Female Frogs Use Video to Demonstrate Defense Mechanisms Against Unwanted Mating

The date of the article is October 21, 2023.

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The authored by Stephanie Baum and was published on Phys.org.

The behavior of different species varies, especially with regards to breeding preferences and techniques between males and females. Instances of aggressive behavior from males towards unresponsive females could lead to a failure to reproduce or, in extreme situations, fatalities to the female. These issues are widespread among the anurans species, a category that includes around 4500 kinds of frogs and toads, where it is reported that females mating have a higher mortality risk.

Species that breed explosively, including the European common frog, Rana Temporaria, have a short and highly competitive reproductive season. This differs from prolonged breeders that reproduce over a more extended period. These males usually establish separate territories which they defend against other males and use mating calls to attract females.

For R. Temporaria, previous studies show mixed results on whether unresponsive females can repel unwanted male advances. Such advances may include harassment, intimidation, and forced copulation (amplexus) by one or numerous males at once. In a recent study, it indicates that females lack the means to prevent this aggression and describes their reproductive behavior as passive, contrary to earlier research.

In a new study titled, 'Drop dead! Female mate avoidance in an explosively breeding frog,' researchers from the University of Jyväskylä in Finland and Germany's Leibniz Institute for Evolution and Biodiversity Science and Berlin–Brandenburg Institute of Advanced Biodiversity Research have investigated female mate avoidance tactics among R. temporaria. The study is published in Royal Society Open Science.

From their prior research on mate selection among mating R. Temporaria males, the researchers utilized video recordings to document the frogs' mating behavior. They made observations and quantifications of the female avoidance tactics, which led to this new study.

The researchers discovered three mating avoidance tactics that unreceptive females used. These behaviors were observed in the water, and in one case, on land. Among the 54 amplexed females, 83% exhibited rotation alone or in combination with one or two other strategies. About half (48%) of amplexed females emitted release calls, always in conjunction with rotation. A third (33%) of all clasped females exhibited tonic immobility, usually paired with rotation and calling.

Interestingly, smaller females utilized all three tactics, while larger females generally combined rotation with calling. About 46% of amplexed females demonstrating these strategies managed to escape, with smaller females showing a higher likelihood of success.

The researchers also proposed explanations for why rotating is a popular tactic of mating avoidance among females. They mention that mechanical grip characteristics might clarify why smaller females more easily escape amplexus by rotating than larger ones. Additionally, they suggest tonic immobility across numerous species could be linked to stress, which may interfere with mating activity and induce longer periods of tonic immobility.

From their observations, the researchers suggest that the age or expertise of the females might influence this behavior. They also note the prevalence of multiple males creating 'mating balls' with a single female as a potential stressor, which could result in the death of both males and females through drowning.

The larger question, why female frogs might accept or reject certain males as mates, remains unanswered.

The research states, 'It remains to be determined which (phenotypic or genotypic) trait(s) is (are) under selection, leading to mate acceptance or rejection, and what benefits and costs, if any, are associated with mate choice by female R. temporaria. There seem to be no obvious direct benefits of choosing a particular male, as males provide no parental care or defend any resources.

'Direct benefits, such as increased fertilization success by larger or size-assorted males have been disproved; in fact, fertilization success has been found to be independent of size assortment.'

Nevertheless, it is clear that mating is not a simple process in R. temporaria, and that females do not always remain passive when it occurs. Future studies, the researchers suggest, could focus on comparing females' stress corticosterone levels to age and sex ratios, as well as how age ratios, sex ratios, and varying frog densities affect mate selection and the risks of reproduction.

Journal information: Royal Society Open Science

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