Caring glass frog fathers have smaller testes

Males of the glassfrog species Hyalinobatrachium valerioi are very dedicated fathers.
Photo Credit: © Francesca Angiolani

An international team of researchers including the University of Bern shows in a new study that male glass frogs that care for their offspring have smaller testes than males of species that do not provide any brood care. This indicates an evolutionary trade-off between sperm production and parenting.

Living in the tropical rainforests of Central and South America, frogs of the glass frog family are fascinating because of their transparent skin on their belly, which reveals their internal organs. However, it is not only the appearance of these amphibians that is remarkable, but also their social behavior. In many - but not all - glass frog species, the males remain with the clutch after mating and guard and care for their offspring. In a new study, an international team, including researchers from the Institute of Ecology and Evolution at the University of Bern, shows that there is a link between this paternal care and the testes size of glass frogs. The results were recently published in the journal Proceedings of the Royal Society B.

Left: Dr. Anyelet Valencia Aguilar, first author of the study, who until recently worked at the Institute of Ecology and Evolution at the University of Bern and is now conducting research at the Justus Liebig University in Giessen as part of a Swiss National Science Foundation (SNSF) postdoctoral mobility grant.
Photo Credit: © Max Ringler
Right: Prof. Dr. Eva Ringler, Head of the Division of Behavioural Ecology, Institute of Ecology and Evolution, University of Bern.
Photo Credit: © University of Bern / Adrian Moser

37 different species of glass frogs

"The study involved several months of field research in the rainforests of Brazil and Ecuador, collecting data on the behavior and physical characteristics of different species of glass frogs," says Eva Ringler, Professor and Head of the Department of Behavioral Ecology at the Institute of Ecology and Evolution at the University of Bern. The researchers also performed a phylogenetic analysis of the entire glass frog family tree. Such analysis can provide key information about evolutionary transitions in certain physical as well as behavioral traits, such as for example parental care. A total of 37 different glass frog species were studied. In 11 of these species, males care for their offspring; in the others, males do not provide any brood care. "Our study is an important contribution to a better understanding of the evolution of male parenting," says Cynthia Prado, last author of the study and professor at São Paulo State University.

Male glassfrog Hyalinobatrachium valerioi caring for three clutches of eggs.
Photo Credit: © Francesca Angiolani

An evolutionary trade-off

In the study, the team showed that male glass frogs that care for their offspring have proportionally smaller testes than males of species that do not care for their offspring. These findings shed light on the various strategies that different species use to maximize their reproductive success. "Our results suggest an evolutionary trade-off between testes size and paternal care in glass frogs," explains Anyelet Valencia Aguilar, first author of the study, who until recently worked at the Institute of Ecology and Evolution at the University of Bern and is now conducting research at the Justus Liebig University in Giessen as part of a Swiss National Science Foundation (SNSF) postdoctoral mobility grant. To explain this trade-off, it is important to know that in glass frogs, as in all amphibians, the female's eggs are fertilized externally: During mating, the female releases unfertilized eggs on which the male immediately deposits his sperm.

This external fertilization carries the risk of "clutch piracy": male frogs that manage to get their sperm onto a fresh clutch of eggs, thus fertilizing those eggs that have not yet been fertilized by the sperm of the original male. "When a male guards his clutch, he reduces the risk of fertilization by other males and thus reduces the selective pressure for increased sperm production – leading to relatively smaller testes of males in species that perform brood care. Our study thus highlights the important role of paternal care in the evolution of testes size in species with external fertilization," Ringler says.

Male glassfrogs of the species Hyalinobatrachium valerioi continue to attract mating partners while caring for clutches.
Photo Credit: © Francesca Angiolani

Similar patterns known in primates

"We were surprised how clear these trade-offs are present in glass frogs," says Ringler. Previously, such evolutionary relationships between male genitalia and reproductive behavior were mainly known in primates: Males of harem-forming species have big body sizes and strength to keep rivals away, which also leads to smaller genitalia because they primary access to mating with females in their group. In contrast, males of species that live in mixed-sex groups with promiscuous mating have relatively large genitalia to maximize fertilization success, as large genitalia are usually associated with large sperm production. In these species, male-male competition does not take place before mating, but rather afterwards, at the sperm level. "It is very exciting to find similar evolutionary relationships in glass frogs. In recent years, it has become evident that there are still so many fascinating aspects to be discovered about glass frogs," says Ringler.

Glass frog research continues: As part of a research project funded by the Swiss National Science Foundation (SNSF), Eva Ringler and her team are currently investigating the relationships between parental care behavior, spatial use, cognition and communication in various glass frog species. "We still know very little about the social and reproductive behavior of many species and the associated cognitive abilities. Our project should make an important contribution to a better understanding of the relationships between behavior, reproduction and cognition," Ringler concludes.

Funding: The study was financially supported by the São Paulo Research Foundation (FAPESP), the Universidad San Francisco de Quito (USFQ) and the Swiss National Science Foundation (SNSF).

Published in journal: Proceedings of the Royal Society B

Title: Evolutionary trade-offs between testes size and parenting in Neotropical glassfrogs.

Authors: Anyelet Valencia-Aguilar, Eva Ringler, Stefan Lüpold, Juan M. Guayasamin and Cynthia P. A. Prado

Source/Credit: University of Bern

Reference Number: bio022224_01

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