Small, spherical glass frogs are better fathers

Important information for anyone who kisses frogs looking for a prince. If you’re hoping to start a family, measure the testicles first. The bigger the balls a glass frog carries, the less likely it is to become a good father. The finding is consistent with evolutionary models, and indeed with evidence from some other species, but the pattern is not universal.

In addition to the survival of the fittest, evolution is partly driven by sexual selection. The animal kingdom has found many different ways to make this work, usually – though not always – among men.

In some species, males seduce females with bright colors or dancing, in others they fight for dominance. Then there are the species where sperm competition is the key to passing on one’s genes, usually identified by the size of the cojones. This has now been verified in Neotropical glass frogs, and the findings were published on Valentine’s Day. But there’s a price to pay for all that testicle endowment, and it comes when it’s time to raise the young.

Many Centrolenidae, colloquially known as glass frogs because of their transparent abdominal skin, live in the tropical rainforests of Central and South America, when they are not The Muppet Show. The females lay eggs on leaves hanging above the water before moving to greener ponds. The males jump in to fertilize after the female is gone, meaning she has no choice over the father, and there can be a lot of competition over a newly laid clutch.

How this competition plays out varies by species, giving Anyelet Valencia-Aguillar of São Paulo State University and collaborators the opportunity to explore strategies.

Males of some glass frog species have spines protruding from their upper arms, which they use as weapons to fight over who gets to spread their seed. Others guard the eggs, both against rivals and sources of danger.

Another form of competition – we recommend not reading the next bit while eating – is to maximize sperm production, so that if multiple males all ejaculate over the same set of eggs, those with the most sperm will have the most tadpoles. Obviously, bigger testicles are the key to winning the sperm wars.

This raises the question to what extent frogs adopt a comprehensive strategy, or whether there is a trade-off between approaches. An alternative idea is that growth in testicle size is not driven by competition, but by females producing larger clutch sizes. In this scenario, frogs need large balls to handle an entire clutch.

Rainforests lend themselves to species diversification, and glass frogs are no exception. Comparing the characteristics of 37 species, the researchers concluded that testicles were larger in species that (say) eat, shoot and leave than in species that hang around to keep the eggs safe.

On the other hand, regarding the upper arm, the authors also expected that men with fighting spines below would be less rested, but found no statistically significant relationship.

Apparently fighting ability only gets you so far in the frog world, and even members of species with arm spines need a backup strategy in the Crown Jewels. Also, males were not much larger than females in small-ball species, a pattern often seen in other animals such as great apes, where gorillas do not need to engage in sperm competition and use their size and strength to fend off rivals.

“Paternal efforts could lead to a decrease in gonadal investment, because if a male invests more in guarding the clutch, he will have less energy to invest in survival, growth, or gamete production,” the authors note on. However, this raises the question of why other strategies, such as controlling spines, do not show the same correlation.

The authors speculate that testosterone affects both testicle size and fighting ability, so these may go together, but the hormone is less compatible with parental care. However, this theory has yet to be tested. Alternatively, males who spend time caring for a clutch can probably fertilize just one per mating season, rather than needing a lot of sperm in the hope of getting several.

Certainly, the hypothesis that testicular size was a response to larger clutches failed. In fact, the opposite was the case: larger clutches were associated with more parental care, but had no relationship with testicle size.

Frogs are far from the first animal type on which such studies have been done. In bats, there is a trade-off between testicle and brain size. Like airplanes, flying species cannot carry too much baggage and a club with large balls on board must throw brain weight to stay in the air. Those who invest in intellect are likely to live longer, but may also have better strategies for courting mates.

On the other hand, studies of parental care and testis size have yielded different results depending on the branch of the animal’s pedigree examined.

Either way, we definitely think someone should tell Miss Piggy to look under the… whatever it is that Muppets have.

The research has been published in Proceedings of the Royal Society B.

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