Female Frogs Prefer Deep-Voiced Males - Who Also Carry More Parasites

Florida Atlantic University

On warm evenings during breeding season, male green treefrogs gather around ponds in loud choruses, inflating their vocal sacs to produce repeated honking calls from nearby vegetation. Females listen carefully. They tend to favor lower-frequency, faster, and sometimes longer calls - acoustic traits that typically signal a larger, healthier male. Then they approach their chosen mate.

But there is a complication. The largest males - the ones producing those attractive low-frequency calls - also tend to carry more parasites. A study published in Current Zoology by researchers at Florida Atlantic University reveals that the relationship between parasites, mating calls, and female choice in green treefrogs (Dryophytes cinereus) is far more complex than previous theories predicted.

Worms in the throat, changes in the call

The parasites in question are oral frog tongueworms (Halipegus occidualis), flatworms that take up residence in a frog's mouth and throat. Frogs acquire them through their diet - the parasites are transmitted through the food web, meaning frogs that eat more prey accumulate more worms over time.

This creates a paradox. The very behaviors that make a male attractive - being large, being a successful forager - also make him more likely to be parasitized. Females evaluating male calls are, in effect, reading a signal that carries information about both desirable traits and infection risk simultaneously.

The researchers recorded calls from wild male green treefrogs and counted the number of tongueworm parasites in each frog's mouth. They analyzed the recordings using audio software to measure call features including frequency, length, and pulse structure. The frogs were grouped into three infection categories: uninfected, moderately infected (five to eight adult worms), and heavily infected (more than nine adult worms).

Not weaker signals, but different ones

The straightforward prediction - based on the decades-old Hamilton-Zuk hypothesis - would be that parasites degrade male sexual signals. Infected males should produce worse calls, and females should prefer uninfected males. The data told a different story.

Heavily infected frogs produced lower-frequency calls, a trait females usually prefer. But their calls were shorter, which can signal lower stamina. The parasites were not simply weakening the signal - they were reshaping multiple call traits in different directions, creating a mixed message.

Females balance attraction against risk

To test how females actually responded, the researchers conducted two-choice playback experiments, broadcasting pairs of recorded male calls and observing which one the females approached.

The results were nuanced. Females avoided the most heavily infected males but often preferred males with moderate infections over uninfected ones. This suggests females are not simply selecting for the absence of parasites. Instead, they appear to weigh multiple signals at once - balancing traits linked to size and attractiveness against cues that hint at 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 energy reserves, signaling vigor and lower infection risk. But the relationship was not entirely straightforward: some moderately infected males produced longer calls than uninfected males, possibly because successful foragers accumulated both energy reserves and parasites.

Challenging a classic hypothesis

First author Sarah R. Goodnight noted that parasites do not always tell a simple story about health or weakness. 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.

Co-author Michael W. McCoy emphasized that mate choice is rarely based on a single trait, and that parasites can reshape the information animals use when choosing partners by subtly changing multiple aspects of a male's call. Understanding that complexity is critical for explaining how sexual selection actually works in natural populations.

One species, one parasite, natural conditions

The study examined one frog species and one parasite species under natural field conditions. Whether similar patterns occur in other host-parasite systems is an open question. The Hamilton-Zuk hypothesis has been tested across many taxa with mixed results, and this study adds another data point suggesting the reality is more complex than the original theory allows.

The playback experiments, while well-controlled, use recorded calls that strip away other sensory information females might use in natural mate choice - visual cues, chemical signals, spatial position in the chorus. The isolated acoustic comparisons test one dimension of female preference but may not capture the full decision-making process.

Sample sizes for the infection categories are not specified in the available information, and the categorization thresholds (five to eight worms as moderate, nine or more as heavy) are somewhat arbitrary. Different thresholds might produce different patterns.

The correlation between male size and parasite load is suggestive but does not establish that foraging behavior specifically causes higher infection rates. Other factors correlated with body size - age, territory quality, immune investment - could also contribute to the pattern.

The croak conundrum, unresolved

The study leaves female treefrogs with a genuine dilemma. The call traits they are attracted to - low frequency, signals of large body size - come bundled with a higher probability of parasitic infection. The females appear to navigate this trade-off by integrating multiple call features rather than relying on any single trait, preferring moderately infected males who combine some attractive qualities with evidence of adequate energy reserves.

It is, as the researchers describe it, a croak conundrum - and one that likely plays out in analogous ways wherever parasites manipulate the signals that drive mate choice across the animal kingdom.