find a potentially receptive mate by sniffing out the pheromones that female spiders leave on their silky threads.

Wasted presents

But in the lab, Costa-Schmidt found that males struggled to identify females of their own species. They frequently wasted their gifts on an incompatible mate.

The cost of these mistakes is often dangerously high, particularly for P. ornata males. They are significantly smaller than P. azul females, and often lose both their gifts and their lives in their failed courtships. The size difference is more likely to work in favour of P. azul males, though, since the P. ornata females see them as bigger and more impressive mates than males of their own species. Consequently, they are less likely to wind up dead after courtship confusion.

The difference probably explains why P. ornata males are somewhat better than P. azul males at pinpointing which female is which.

Costa-Schmidt points out that, in addition to pheromones, male spiders in the wild probably make use of other signals to identify potential mates, so they may fare better than those in the lab. But if the lab results are representative, the males of both species would do well to keep their many appendages crossed when they feel the urge to mate.

Journal reference: Animal Behaviour, DOI: 10.1016/j.anbehav.2012.08.026

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