The new title for world's biggest testicles (relative to body weight) goes to the tuberous bushcricket, a type of katydid, according to a new study.
The sperm-producing organs account for 14 percent of the body mass of males of this bushcricket species. The previous record holder's testicles—belonging to the fruit fly Drosophila bifurca—tipped the scales at about 11 percent of its body mass.
"I was amazed by the size of the testes—they seemed to take up the entire abdomen," said study leader Karim Vahed, a behavioral ecologist at the University of Derby in the U.K.
But the new heavyweight champion doesn't pack much of a punch. The team was surprised to discover that tuberous bushcrickets have smaller ejaculations than bushcricket species with smaller testicles.
Bushcrickets Ideal Study Subjects
For the testicle study, Vahed and colleagues dissected specimens from 21 bushcricket species, collected around Europe.
The insects are ideal for studying reproductive evolution because of their efficient mating process, Vahed noted.
For one thing, the male bushcricket transfers his sperm to the female in a "neat packet" that's easily retrievable by researchers—"whereas in mammals, you'd have to provide some sort of condom to measure the ejaculate," he said.
Likewise the female stores each male's sperm packet in a separate pouch, enabling scientists to count how many times a female has mated in her lifetime. (See insect-egg pictures in National Geographic magazine.)
Predictably, the team found that the species whose females mate the most has the males with the biggest testicles, according to the study, published November 10 in the journal Biology Letters.
But among the 21 bushcricket species, the study showed that, as testicle size increases, ejaculation volume decreases.
The discovery runs counter to previous findings in other species—especially mammals. Usually the male with the biggest testicles has more sperm in each ejaculation, thus earning him more tickets in the lottery of fertilizing females, Vahed explained.
Bigger Testicles, More Sperm Supply?
A possible explanation, he said, is that, in societies with promiscuous females, large testicles give males a more plentiful sperm reservoir for multiple matings. Female tuberous bushcrickets mate an average of 11 times in their two-month life spans.
This alternative explanation for large testicles may even make scientists revisit some of their studies on vertebrates, he added. It's usually the other way around.
"It's clear that insects are one of the major types of organisms on the planet Earth, [but] the tendency is to draw conclusions from studies of vertebrates and generalize them as if they apply to everything," he said.
Indeed the take-home message for scientists is that mating rate needs to be taken into account when investigating testicle size, according to David Hosken, chair in evolutionary biology at the U.K.'s University of Exeter.
Overall the findings are not that surprising, he added via email.
"Higher mating rate selects for larger testes, but across other species, sperm competition risk seems to have a greater effect than mating rate."
Bushcricket Titillator Mystery
Next, Vahed plans to shift his focus to "titillators," the hard, penis-like part of male bushcricket genitalia that's inserted into the female. These poorly studied—and often spiny—parts may stimulate the female, allow the male to hang on, or both.
And, as it turns out, the tuberous bushcricket isn't quite so well endowed in this arena. The species' parts, he said, "don't seem to be as outlandish as some species that have double sets of spiny titillators."