カブトムシ雌の闘争行動と同性間マウンティング

井口豊（生物科学研究所）

Research Report of Laboratory of Biology
March 6, 2024

Female-female fighting and mounting in the horned beetle Trypoxylus dichotomus septentrionalis

Yutaka Iguchi
Laboratory of Biology

Abstract

Females of the Japanese horned beetle, Trypoxylus dichotomus septentrionalis, have only small horns, but they often fight over access to feeding sites by head-butting each other (Iguchi, 2007, also see Fig. 1 and the video). However, they also show intrasexual mounting (so-called homosexual behavior or same-sex behavior). In this case, the mounting female extends her ovipositor towards the mounted female's genital organ as if she copulated (Fig. 2).

Female-female mounting in this species was first reported by Iguchi (1996), and then by Tanaka (1999) and Abe et al. (2006).

My recent paper examined the relationship between the body size of females and the outcome of their fights and mountings (Iguchi, 2010). The results showed that female body size had different effects on the outcome of their fights and mountings. It seemed natural that large females more often won fights to gain access to food than small females (Fig. 3). However, it was surprising that small females more often mounted large females (Fig. 4).

Small females mounted large females significantly more often after losing than after winning fights (Fig. 5). It was interesting that small females that mounted large females after losing a fight were as successful at gaining access to food as those that won fights (food acquisition after mounting, 4/19 = 21%; after winning, 10/38 = 26%; Fisher's exact test, p = 0.75).

These results suggest that female mounting may function as a tactic for reducing intrasexual fighting between small and large females.

References

Abe H., Fujii T., Suzuki A., Sato T., Iwabuchi K. & Obara Y. (2006) Elicitation of male mating behavior in females in insects: The cases of Japanese horned beetle, a long-horned beetle, a silk worm and other. Japanese Journal of Sexuology, 24: 7-16 [in Japanese with English abstract].

Iguchi Y. (1996) Sexual behavior of the horned beetle, Allomyrina dichotoma septentrionalis (Coleoptera, Scarabaeidae). Japanese Journal of Entomology, 64: 870-875.

Iguchi (2007) Female-female fighting behavior of the Japanese horned beetle. Movie Archives of Animal Behavior, Data No. momo071108td01b

Iguchi Y. (2010) Intrasexual fighting and mounting by females of the horned beetle Trypoxylus dichotomus (Coleoptera: Scarabaeidae). European Journal of Entomology, 107: 61-64.

Tanaka Y. 1999: Pseudocopulation behavior in female Allomyrina dichotoma septentrionalis. Gekkan-mushi, 346: 37-38 [in Japanese].

Fig. 1. Female-female fighting over food in Trypoxylus dichotomus septentrionalis (after Iguchi, 2007). You can see the original video on Movie Archives of Animal Behavior.

Female-female homosexual mouniting in beetles

Fig. 2. Intrasexual mounting by females of Trypoxylus dichotomus septentrionalis. This is also called homosexual behavior or same-sex behavior. The mounting female is extending her ovipositor towards the mounted female's genital organ as if she were copulating (after Iguchi, 2010).

Fig. 3. The effect of female body size on the outcome of intrasexual fighting (after Iguchi, 2010).

Smaller females show homosexual behavior.

Fig. 4. The effect of female body size on the outcome of intrasexual mounting (after Iguchi, 2010).

Small females mount large females after losing fights.

Fig. 5 The effect of female intrasexual fighting on intrasexual mounting (after Iguchi, 2010).

カブトムシ雌の同性間闘争に対するマウンティングの役割

日本のカブトムシ Trypoxylus dichotomus septentrionalis では，エサを得るために，雄だけでなく雌もまた同性間で闘争をする（Fig. 1)，井口，2007も参照）。しかしながら，そのとき雌は，同性間でマウンティング（いわゆる同性愛行動）も示す（Fig. 2）。このとき，マウンティングする雌は，まるで交尾するかのごとく，マウンティングされる雌に産卵管を伸ばす。

私は，このようなカブトムシ雌の同性間マウンティングを初めて報告し（Iguchi, 1996），その後，田中（1999），阿部ら（2006）がこの行動を報告した。

最近，私は，カブトムシ雌のサイズが，同性間闘争とマウンティングに与える影響について調べた（Iguchi, 2010）。その結果，サイズが両者に全く異なる影響を与えることが分かった。

大きな雌が小さな雌との闘争に勝ち，エサを得るのは当然の結果と言えよう（Fig. 3）。しかし，小さな雌ほど，大きな雌にマウントする傾向があるのは興味深い（Fig. 4）。

さらに，小さな雌は，大きな雌との闘争に勝った後よりも，負けた後にマウンティングを示すことが分かった（Fig. 5）。闘争に負けた後にマウンティングを示した小さな雌は，闘争に勝った小さな雌と同じくらいの割合でエサを得るのに成功した（マウンティング後のエサの獲得, 4/19 = 21%; 闘争に勝った後のエサの獲得, 10/38 = 26%; Fisher's exact test, p = 0.75）。

これらの結果は，カブトムシ雌の同性間マウンティングが，小さな雌と大きな雌の間の闘争を軽減する役割を果たしているかもしれないことを示唆する。

カブトムシの場合，雄は，雌のサイズにかかわらず，配偶行動を示すことが示唆されている（井口，2001）。小さな雌が頻繁に同性愛行動を示すのは，それが餌場に残り，雄と交尾をする上での利点となるかもしれない。