Fighting and aggressive sound determines larger male to win male-male competition in a bark beetle.

Intrasexual selection occurs in male-male competition over access to females and usually results in the larger male winning. While much research has documented that size matters, little is known about how the larger male wins. Dendroctonus valens is an aggregating monogamous bark beetle in which males have large variation in body size and display intense competition over females. Behavioral observation showed two males fight each other within the gallery by pushing/shoving and stridulated more when two males encountered each other. Experiments using two different-sized males synchronously competing showed that larger males won 95% of contests. Reciprocal displacement experiments using muted and intact males of different or equal size were used to simulate male-male competition. Larger males displaced the smaller resident male in 90% of contests, while smaller males prevailed over larger residents in 6.7% of contests. With both males silenced, larger males displaced smaller males in 80% of contests, while smaller males prevailed in 8% of contests. Further experiments using equal-sized males showed aggressive sound-emitting males displaced muted males in 67% of contests, yet intact males displaced other intact males in only 37.5% of contests. Sound analysis showed sound pressure level is an honest signal of body size and males chose soft sounds over loud aggressive sounds in assays. Therefore, D. valens males have evolved dual behaviors, fighting and aggressive sounds associated with body size, to assess rivals to compete for a partner, gaining insights in male-male competition for this species and for other animals.