The ‘Croak’ Conundrum: Parasites Complicate Love Signals in Frogs
Male green tree frogs use loud, repeated “honking” calls to attract mates.
Study Snapshot: Across the animal kingdom, sound is more than communication – it’s a signal of survival and success. Male green treefrogs use loud, repeated “honking” calls to attract mates, with females favoring lower-frequency, faster or longer calls that often signal size, health and stamina. 鶹ƷƵresearchers investigated how oral tongueworm parasites affect these mating signals. By recording calls in the wild, counting parasite loads, and analyzing call features, they found that heavily infected males produced lower frequency but shorter calls. Playback experiments showed that females avoided the most heavily infected males but sometimes preferred moderately infected ones, suggesting they weigh multiple cues simultaneously.
These findings challenge the long-standing idea that parasites simply weaken sexual signals. Instead, infections create a complex mix of information about size, condition and infection risk, subtly reshaping how females evaluate potential mates. Call duration emerged as particularly important, reflecting both energy reserves and parasite load, while other call traits signaled body size or attractiveness. The study highlights the nuanced ways parasites influence sexual selection and mate choice in natural populations.
Across the animal kingdom, sound is more than communication – it’s a signal of survival and success. From birds and primates to insects, fish and amphibians, animals broadcast acoustic “advertisements” to defend territory, attract mates and reveal their physical condition. Because these calls can reflect traits such as body size, strength or health, they play a powerful role in sexual selection and help shape how species compete and reproduce.
Parasites can influence these mating signals. Infections drain energy and trigger immune responses that weaken the body, altering traits tied to mating success, such as stamina and the quality of acoustic calls, sometimes disrupting how sounds are produced or perceived.
Adding to the complexity, some parasites infect hosts through predator-prey interactions. This means individuals that are larger or more effective at foraging – qualities often preferred by potential mates – may actually face a higher risk of infection. However, studies in amphibians have produced mixed results.
To explore this paradox, 鶹ƷƵ researchers studied green treefrogs (Dryophytes cinereus) and oral frog tongueworm parasites (Halipegus occidualis) that live in the mouth and throat of frogs, to test whether food-web–transmitted parasites influence mating calls and female mate choice in a natural population.
During the breeding season, male green treefrogs gather in loud choruses around ponds, inflating their vocal sacs to produce repeated “honking” calls from nearby vegetation. Females use these calls to choose mates, typically favoring lower-frequency, faster and sometimes longer calls – traits that often signal a larger or healthier male. Pulse patterns in the calls also help females recognize their own species.
Researchers recorded the calls of male green treefrogs in the wild and counted the number of tongueworm parasites in each frog’s mouth. They then analyzed the recordings using audio software to measure features of the calls, such as frequency, length and pulse structure. They aggregated calls into three infection categories: uninfected, moderately infected (five to eight adult worms), and heavily infected (more than nine adult worms).
To see how females responded, the team conducted two-choice playback experiments, broadcasting pairs of male calls and observing which one they approached.
Results of the study, published in the journal , suggest that choosy female green treefrogs may face a croak conundrum: the call traits they prefer – such as lower frequencies – are typically produced by larger males, which may also be more likely to carry parasites.
Tongueworm infections do influence the calls males use to attract mates, but not in the simple way scientists expected. Rather than just weakening signals, the parasites altered several call traits, creating a complex pattern that can change how females evaluate potential partners.
“Parasites don’t always tell a simple story about health or weakness,” said Sarah R. Goodnight, Ph.D., first author, a Ph.D. graduate of 鶹ƷƵHarbor Branch, and a postdoctoral fellow at the Smithsonian Environmental Research Center. “In this system, the frogs most successful at finding food may also be the ones most likely to pick up parasites. That means females are evaluating signals that can simultaneously advertise both strength and risk.”
The findings challenge the long-standing Hamilton–Zuk hypothesis, which predicts that parasites reduce the quality of sexual signals and that females should prefer less-infected males. Instead, the pattern was more complex.
Larger male frogs – typically favored by females – also carried more tongueworm parasites, likely because males that eat more prey accumulate infections over time. Parasites subtly reshaped male calls: heavily infected frogs produced lower-frequency calls, a trait females usually prefer, but their calls were shorter, which can signal lower stamina.
Playback experiments revealed a similar pattern. Females avoided the most heavily infected males but often preferred males with moderate infections over uninfected ones, suggesting they weigh multiple signals at once – balancing traits linked to size and attractiveness against the risk of parasite infection.
Call duration appeared to play a particularly important role in this decision-making. Longer calls generally came from males with fewer parasites and greater energetic reserves, signaling vigor and lower infection risk. However, the relationship wasn’t entirely straightforward: some moderately infected males produced longer calls than uninfected males, possibly because successful foragers accumulated both energy reserves and parasites.
“Mate choice is rarely based on a single trait,” said Michael W. McCoy, Ph.D., co-author, associate director, 鶹ƷƵSchool of Environmental, Coastal, and Ocean Sustainability, and professor of quantitative ecology, , 鶹ƷƵCharles E. Schmidt College of Science and 鶹ƷƵHarbor Branch Oceanographic Institute. “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 study reveals that parasites influence mate selection by altering multiple traits in male calls, creating a complex signal environment. Rather than just diminishing attractiveness, infections introduce nuanced cues that females must interpret, revealing how parasites subtly guide mating decisions and shape sexual selection in wild populations.
Study co-author is Ellen F. Titus with The Nature Conservancy.
The research was supported by the International Herpetological Symposium awarded to Goodnight, The Sigma Xi Scientific Research Honor Society awarded to Goodnight, and the National Science Foundation awarded to McCoy.
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