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Beetle Battles: The Secret World of Leg Wrestling and Abdomen Squeezing

The views expressed are those of the author and are not necessarily those of Scientific American.


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frog-legged-leaf-beetle-sagra

Credit: iStockphoto

This is the frog-legged leaf beetle (Sagra buqueti), and there’s a good chance those gigantic gams are his weapons.

Found in the jungles of Southeast Asia, this brightly coloured, iridescent species can grow up to 5 cm long. Unlike its namesake, it doesn’t use its hind legs for jumping, instead they’re used to cling onto stems and foliage while it eats, its grip aided by scores of tiny hair follicles that cover the surface of the leg. But there could be more to those legs than just grip, because look at the difference between the males and females:

If both the males and females move around the trees in much the same way to search for food, there must be a reason why the males ended up with such monstrous hind legs, which has led researchers to suspect that they could be a sexually selected trait that evolved as the result of male-on-male contests over females. We’re yet to see a leg battle between two frog-legged leaf beetles, but this behaviour has been observed in several beetles belonging to a different family – Coreidae – which features similarly oversized hind legs.

Sagra.buqueti

Mounted Sagra buqueti Credit: Sarefo; distributed under a CC-BY 2.0 license.

Researchers have suggested that the modified legs of the coreid bugs, otherwise known as leaf-footed bugs, play an important role in interspecific interactions, such as males fighting over a females or females fighting over a good meal. Typically, the top part of a coreid bug’s hind leg, called the tibia, is enlarged, flattened, and often covered in spines. Sometimes the lower part of the hind leg, called the femora, is enlarged too.

In 1993, Takahisa Miyatake from the Entomological Laboratory of the University of the Ryukyus in Japan caught a bunch of passionvine bugs (Leptoglossus australis) on Okinawa Island and observed their behaviour in a lab setting. Because he saw the bugs mating in the wild on sponge gourds (Luffa cylindrical), Miyatake strung some gourds up in his lab to see if this would elicit any competitive behaviour between the males.

Leptoglossus australis

The passionvine bug (Leptoglossus australis) and its enlarged hind legs. Credit: Changhua Coast Conservation Action (flickr.com/photos/waders)

Reporting in the Journal of Ethology, Miyatake described what is basically beetle leg wrestling. It beings with male #1 noticing male #2′s approach, causing him raise one of his huge, hind legs into the air, and waggle it at male #2. If male #2 ignores this threat and continues his approach, male #1 will initiate a “one side attack”, as Miyatake called it, which could go one of two ways – either he will try to hook and pull male #2′s thorax (the upper-middle part of the body) with his forelegs to drag him away, or he will repeatedly beat him with one of his hind legs.

If male #2 still hasn’t had enough, male #1 will spread his hind legs, and then turn around so his butt (or the rear-end of his abdomen) is facing male #2. Male #2 will do the same, and an intense fencing match, or the leg version of a thumb war, ensues, during which both males will attempt to wrap their spiny hind legs around the abdomen of their opponent. Sometimes things get weird when one of the males will try to further assert their dominance by mounting his opponent, but not super-weird, because this mounting is the non-sexual kind. The losing male will wait for the winner to finish mounting him, and then retreat when released.

Out of the 49 encounters Miyatake observed between passionvine bug males, almost half of them were resolved at the “one side attack” stage. A third of the leg-waggling threats were taken seriously by the rivals, causing them to retreat. Larger males won the majority of the battles.

leaf-footed beetle-battles

A) The males back toward the other with spread hind legs, possibly to align their abdomens. (B) Both males raise their abdomens and make contact with their hind legs. (C) The males grasp and squeeze the other's abdomen with their hind legs. Credit: Takahisa Miyatake 1997

In 1998, William Eberhard from the Smithsonian Tropical Research Institute and the Universidad de Costa Rica observed the sexual behaviour of another coreid bug, the giant leaf-footed bug (Acanthocephala declivis), in the wild in Mexico. Eberhard reported in Annals of the Entomological Society of America that the females used their hind legs to push the males away if they did not want to mate, but if the female was receptive, the two of them would mate, very quietly, and sometimes one or both of them would feed from the tree at the same time. One couple mated for more than 90 minutes.

