Popillia japonica, the Japanese beetle is an invasive species here, and we can find it in incredibly amazingly high numbers… Some plant species are particularly sensitive, and I am not worrying about gardener’s roses here, I was really thinking about some indigenous vines (but not the poison ivy of course!), which are litterally left leafless, to the exception of a few leafstalks.
But still, this species is of interest because it exhibits a very easily observed mating behaviour: males are staying onto their mate, to keep other males from trying to copulate with her. This behaviour is known as “mate guarding”. It is not difficult to understand that it is beneficial for males to do so: it does indeed increase their reproductive success by decreasing sperm competition.
One doesn’t need a deep understanding of evolutionary theory to make sense of this: males that guarded females outcompeted those that did not, leaving more offspring so that the behaviour is a norm in this species today.
Of course, it implies that guarding actually results into reproducing more than not guarding. One may think that time spent into guarding is also time lost to look for other mates. But does the number of mating events really count when your gametes will compete with many many others? Actually, we don’t know (yet), but we suspect that competition between males (and gametes) is indeed huge in this species. Simply because mating occurs at feeding sites, where you expect to find many guys and gals altogether.
But some scientists worked out many interesting facts about the Japanese beetle’s mating behaviour:
- first it seems those guys are not really good at spotting which ones of the other beetles are female, thus leading sometimes to some confusion, particularly in the morning (I suspect this is because they don’t drink enough coffee!) ;
- one way to avoid this is to find a way to find out a cue. This species has a slight size dimorphism, with females being slightly bigger on average than males. Males tend to choose bigger female as mates  (and indeed, this also explains a pattern found in  where homosexual pairs are formed with smaller males attempting on bigger ones);
It’s amazing, because this kind of cue is also a quite good predictor of female potential fecundity (though apparently not the only one) since bigger females generally lay more and larger eggs. The ability to tell apart based on size assessment would thus probably have been selected for. Guarding duration is also longer when males mated with larger females, suggesting that finding a large female is very rewarding, with regard to descent.
- guarding behaviour is also longer when the local density of beetles is high , irrespective of the actual sex-ratio: even when males are surrounded with many more females, they just keep on guarding. The mating behaviour of this beetle is obviously not optimized… But it’ still meaningfull with regard to an intrinsic difficulty to tell sex apart for beetles in this species;
- guarding is not free of pain, as illustrated with this picture, since it is not working all the time. But I guess that disrupting actual mating pairs is a good way to tell apart true females. Contest would then be a rational expectation, thus enforcing the need for guarding after successfully mating. But, moreover, guarding has a cost: when guarding, you don’t feed. You also get thermoregulating problems (guarding males were found to have higher thoracic temperature than non-guarding males ). So that there is probably a time when guarding is not usefull anymore, and you better have to look for food instead… Some don’t like it hot!
- Switzer, P. V., Forsythe, P. S., Escajeda, K. and K. C. Kruse. 2004.Effects of environmental and social conditions on homosexual pairing in the Japanese beetle (Popillia japonica Newman). Journal of Insect Behavior, 17(1): 1-16.
- Saeki, Y., Kruse, K. C., and P. V. Switzer. 2005. Male preference for large females and female reproductive condition in the Japanese beetle Popillia japonica Newman (Coleoptera: Scarabaeidae). Journal of the Kansas Entomolgical Society, 78(1): 13-19.
- Saeki, Y., Kruse, K. C., and P. V. Switzer. 2005. The social environment affects mate guarding in Japanese beetles, Popillia japonica. Journal of Insect Science, 5 Art. #18.
- Saeki, Y., Kruse, K. C., and P. V. Switzer. 2005. Physiological costs of mate guarding in the Japanese beetle (Popillia japonica newman). Ethology, 111(9): 863-877.