Yamaguchi University research reveals female rhinoceros beetles mate only once in their lifetime

Even familiar creatures can harbor surprising behaviors. A research group from Yamaguchi University, led by Associate Professor Wataru Kojima from the Faculty of Science, Undergraduate Student Asuka Maruo (fourth year at the time of the research), and Master's Student Souta Yamate (second-year) from the Graduate School of Sciences and Technology for Innovation, has conducted a joint study with the University of Montana of the United States and revealed that female Japanese rhinoceros beetles (Trypoxylus dichotomus) mate only once in their lifetime. Their findings were published in Behavioral Ecology and Sociobiology.

The rhinoceros beetle is one of Japan's most familiar insects. Yet its ecology and physiological processes following mating had remained largely unexplored.

The research group conducted experiments using 85 virgin females, and all of them accepted mating with the first male they were paired with. However, when introduced to a different male 1 to 28 days later, the females aggressively kicked the males away and refused to mate. Given the typical lifespan of wild rhinoceros beetles, this prolonged rejection period of 28 days means that females mate only once in their lifetime—a phenomenon known as monandry. Multiple matings are the norm among beetles in the family Scarabaeidae, making this extreme monandry a remarkably rare occurrence.

What mechanism causes females to refuse remating? In some insect species, males transfer a capsule called a "spermatophore," which contains sperm and nutritional substances such as proteins, into the female's reproductive tract during copulation.

To determine whether the cause was chemical substances in the spermatophore or the physical stimulation of copulation, the group conducted two experiments. The first involved injecting aqueous extracts of spermatophore directly into the female hemocoel. While this method suppresses remating in some other insects, females of the rhinoceros beetle still accepted mating after the injection. The second experiment involved artificially interrupting copulation: males were forcibly separated from females 15 minutes after genital insertion, and these females went on to mate with a different male the following day. These results show that the mere transfer of chemical substances into the bloodstream, or physical stimulation of the genitalia alone, is insufficient to suppress remating in females.

The researchers then turned their attention to the size of the spermatophore transferred by males. Virgin males produce large spermatophores, but males that had just mated with another female showed a 60% reduction in spermatophore size. When females received these smaller spermatophores, their remating rate rose slightly from nearly 0% to 15%. Yet remarkably, females that received a small spermatophore showed no difference from those that received a large one in terms of egg-laying count, egg hatch rate, or even lifespan.

In other words, a single mating—even with a spermatophore reduced by 60%—provides females with more than enough sperm for lifetime reproduction. The reason virgin males nonetheless produce large spermatophores is not to deliver more sperm or nutrition to the female, but rather to physically fill the female's reproductive tract with an oversized capsule, thereby preventing remating with other males entirely and ensuring that only their own offspring are produced.

Journal Information
Publication: Behavioral Ecology and Sociobiology
Title: Monandry and its underlying mechanisms in the rhinoceros beetle Trypoxylus dichotomus
DOI: 10.1007/s00265-026-03708-6