Male dragonflies are not as violent as thought

ResearchBlogging.orgby Piter Kehoma Boll

Males and females are defined by their gametes. Males have tiny, usually mobile gametes, while females have very large gametes that usually do not move. This means that females produce less gametes, but put a lot of resources in each one, i.e., female gametes are expensive. On the other hand, male gametes are very cheap, small and produced in large quantities. As a result of these differences, males and females have different interests during sex.

As females produce more expensive and less numerous gametes, they tend to be very selective on who they let fertilize them. But males benefit from fertilizing every female gamete they find in their way. In other words, females want quality and males want quantity. This difference in interests is called sexual conflict and is a strong evolutionary force.

One evolutionary adaptation that has been seen as resulting from sexual conflict is the mating system in odonates (dragonflies and damselfies). During sex, the male dragonfly grasps the female neck using a grapsing apparatus at the end of its abdomen. The female is then induced to connect the tip of its abdomen to the second and third segments of the male’s abdomen, where sperm is stored. The couple than flies together in a heart-like formation.

Two dragonflies of the species Rhionaescna multicolor copulation. The male is the blue one, which is grasping the female's neck and making her touch the tip of her abdomen to his second and third abdominal segments, where sperm is stored. Photo by Eugene Zelenko.*

Two dragonflies of the species Rhionaeschna multicolor copulating. The male is the blue one, which is grasping the female’s neck and making her touch the tip of her abdomen to his second and third abdominal segments, where sperm is stored. Photo by Eugene Zelenko.

It was thought that the male grasping apparatus forced an unwilling female to copulate with him, suggesting that the organ evolved through sexual conflict. The fact that males usually grab females way before they accept to mate and continue to hold them for a long time after the mating has finished (preventing her from mating with other males) seem to be good evidence for this theory. If this is true, than the female would try to get rid of the male, selecting stronger and bigger grasping apparatuses in males, as those would be more efficient in holding the female and, as a result, would lead to more descendants.

A study published last year tested this hypothesis. Córdoba-Aguilar et al. (2015) evaluated the allometry (the proportional size of a structure with respect to body size) of the male grasping apparatus in several dragonfly species. If males forced females to copulate, a hyperallometric relationship should be expected.

What does that mean? Well, let’s try to explain it the simplest way. When you plot data on the size of a structure according to the size of the body as a whole on a graph, using values that lead to a linear relationship, you may have different results. The structure may increase in size in the same way as the body, in a 1:1 relationship. In this case, the line in the graph is said to have a slope equal to 1 and there is an isometric relationship of the structure to the body. If the slope is greater than one, this means that the structure grows faster than the body, having a hyperallometric relationship. If the slope is smaller than one (but greater than zero), the relationship is hypoallometric and the structure grows slower than the body.

allometry

The measurements of the grasping apparatus in dragonflies in general showed an isometric relationship. So, according to this approach, the structure did not evolve as a “weapon” to subdue females. But which other explanations may exist then? It could be used as a courtship tool, a way for the male to convince the female to mate with him. It could also be a way to avoid interspecific mating, as the grasping apparatus has a strong specificity in shape to the female neck of the same species. A male dragonfly cannnot grasp a female of other species because the grasping apparatus simply does not fit in the female’s neck.

Both alternative hypotheses for the evolution of the apparatus are possible, but further studies are needed to test them.

– – –

References:

Chapman, T., Arnqvist, G., Bangham, J., & Rowe, L. (2003). Sexual conflict Trends in Ecology & Evolution, 18 (1), 41-47 DOI: 10.1016/S0169-5347(02)00004-6

Córdoba-Aguilar, A., Vrech, D., Rivas, M., Nava-Bolaños, A., González-Tokman, D., & González-Soriano, E. (2014). Allometry of Male Grasping Apparatus in Odonates Does Not Suggest Physical Coercion of Females Journal of Insect Behavior, 28 (1), 15-25 DOI: 10.1007/s10905-014-9477-x

– – –

Creative Commons License
All images are licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Advertisements

3 Comments

Filed under Behavior, Entomology, Evolution

3 responses to “Male dragonflies are not as violent as thought

  1. Are these dragonflies also available in Africa!

  2. Pingback: Badass females are unpopular among praying mantids | Earthling Nature

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s