Study shows rhinoceros beetle horns evolved to accommodate species-specific fighting styles

September 9, 2014 by Marcia Malory report

(Phys.org) —Male rhinoceros beetles have elaborate horns, which they use when fighting for mates. The shape and number of horns differ from species to species. Erin McCullough of the University of Montana at Missoula and her colleagues have discovered that horns evolved to have shapes that best suit each species' fighting style. The research appears in the Proceedings of the National Academy of Sciences.

Considered the strongest animals in the world, male rhinoceros beetles compete for females by attempting to remove their opponents from trees and shoots and throw them onto the ground. They use their as weapons. Horns come in a wide variety of shapes.

McCullough and her team wanted to find out why so many different horn types had evolved. They did not find any evidence that females chose mates based on horn size or shape, so they hypothesized that horn shapes evolved to improve ability. Previous studies on the horns and antlers of ungulates supported this hypothesis. Among ungulates, males with long horns tend to fence or wrestle, males with curved horns tend to ram and males with short, smooth horns tend to stab.

To see if rhinoceros beetles also had horns suited to their particular fighting styles, the team used a type of computer modeling system, known as finite element analysis, to calculate the stresses and strains the horns would experience when used for different methods of fighting. They studied three : Trypoxylus dichotomus, Golofa porteri, and Dynastes hercules.

All of these species have differently shaped horns, which they use in different ways. Trypoxylus has pitchforked-shaped horns that it uses to pry and twist opponents off tree trunks and branches. Golofa has long, slender horns that it uses like fencing swords, lifting opponents and throwing them off balance when fighting on narrow shoots. The head and thoracic horns of Dynastes form pincers, which the beetle uses to lift and squeeze opponents, then toss them to the ground. Models showed that a species' horns experienced the least stress and strain under conditions that mimicked species-typical fights.

Rotating view of the 3D horn models used in the finite element analyses. Credit: Erin L. McCullough, PNAS, doi: 10.1073/pnas.1409585111

The team predicted the cross-sectional shapes of the horns of each species, considering which shapes would maximize the ability to dislodge opponents, using that species' style of fighting. They thought cross sections of Trypoxylus horns would be triangular, those of Golofa would be circular and those of Dynastes would be elliptical. Microcomputed tomography scans proved their predictions correct. The team suggests researchers perform further studies on the effect of horn shape on other measures of fighting performance, such as grip and stability.

Animations of the Von Mises stress distributions from finite element models of a Trypoxylus dichotomus horn loaded under vertical bending, lateral bending, and twisting. Credit: Erin L. McCullough, PNAS, doi: 10.1073/pnas.1409585111


Explore further: Researchers unlock genetic twist in differences in horn size with sheep

More information: Structural adaptations to diverse fighting styles in sexually selected weapons, Erin L. McCullough, PNAS, DOI: 10.1073/pnas.1409585111

Abstract
The shapes of sexually selected weapons differ widely among species, but the drivers of this diversity remain poorly understood. Existing explanations suggest weapon shapes reflect structural adaptations to different fighting styles, yet explicit tests of this hypothesis are lacking. We constructed finite element models of the horns of different rhinoceros beetle species to test whether functional specializations for increased performance under species-specific fighting styles could have contributed to the diversification of weapon form. We find that horns are both stronger and stiffer in response to species-typical fighting loads and that they perform more poorly under atypical fighting loads, which suggests weapons are structurally adapted to meet the functional demands of fighting. Our research establishes a critical link between weapon form and function, revealing one way male–male competition can drive the diversification of animal weapons.

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verkle
Sep 09, 2014
This comment has been removed by a moderator.
tadchem
1 / 5 (6) Sep 09, 2014
There is an old saying: "When the only tool you have is a hammer, everything begins to look like a nail."
For a rhinocerous beetle, the 'fighting style' would be learned from practice with the tool it was born with. I contend that the fighting style is more likely to have 'evolved' to maximize the effectiveness of the tool than vice versa.
Vietvet
4.2 / 5 (10) Sep 09, 2014
Crazy logic. Study shows rhinoceros beetle horns evolved? No. It only shows that rhinoceros species have various horns.


There you go again, denying evolution because of your religious beliefs.

Are you merely dishonest or is your faith that weak?
Plantastic
1.8 / 5 (5) Sep 09, 2014
Amazing how scientist can figure this all out.
Still they haven't figured out exactly how or why the pet TickleMe Plant moves and closes its leaves when you Tickle it!
http://www.ticklemeplant.com

OZGuy
4.6 / 5 (9) Sep 09, 2014
@verkle
You're still posting drivel.regarding subjects you obviously have zero knowledge of and ZERO interest in learning.
You really should pick a new hobby as you suck at this one.
mooster75
2 / 5 (8) Sep 10, 2014
I'm sorry, but the way this is worded makes me side with verkle this time. The way this article is written is just pure BS. The phrase "horns have evolved to..." do anything is the height of ignorance. Do the horns have little brains inside to come up with this brilliant scheme? Nothing evolves "to do" ANYTHING!! If a new horn works better for the fighting style, then the beetle has an advantage. Nothing is planned, there is no grand scheme. It just happens. To say that the horn evolved to work better is simple a clumsy version of Intelligent Design.

I know that this sounds like some out of control pet peeve rant, but I'm firmly convinced that a lot of the people out there who "don't believe" in evolution make that decision as a result of this clumsy misrepresentation. When you say things like this, evolution just sounds stupid.
yep
1 / 5 (2) Sep 10, 2014
Oh, your fighting style is weak, my Golofa technique is much more powerful.
verkle
Sep 10, 2014
This comment has been removed by a moderator.
verkle
Sep 10, 2014
This comment has been removed by a moderator.
rockwolf1000
5 / 5 (6) Sep 10, 2014
Hi OZGuy, Thanks for just attacking my character, because you probably have no capability to address my comments in a scientific and honorable way.


Why would anyone waste their time doing that?

Hi Vietvet,
Yes, because I don't see science backing up the myriad claims of this debunked theory. It is a nonsense theory.


Show how it has been debunked.

Show that you know what the word "debunked" really means.

Its readily apparent you have no idea what "debunked" means. If the theory of evolution had ever been debunked, which it hasn't, I'm sure it would make headline news around the world.

Links?
Mike_Massen
5 / 5 (6) Sep 10, 2014
verkle didnt know what he said with
..It only shows that rhinoceros species have various horns.
Thanks, you said it - variation, one of the keys of evolution. Its obvious environments are not static both on local & global scales all is in motion all the time its just not obvious to the uneducated as those people crave determinism - really naive !

Why should DNA be static and 'never' change - in your thoughts of the universe ?

verkle earlier muttered
Crazy logic..
Don't you see your god has this clear attribute, he/she/it punished ALL of creation because a young woman was manipulated by a powerful angel which your god MUST have been able to foresee, otherwise NOT a god = setup !

Don't you see the obvious crazy flawed logic of the bulk of Moses attempt to explain why humans suffer just the SAME as ALL other life ?

Why are all gods made by mne & oh so nasty they also have problems with women ?

And you claim its crazy logic to appreciate the world is not static !

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