Evol Ecol Res 8: 1129-1137 (2006)     Full PDF if your library subscribes.

Are beetle horns costly to produce?

Yutaka Iguchi*

Laboratory of Biology, Yamashita-cho 1-10-6, Okaya City, Nagano 395-0005, Japan

e-mail: bio-igu@f8.dion.ne.jp


Question: Do male beetles pay costs of producing horns before becoming adults?

Hypotheses: (1) Smaller males emerge earlier in the season. (2) Smaller males pupate earlier in the season. (3) Males require longer pupal duration than females of the same size. (4) Horn size affects pupal duration independently of body size. (5) Males require more final-instar mass to become adults than females of the same size. (6) Horn size depends on final-instar mass independently of body size.

Organism: The Japanese horned beetle Trypoxylus dichotomus septentrionalis (Kono) (Coleoptera: Scarabaeidae). Males produce horns, but females do not.

Methods: I reared larval beetles under nearly natural conditions until they became adults. I recorded final-instar mass, pupation date, pupal duration, adult emergence date, adult body size, and horn size.

Results: (1) Larger males emerged earlier in the season. (2) Larger males pupated earlier in the season. (3) Males required only the same pupal duration as females of the same size. (4) Horn size did not affect pupal duration independently of body size. (5) Males required less final-instar mass than females of the same size. (6) Horn size increased with increasing final-instar mass independently of body size.

Conclusions: The results provide support for the sixth hypothesis, but not the other hypotheses. Beetle horns are not so costly to produce as entomologists expect them to be.

Keywords: adult emergence, body size, horned beetle, horn cost, horn size, larval mass, pupal duration, pupation, Trypoxylus dichotomus septentrionalis.

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