Growing up to two-thirds of their body length, the horn of the male rhinoceros beetle (Trypoxylus dichotomus) looks like a heavy burden to bear. The large and elaborate horn is a sexually selected trait used in male-male competition to gain control of the best feeding territories and access to females.
But according to recent research the cost of possessing a large horn is less than first predicted. Experimental evidence has found that males with larger horns do not suffer from reduced locomotor abilities, impaired immune systems or reduced resource allocation for other body functions.
The males' horn is surprisingly lightweight; causing little difference to total body mass and drag when flying. In all males, both the weight and drag of the horn increased the force required to fly by less than 3 %. Males were also able to fly as fast as females thanks to their extreme angle of flight (Flight video).
One suggestion for the lack of locomotor costs is that horns were costly in the evolutionary past of the species but compensatory traits have since evolved to reduce these costs. Some evidence has indicated that males with long horns for their body size had larger wings than males with relatively short horns. Relative to body size, all males had larger wings and flight muscles than females.
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| Rhinoceros beetle in flight. The sharp angle helps reduce the aerodynamic costs to locomotion. Image credit: Tetsuya Shimizu |
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| The rhinoceros beetles horn can almost more than half the length of its body Image credit: Wikipedia (some rights reserved) |
The males' horn is surprisingly lightweight; causing little difference to total body mass and drag when flying. In all males, both the weight and drag of the horn increased the force required to fly by less than 3 %. Males were also able to fly as fast as females thanks to their extreme angle of flight (Flight video).
One suggestion for the lack of locomotor costs is that horns were costly in the evolutionary past of the species but compensatory traits have since evolved to reduce these costs. Some evidence has indicated that males with long horns for their body size had larger wings than males with relatively short horns. Relative to body size, all males had larger wings and flight muscles than females.
Find Out More:
Rhinoceros beetles' horns are not costly during flight - BBC Nature summary
Sources:
- MCCULLOUGH, E. L., WEINGARDEN, P. R. & EMLEN, D. J. 2012. Costs of elaborate weapons in a rhinoceros beetle: how difficult is it to fly with a big horn? Behavioral Ecology, 23, 1042-1048.
- MCCULLOUGH, E. L. & TOBALSKE, B. W. 2013. Elaborate horns in a giant rhinoceros beetle incur negligible aerodynamic costs. Proc. Royal Soc. B, 280, 1-5
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