Glass Frogs Paternal Care Linked To Reduction In Testes Size, Study Finds

Experts said that male glass frogs that usually help care for offspring tend to have smaller testes than the species whose males do not help care for their offspring.

Behavior Of Glass Frogs

They went into the tropical rain forests in Ecuador and Brazil in order to study the behavior of glass frogs over the course of several months.

In previous studies, it was found out that in some species of glass frogs, males often help females care for and protect their offspring. In the latest study, the researchers wondered what biological differences might account for the different behaviors between species.

In males, large testes size signifies high sperm production and is commonly linked to heightened sperm competition levels. It may also evolve as a response to an elevated risk of sperm depletion due to multiple mating or large clutch sizes.

Conversely, weapons, mate or clutch guarding may allow individuals to monopolize mating events and preclude sperm competition, thereby reducing the selection of large testes. In the study, scientists examined how paternal care, sexual size dimorphism (SSD), weaponry and female fecundity are linked to testes size in glassfrogs.

''We found that paternal care was associated with a reduction in relative testes size, suggesting an evolutionary trade-off between testes size and parenting. Although females were slightly larger than males and species with paternal care tended to have larger clutches, there was no significant relationship between SSD, clutch size and relative testes size,'' the experts said.

These findings suggested that the evolution of testes size in glassfrogs is influenced by sperm competition risk, rather than sperm depletion risk.

Experts inferred that clutch guarding precludes the risk of fertilization by other males and consequently diminishes selective pressure for larger testes.

The study highlighted the prominent role of paternal care in the evolution of testes size in species with external fertilization.

Glassfrogs may evolutionarily trade-off relative testes size with paternal care.

A similar pattern regarding the presence of humeral spines cannot be rejected but received less support in the dataset.

Moreover, species with relatively large clutches were more likely to show paternal care than those with smaller clutches, but there was no significant relationship between clutch size and relative testes size.

Sperm Competition

Sperm competition has been playing an important role in the evolution of paternal care, as the likelihood of males investing in parental care generally declines with a decrease in their probability of paternity.

In general, the likelihood of paternal care evolving is expected to increase with the certainty of paternity.

Different forms of parental care vary in their temporal and energetic costs to the carer, and more costly forms might constrain male investment in fertilization effort and consequently in testes size.

Besides sperm production, testes size has also been associated with testosterone levels, which play a central role in the expression of numerous sexual traits and behaviours.

However, although levels of circulating testosterone are often positively correlated with male testes size and aggressive behaviors, elevated androgen levels might also interfere with male parental care.