Eberhard also found that the female giant leaf-footed bugs were usually quite aggressive towards each other in a good feeding area, and would use their hind legs to wave threats and beat each other, just like the passionvine bugs. Sometimes a female would turn around and wave her hind leg and her butt to really drive home the message. The opposing female would either raise her own leg in response as she retreated, or would turn around and engage in a butt-to-butt leg beating battle. In one instance reported by Eberhard, a female gave such a beating that her opponent fell clean off the tree.

Acanthocephala declivis

The giant leaf-footed bug (Acanthocephala declivis). Credit: Jason Sharp (flickr.com/photos/78235221@N05)

Because the females tended to keep some distance between their butts as they battled, the most they could do was beat or lightly grasp at each other with their hind legs. The males, on the other hand, get right up in there, walking backwards towards each other with their hind legs outstretched and held relatively low, not stopping till the tips of their butts are touching. They then engage in a heated leg battle, and the objective here is to grab hold of your opponent’s abdomen and give it a few quick and powerful squeezes with your hind legs. Sometimes this battle would cause both males to fall off the tree mid-squeeze, prompting them to separate in mid-air, and one or both would fly back to their original spot on the trunk. Eberhard noticed a few examples of wear and tear on the males’ legs as a result of battles – some walked with a limp while others were completely missing a leg.

While access to a female appears to be the primary reason for these battles, Eberhard noticed that the females will often be completely forgotten by the males in the heat of the battle. So even if a male wins, instead of heading straight to the awaiting female, he will pursue the loser up to a metre away before returning to his original spot to scout for more rivals. This suggests that it’s more about defending one’s bachelor territory, says Eberhard, who was quick to point out how ineffective this beahviour got when there were multiple leaf-footed bugs in the same area:

“The most dramatic illustration of ineffective male aggression occurred when one large male spent much of four hours threatening and attacking other males on a two-metre segment of the trunk. Although this male won all aggressive interactions, he failed to obtain a single copulation, even though 10 copulations involving other males occurred on this portion of the trunk during this time.”

DUDE. COME ON.

Ebherhard suggests that because the males don’t use their large hind legs for courting females, they probably represent adaptations for delivering strong, punishing squeezes to other males. So could the beautiful frog-legged leaf beetle be doing the same? Just look at those things!

frog-legged-leaf-beetle-legs

Frog-legged leaf beetle (Sagra buqueti). Credit: Smithsonian Institution NMNH Insect Zoo (flickr.com/photos/88896512@N03)

Papers cited:

Emlen, Douglas J. (2008) The Evolution of Animal Weapons. , 39(1), 387-413

Eberhard, William (1998). Sexual Behavior of Acanthocephala declivis guatemalana (Hemiptera: Coreidae) and the Allometric Scaling of their Modified Hind Legs. Annals of the Entomological Society of America, 91 (6), 863-871

Miyatake, Takahisa. (1997) Functional morphology of the hind legs as weapons for male contests in Leptoglossus australis (Heteroptera: Coreidae), Journal of Insect Behavior, 10(5), 727-735

Takahisa Miyatake (1993) Male-male aggressive behavior is changed by body size difference in the leaf-footed plant bug, Leptoglossus australis, Fabricius (Heteroptera: Coreidae) Journal of Ethology, 11 (1), 63-65

Related posts:

Dancing woolly aphids will probably stab you
The leggiest animal on Earth gets another look
Glad you ditched the anal fork, Golden Tortoise Beetle

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Bec Crew About the Author: Bec Crew is a Sydney-based science writer and award-winning blogger. She is the author of 'Zombie Tits, Astronaut Fish and Other Weird Animals' (NewSouth Press). Follow on Twitter @BecCrew.

The views expressed are those of the author and are not necessarily those of Scientific American.





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