Sexual dimorphism comes in many forms and the three-spined stickleback represents one of the most interesting examples. A recent PLoS ONE paper (link here) reports that male three-spined sticklebacks have 23% heavier brains when compared to females! While there is no difference in body size there are massive behavioral differences that may account for the neural size disparity. Male sticklebacks court the females, create fancy nests and are the sole care providers for the kids.
Left: female Right: male
There is also an older paper showing a correlation in male bower bird brain size and the complexity of the nest. Check it out here- Sex, bowers and brains: J. Madden
Males will often do whatever it takes to get ahead. Evolution is the daytime drama we are privy to every day. Getting ahead or being successful means passing on your genetic material as often as possible. For the majority of cases in the animal kingdom, males mate with many females and females are able to exercise some sort of choice. Males employ various strategies to out-compete the next guy and one such obvious strategy is ornamentation.
As illustrated in the previous post regarding stalk-eyed flies, males often develop extreme ornamentation as a result of sexual selection. This ornamentation may come in the form of plumage like the peacock to catch a females eye or it may aid in fighting other males like the horn of the rhinoceros beetle. Both cases give males an advantage when vying for females and both can be costly to the males. Consider the weapons associated with male-male competition: large mandibles and horns are energetically costly to develop and carry around. Likely as a result of this greater cost, supportive traits develop to compensate.
A number of past studies have examined fight outcome and the relationship to horn, mandible or rear leg size for a number of insects. Interestingly, a recent paper examined the effect of supportive traits on fighting success (Y. Okada et al., 2012).
The study examines two species, the broad-horned flour beetle, Gnatocerus cornutus, and the bean bug, Riptortus pedestris. Both of these have exaggerated ornaments which are utilized in fighting. The broad-horned flour beetle has large mandibles and the bean bug has large hindlegs used for fighting. The authors measured the ornaments and supportive traits (i.e. head width, prothorax width, thorax length). Supportive traits are the structures in the body that would aid in compensation for the energetically costly weapon. For example, the male stalk-eyed fly has larger wings to compensate for exaggerated eye stalks. Furthermore, the authors also measured fighting success by pairing males in a small arena and determining the winner of a staged bout. This success was then correlated with the weapon size and supportive trait measurements. Specifically, this study employed principal components analyses and generalized linear models to examine phenotypic correlations.
Results indicate that in the case of both insects supportive traits significantly increased fighting success. This is not all that surprising, but this paper has been the first to demonstrate the contribution of supportive traits to fighting outcomes. The authors continue in their discussion to suggest that these characters may be tied together in correlational selection and phenotypic integration. Consider that sexual selection acts initially when males are developing more exaggerated weapons, the more exaggerated the weapon the more difficult it will be to flee from predators or actually even win a fight. One may have the biggest mandibles, but may not be able to fight with them effectively and in this case is it really an honest signal?
Okada, Y., Suzaaki, Y., Miyatake, T., and K. Okada. 2012. Effect of weapon-supportive traits on fighting success in armed insects. Animal Behaviour article in press
Male stalk-eyed flies have exaggerated ornamentation in the form of the eyestalks when compared to females (figure below). While males also have larger wings, this is most likely to compensate for the increased energetic cost of flying with the larger ornaments.
photo credit: Jerry Husak
If you aren’t familiar with the stalk-eyed fly, then watch the video below to see how the stalks develop.
Video Credit: BBC’s Life series with David Attenborough
Check out this link to learn more about current research from Dr. Jerry Husak.
When we think of sexual selection we usually think of 1) female choice and 2) male-male competition. However, if these were the only two phenomena present one wouldn’t expect to find such a high degree of male variation in nature. Alternative reproductive tactics can explain this variability and one example is the instance of male morphs. These “sneakers” or “satellite males” exist across several taxa and have evolved many tricks to pass on their genetic material without the risk of being the alpha male.
Cuttlefish sneakers make themselves look like females.
Green treefrog sneakers wait for the bigger male to call and when a female approaches the male jumps in.
The scarab beetle has two (in some cases three) male morphs with different horn sizes and reproductive tactics. The males with large horns guard the females they mate with and fight off other large males. The males with small horns sneak by and mate with females, but waste no energy in guarding her.
Feel free to comment with other examples.