Parasites Complicate Mate Choice for Florida's Green Treefrogs

FAU researchers have delved into the intriguing influence of food-web-transmitted oral frog tongueworm parasites (Halipegus occidualis) on the mating calls and female mate choice of green treefrogs (Dryophytes cinereus) in a natural setting. The study sought to uncover how these parasites might subtly alter the acoustic advertisements males use to attract mates.

Research Methodology

During the breeding season, male green treefrogs emit calls to attract females. Females typically favor calls that are lower-frequency, faster, and longer, which often indicate a larger or healthier male.

Researchers meticulously recorded calls from wild male green treefrogs and concurrently counted the number of tongueworm parasites present in each frog's mouth. Advanced audio software was employed to analyze key call features such as frequency, length, and pulse structure.

Calls were then categorized based on the male's infection level:

• Uninfected
• Moderately infected (five to eight adult worms)
• Heavily infected (more than nine adult worms)

To gauge female response, two-choice playback experiments were conducted, carefully observing which call pairs females approached, providing insight into their preferences.

Key Findings

The study, published in the journal Current Zoology, revealed that female green treefrogs may face complex decisions when choosing mates. Preferred call traits, such as lower frequencies, are often produced by larger males, which were also observed to carry a higher parasite load.

Tongueworm infections were found to affect male mating calls in nuanced ways. Rather than simply weakening signals, parasites altered several call characteristics. Heavily infected frogs produced lower-frequency calls—a trait generally preferred by females—but their calls were also shorter, which can signal lower stamina.

Playback experiments demonstrated that females avoided the most heavily infected males. However, surprisingly, they often preferred males with moderate infections over uninfected ones.

This suggests that females evaluate multiple signals simultaneously, weighing desirable traits like size against the potential risk of parasite infection.

Call duration appeared to be a significant factor in female decision-making, with longer calls generally originating from males with fewer parasites and greater energetic reserves.

Challenging Hypotheses

The findings challenge the long-standing Hamilton–Zuk hypothesis, which posits that parasites reduce the quality of sexual signals and that females should therefore prefer less-infected males.

This research indicates a more intricate relationship where parasites introduce complex cues that females must interpret. The study concludes that understanding this complexity is essential for explaining the mechanisms of sexual selection in natural populations.