Eunuch spiders still have balls

THE male Malabar spider may lose its genitals, but no one can accuse it of needing to grow a pair. Even after its reproductive organs snap off, the spider is still willing to take down rivals.

Genital loss is known to make Nephilengys malabarensis a more aggressive fighter. Simona Kralj-Fiser from the Slovenian Academy of Sciences and Arts in Ljubljana thinks the explanation is that the eunuch male has everything to gain and little to lose in seeing off rivals. That's because male Malabar spiders leave their two palps behind after inseminating the female. These block the female copulatory opening but can be removed by a rival male, who then impregnates the female.

In a related species, N. livida, genital loss does not turn males into fighting machines. Kraj-Fiser and Matjaz Kuntner from the National Museum of Natural History in Washington DC staged fights between virgin and eunuch N. livida males. They found that virgin males were more aggressive than the eunuchs, who did not even guard the females.

Kralj-Fiser and Kuntner suggest this is because the sperm plugs of N. livida are more efficient and so "guard" the female for the eunuch (Naturwissenschaften, DOI: 10.1007/s00114-011-0873-1).

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