Morphological Traits Relate to Mating Strategies in Male Japanese Stag Beetles

Prosopocoilus inclinatus

Male Japanese stag beetles (Prosopocoilus inclinatus) battle to defend territory, but their differing morphological traits correlate to differing “weapon” sizes and differing battle styles. (Photo credit: Eisuke Hasegawa, Ph.D.)

By John P. Roche

Male Japanese stag beetles (Prosopocoilus inclinatus) have battles to defend territories to earn the opportunity to mate with females. They use enlarged mandibles that look like the horns of male deer to battle for these matings—hence the name stag beetles. Stag beetles that win male-male contests usually succeed in mating, and thus there is strong sexual selection for success in battle.

The beetle battles by trying to grab an opponent with its enlarged mandibles and then throw the opponent into the air. Male stag beetles show two morphological types associated with different mating strategies: The strategy of males with large mandibles is to fight off other males to mate, and the strategy of males with smaller mandibles is to sneak matings (though smaller males still have enlarged mandibles and still readily engage in fights). Previous studies have examined these morphs in Japanese stag beetles, but few have looked at how different morphological traits correlate within the two male morphs. Eisuke Hasegawa and colleagues at Hokkaido University in Japan helped to fill this gap by investigating differences in how different morphological traits of Japanese stag beetles correlate within the large-mandible morph and within the small-mandible morph. Their study is reported in the Annals of the Entomological Society of America.

In studies with other beetle species—Onthophagini scarab beetles and Gnatocerus cornutus flour beetles—investigators found that eye size was negatively correlated with mandible size. That is, when mandibles are bigger, eyes are smaller. Multiple hypotheses could explain this pattern. One potential explanation is a developmental tradeoff: During development, the resources required to produce big mandibles might make it difficult to also make big eyes. An alternative hypothesis is that, in species that have big mandibles but small eyes, large eyes are not selected for because they are not needed. These two hypotheses led Hasegawa to become interested in how traits correlated in the two morphs of Japanese stag beetle.

Hasegawa and colleagues collected 150 adult P. inclinatus stag beetles in the wild in Hokkaido, Japan; froze the specimens; and categorized them into two morphs by measuring the length of a structure that correlates with mandible size: the protective shield called the elytra that covers their wings. If the elytra was longer than 19.70 mm, the specimens were categorized as major males; if the elytra was shorter than 19.70 mm, the specimens were categorized as minor males. Then, the researchers randomly selected 50 males from each morph and conducted a multivariate morphological analysis.

One of their discoveries showed an interesting difference between P. inclinatus and both Onthophagini beetles and Gnatocerus cornutus. Hasegawa said, “We tested whether there were morph-specific traits that correlate with the battle mode of each morph, and we found that eye size correlates positively with weapon size in the large morph of P. inclinatus. “That is, in major males, when mandibles were larger, eyes were also larger. Hasegawa concludes that the small eyes seen with large mandibles in Onthophagini beetles and Gnatocerus cornutus beetles might have evolved because these species fight in dark sites underground or in wheat flour, where vision is not as useful. “But the battle sites are quite different for Japanese stag beetles,” Hasegawa said in an interview, “because they wage battle on tree surfaces at night where starlight or moonlight exists.” With some light present, vision can be used and larger eyes could potentially provide a fitness benefit.

Another discovery in the study was that, in minor males, mandible size correlated positively with length of forelegs but correlated negatively with abdomen size. Hasegawa and colleagues conclude that the correlation of larger forelegs with larger mandibles could be explained in minor males because the minor male uses a strategy of attacking an opponent from the side, planting its forelegs on a branch, and using the forelegs for leverage to throw the opponent into the air. This is just one potential hypothesis explaining this finding, but it is consistent with the result.

Hasegawa concluded that mandible size might correlate negatively with abdomen size in minor males because minor males rely on sneaking matings, and a larger abdomen allows for larger testes and thus greater sperm production that could increase the likelihood of fertilizing females in sneaked matings. This pattern has been confirmed in other beetle species and is something that Hasegawa and colleagues plan to test in P. inclinatus.

When asked what next steps he envisions for his research, Hasegawa commented, “Because the morph of a P. inculinatus male is influenced by their nutritional condition at the larval stage, we plan to test if there is a genetic component of the morph-specific trait correlations by raising male beetles and comparing traits in sibling pairs. Japan is the most advanced country for captive rearing of stag beetles, so we will easily be able to rear many larvae of P. inculinatus in our laboratory.”

Stag beetles are popular as pets in Japan, and the country imports more than 1 million stag beetles each year. When some of these imports end up in the wild, they could lead to competition with native species, introduction of non-native parasites, and genetic hybridization. Because of these ecological risks, research on the morphology and mating strategies of Japanese stag beetles could someday be valuable for management and preservation of stag beetle populations.

Read more: “Morph-Specific Weapon-Correlated Traits in a Male Dimorphic Stag Beetle Prosopocoilus inclinatus, (Coleoptera: Lucanidae),” Annals of the Entomological Society of America


John P. Roche is a science writer and author with a Ph.D. in the biological sciences. He has served as a senior scientist and adjunct professor at Boston College, as an editor-in-chief of periodicals at Indiana University and Boston College, and as a science writer at Indiana University and the University of Massachusetts Medical School. He has published more than 160 articles and has written and taught extensively about science. For more information, visit http://authorjohnproche.com.

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