Do the horniest beetles have the smallest testes?

[Wise Young]

A very interesting news article piqued me (by the way, the verb pique is often mispelled as picque and means "to cause resentment or indignation"). Entitled "Horny Beetles have Tiny Testes", the article is about a study published recently in the Proceedings of the National Academy of Science (PNAS). PNAS is one of the most prestigious science journals in the world.

http://www.abc.net.au/science/news/s...06/1766434.htm

Horny beetles have tiny testes
Judy Skatssoon
ABC Science Online

Tuesday, 17 October 2006

Horned beetle


Horned beetles can have big weapons or big testes, not both (Image: O Helm & DJ Emlen)
Beetles with the biggest horns have the smallest testes, say scientists who show that in evolutionary terms you can't have it all.

They say their finding is clear evidence of an evolutionary trade-off between the ability to fight off sexual competitors and reproductive potency.

Or put simply, the ability to find a mate and the ability to fertilise her.

Professor Leigh Simmons of the University of Western Australia and US researcher Professor Douglas Emlen of the University of Montana publish their findings online today in the Proceedings of the National Academy of Sciences.

<snip>

Simmons tested the theory by cauterising the area of the developing beetle pupa, where the horns would grow, destroying cells and preventing horn growth.

He found that if horn development was inhibited, the pupa compensated by developing larger testes.

The principle is known as a resource allocation trade-off, he says.

<more>

Simmons says the principle of resource allocation has been demonstrated in other animal studies.

For example, there is some evidence that bats trade the size of their testes for brain power.

And a recent study showed that stalk-eyed flies, in which eye span width is a measure of sexual desirability, trade testes size for the width of their eyes.

"There have been suggestions of a trade-off between gaining mates and gaining fertilisation but our study is probably one of the most conclusive studies to date to have demonstrated this," Simmons says.

So, why am I piqued about this article? It mixes evolution and development for the sake of sensationalism. The fact that you can cauterize the horns of developing dung beetle and they get bigger balls doesn't necessarily warrant the conclusion that there has been evolutionary allocation of resources between the horns and testicles. For example, perhaps when they cauterized the horns, it stimulated testosterone production through inflammation and that resulted in bigger testes. The other examples that Professor Simon cites were evolutionary based correlations. For example, Scott Picknik of Syracuse University had analyzed brain and testes size in 334 species of bats and found an inverse correlation between brain size and testes size, suggesting that there may be evolutionary trade-off between brain size and testes size (Source).

What Simmons and his colleagues showed is a developmental effect and not necessarily an evolutionary effect. In fact, in a followup analysis of 25 species of horn beetles, Simmons, et al. were unable to find an inverse correlation between horn size and testes size (Source):

In a follow-up analysis, the researchers looked at 25 species of Onthophagus beetles. Between these other species, the researchers did not find an inverse relationship between horn and testicle size. They suggest that those species with the most dramatic horns were those that had developed an evolutionary strategy to buffer or protect the development of their testes.

The study also showed that in species where females mate with the most males – where sperm competition is at its fiercest – horns tend not to develop on the thorax. The thorax is closer to the testicles than the head, so a thorax horn might be more likely to divert precious resources from the testes during development, the researchers suggest.

"Trade-offs are fundamental to biology but they continue to surprise us, both in the forms they take, and in the myriad ways they can shape the evolution of organisms," says Emlen.

The lack of an inverse correlation between horn size and testes size argues strongly against an evolutionary trade-off but instead the authors speculate that that big-horned beetles have "developed an evolutionary strategy to buffer or protect the development of their testes". This is not good science. In short, the straightforward answer to the question asked in the title of this topic is a simple "NO, the horniest beetles do not have the smallest testes". The observed phenomenon of bigger testes after cauterization of the horns during development is a developmental phenomenon and not evolutionary. The bigger testes in lesioned beetles are not likely to be passed on to offspring beetles.

By way, it is not clear that this phenomenon in beetles could be generalized to other species or could be used to support a theory of energy constraints on testes development. Upon reading this story, combined with the bat story, some people might be tempted to hypothesize that energy limitations during development are a major constraint on testes development. I am not convinced by this or other studies that such a constraint exists. If this were true, one would predict huge testes in children with cerebral palsy but this is not true.

Imagine a science teacher taking the article back to his or her biology class and showing the study to students, saying that this is an example of evolutionary resource allocation and energy constraints of testicular development. This may be happening in hundreds of classrooms around the world. Anything that has a teensy bit of sex in it, a play of the words such as horny, testes, and dung with an oversimplistic explanation tends to be presented to our youngest and most vulnerable students. It is the responsibility of science journalists and, above all, the scientists themselves to ensure that they are not foisting poorly thought out untruths on the public.

Wise.

[Wise Young]

I just spoke to one of the authors of the paper. I was a little embarrassed because I had been critical of the paper without having read the original paper. They were more circumspect in the paper, something did not come through in the news article that reported the paper.

The hypothesis that there may be an evolutionary trade-off between horn size and testicle size is very attractive and worthwhile investigating. The authors showed that if they removed the horn, testicle size increased. However, when they looked for an inverse correlation between horn and testicle size, they did not find it, to their surprise. It is a reasonable speculation that the beetles had evolved some way of getting around the trade-off.

Of course, it is also possible that removing the horn did something to the developing beetle to cause the testes to enlarge. One way of controlling for this possibility is the remove some other part of the anatomy of the beetle and see if the testes enlargement is specific to horn removal.

It would also be useful to do genetic analyses of the beetles and show that there are certain genes associated with horn size and with testes size, and that the two negatively interact with each other.

Wise.

[Lindox]

Does the horn grow bigger if the testes are removed?

What are the testes of the beetle that is still with horn composed of?

Could it be when the horn is removed the testes become engorged with a hormone that has no place to go anymore?

Did they compare the before and after of horn removal testes?

Is the horn a sexual attractant to female beetles? Thus dependent on the testes feeding it with some type of hormone.

Exactly why is this important?

[Wise Young]

Originally Posted by Lindox

Does the horn grow bigger if the testes are removed?

What are the testes of the beetle that is still with horn composed of?

Could it be when the horn is removed the testes become engorged with a hormone that has no place to go anymore?

Did they compare the before and after of horn removal testes?

Is the horn a sexual attractant to female beetles? Thus dependent on the testes feeding it with some type of hormone.

Exactly why is this important?

Wonderful questions. I hope that Doug Emlen will answer these questions. Wise.

[NoDecafPlz]

Slow day Dr. Young?



J.