The Croak Conundrum: How Parasites Influence Frog Mate Choice

Male green treefrogs employ loud, repeated “honking” calls to attract mates. Females generally prefer calls with lower frequencies, faster rates, or longer durations, traits often indicative of a male’s size, health and stamina. However, modern research from Florida Atlantic University reveals a complex interplay between these preferences and the presence of parasites, challenging long-held assumptions about sexual selection.

The Role of Sound in Animal Communication

Across the animal kingdom, sound serves as more than just a means of communication. it’s a vital signal of survival and reproductive success. Animals utilize acoustic “advertisements” to defend territories, attract mates, and convey their physical condition. These calls often reflect important traits like body size, strength, and overall health, playing a crucial role in sexual selection and species competition.

Parasites and Mating Signals: A Complex Relationship

Parasites can significantly influence mating signals. Infections drain energy and trigger immune responses, potentially weakening traits essential for mating success, such as stamina and the quality of acoustic calls. However, the relationship isn’t always straightforward. Some parasites are transmitted through predator-prey interactions, meaning larger, more successful foragers may be at a higher risk of infection.

FAU’s Research on Green Treefrogs and Tongueworm Parasites

Researchers at Florida Atlantic University investigated the impact of oral tongueworm parasites (Halipegus occidualis) on the mating calls and mate choice of green treefrogs (Dryophytes cinereus). These parasites reside in the mouth and throat of frogs and are acquired through the consumption of infected prey.

Methodology: Recording Calls and Analyzing Features

During the breeding season, male green treefrogs gather around ponds, inflating their vocal sacs to produce distinctive “honking” calls. Researchers recorded these calls in the wild and simultaneously counted the number of tongueworm parasites present in each frog’s mouth. The recordings were then analyzed using audio software to measure call characteristics, including frequency, length, and pulse structure. Frogs were categorized into three infection levels: uninfected, moderately infected (5-8 adult worms), and heavily infected (more than 9 adult worms).

Playback Experiments: Female Responses to Different Calls

To assess female preferences, the research team conducted two-choice playback experiments. They broadcast pairs of male calls and observed which call attracted the females. This allowed them to determine how females responded to calls from frogs with varying parasite loads.

Key Findings: A Nuanced Picture of Mate Choice

The study, published in the journal Current Zoology, revealed a surprising complexity in female mate choice. Although females generally prefer lower-frequency calls – a trait often associated with larger males – the presence of parasites altered the equation. Heavily infected frogs produced lower-frequency calls but with shorter durations, potentially signaling lower stamina.

Challenging the Hamilton-Zuk Hypothesis

The findings challenge the Hamilton-Zuk hypothesis, which posits that parasites reduce the quality of sexual signals and that females should favor less-infected males. Instead, the study suggests that females weigh multiple cues simultaneously, balancing traits linked to size and attractiveness against the risk of parasite infection.

The Importance of Call Duration

Call duration emerged as a particularly important factor in female decision-making. Longer calls generally indicated fewer parasites and greater energy reserves, signaling vigor and a lower infection risk. However, the relationship wasn’t always linear; some moderately infected males produced longer calls than uninfected males, potentially due to a combination of energy reserves and parasite load.

Implications for Sexual Selection

“Mate choice is rarely based on a single trait,” explained Michael W. McCoy, Ph.D., co-author and associate director of FAU’s School of Environmental, Coastal, and Ocean Sustainability. “Our results show that parasites can reshape the information animals use when choosing partners by subtly changing multiple aspects of a male’s call. Females may be responding to several signals at once, some linked to desirable traits like size and others hinting at infection. Understanding that complexity is critical for explaining how sexual selection actually works in natural populations.”

The research highlights that parasites don’t simply diminish attractiveness; they introduce nuanced cues that females must interpret, subtly guiding mating decisions and shaping sexual selection in wild populations.

Study Contributors

Ellen F. Titus with The Nature Conservancy also contributed to the study.

Funding Sources

The research was supported by grants from the International Herpetological Symposium, The Sigma Xi Scientific Research Honor Society, and the National Science Foundation.