Horns vs. Sperm: Male beetles on tight equipment budget

A group of dung beetle species that sprout horns like tiny elk, rhinos, or sci-fi invaders often face trade-offs between horn and testes sizes, say researchers.

Among the 2,000 species of Onthophagus dung beetles, males sport various styles of swooping prongs, with which they wrestle other males for access to females. “That’s like producing another leg and wearing it around on your head for the rest of your life,” says Douglas J. Emlen of the University of Montana in Missoula. His earlier experiments showed that as an individual beetle develops horns, they steal resources from other organs, leading to smaller eyes, antennae, or wings.

To test for trade-offs between horns and testes, Emlen and Leigh W. Simmons of the University of Western Australia in Crawley worked with immature Onthophagus nigriventris and cauterized cells that would have grown into horns. The prongless males grew testes that were about 30 percent larger than those of comparably sized, horned males, Simmons and Emlen report.

That finding, they note, fits with results from another research team, which stopped genital growth in an Onthophagus species. Unusually small males grew horns.

Sperm investment hasn’t gotten its due respect, comments Scott Pitnick of Syracuse (N.Y.) University. He welcomes the beetle research as adding to the growing body of work demonstrating that “sperm production turns out not to be cheap, after all.” Pitnick and his colleagues have reported that among species of small, insect-eating bats, investing in supersize testes tends to correspond to having smaller brains.

However, Simmons and Emlen didn’t find such a straightforward pattern when they analyzed more than 20 species of Onthophagus beetles. Big horns didn’t correspond to smaller testes, as would be expected if there were a simple constraint on growth.

Instead, some beetle species seem to protect their genital development. “Given enough [evolutionary] time, something will come up that’s a way around a constraint,” says Emlen.

The beetle species that broke the expected pattern had protected the development of their testes to an unusual degree. Emlen and Simmons report the findings online this week for an upcoming Proceedings of the National Academy of Sciences.

The relationships between horn and testes sizes “certainly suggest there’s something going on—it’s not random,” says Gerald Wilkinson of the National Science Foundation in Arlington, Va.

The beauty of studying trade-offs between testes and horn sizes is that reproductive pressures drive them both, Wilkinson says. Onthophagus females mate with multiple males, so the competition favors males that deliver abundant sperm. Yet that delivery power doesn’t matter unless a male uses his horns to reach the female.