Rhinoceros beetles' horns not costly

13 March 2013 Last updated at 07:50 By Jeremy Coles Reporter, BBC Nature

Giant rhinoceros beetles' large and elaborate horns are not costly during flight, a US study has found.

Researchers at the University of Montana investigated the costs of horn size and shape to the beetles during flight.

They expected that larger, more elaborate horns would be more costly to produce and carry around.

To the team's surprise, the beetles' horns were lightweight and had little effect on body drag.

The results were published in the journal of the Proceedings of the Royal Society B: Biological Sciences.

Rhinoceros beetle horns are some of the biggest and most elaborate weapons found in the animal kingdom.

The horns of male giant rhinoceros beetles (Trypoxylus dichotomus) can reach two-thirds of their body size and are used in fights with other males for the rights to feeding sites on trees.

Males with larger horns outcompete smaller horned males and have greater reproductive success because they have access to females that come to feed on the sap.

This competition for mates drives the evolution of increasingly larger, more elaborate horns - a process known as sexual selection.

However, PhD student and lead researcher, Erin McCullough explained: "Almost all of the sexual selection literature predicts sexually selected traits (ornaments and weapons) to be costly."

Miss McCullough and the team of researchers wanted to find out the costs involved for giant rhinoceros beetles.

"Given that males have a giant pitchfork of a horn sticking out of the head, I expected that flying would be cumbersome and awkward," Miss McCullough told BBC Nature.

"Quantifying flight costs seemed like a good place to start."

The team predicted that the impressive horns would be costly during flight, the beetles' primary mode of locomotion by increasing body weight, shifting the centre of mass and increasing body drag.

But what the team found surprised them.

The horns were actually lightweight, contributing little to the males' overall body weight and distribution of mass.

And because rhinoceros beetles usually fly with a high body angle at slow speeds, they found the horns had little effect on total body drag.

Both the weight and drag of a horn, even in the largest males studied, only increased the force required for flight by less than 3%.

"I was not expecting the horns to have such a small effect on flight," said Miss McCullough.

"Horns [also] don't appear costly in terms of resource allocation trade-offs, immune costs or survival.

"So if there are costs associated with carrying and/or producing horns, it definitely isn't obvious what these costs might be," she said.

Because horns appear to be relatively "low-cost" structures, they are less constrained by natural selection, and therefore can become very large and take on lots of diverse shapes without negative consequences on fitness, explained Miss McCullough.

"It calls into question whether costs are as important as we once thought."

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Source: http://www.bbc.co.uk/nature/21747494

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