The Bigger Your Weapon, The Smaller Your Balls - If You're A BeetleS

To compete for mates, male beetles often grow fierce mandibles that they use to fight other males. Unfortunately, there's a tradeoff. Researchers in Japan discovered that beetles with longer mandible weapons have smaller testicles and less ejaculate than their brethren.

In a paper published yesterday in the Proceedings of the Royal Society B, charmingly titled "Dispersal and ejaculatory strategies associated with exaggeration of weapon in an armed beetle," the scientists report their findings. They bred three strains of beetle over successive generations: Ones with long mandibles, ones with short mandibles, and a control group. They found that the beetles with long mandibles did win in battle, but they lost the sperm war.

In their comparison of the relative tradeoffs between mandible size and testicle size, the researchers found that, on average, beetles with giant mandibles had small balls:

Multiple comparisons of testis size and wing size showed that the relative line means were significantly smaller in the long selection regime than in the control and short selection regimes, and significantly larger in the short selection regime than in the control.

You really don't want to know how the researchers put these poor beetles' balls under a microscope, but suffice to say their measurements were very exact. In addition, the researchers observed that beetles with large weapons had smaller wings.

Females who mated with the beetles who had short mandibles also retained a lot more sperm (see chart).

The Bigger Your Weapon, The Smaller Your Balls - If You're A BeetleS

I think it's obvious what this research reveals: Big sticks mean little dicks. You may not be surprised to discover that the scientists have a more even-handed and accurate explanation:

Males normally guard territories and mates with their weapons, and there was a positive relationship between weapon investment and fighting success in this beetle. As a result, males with long mandibles have higher remating rates with the same mate. Because these males will therefore face a lower risk of sperm competition, they are unlikely to need to inseminate each female with more sperm per copulation. By contrast, the reduction in mandible investment will decrease the success in male fighting. Males disperse to new territories, which may or may not contain other males, when they fail to guard their territories . Because the females copulate with two or more males, these dispersing males will face a higher risk of sperm competition from guarding males compared with males with long mandibles. Males generally increase their ejaculatory expenditure when the risk of sperm competition is higher. Males with short mandibles potentially need to invest more in testes and wings for mating success. Therefore, this trade-off between the weapon, spermatogenesis and dispersal characters may occur as a result of selection on the mating strategy.

Reference: Dispersal and ejaculatory strategies associated with exaggeration of weapon in an armed beetle, via Royal Society