As I mentioned in ‘A Trip to Canyon Creek’ I had no idea of what this was when I encountered it. And I have to admit that, once I’d extracted it from the net and was peering into the container, my first thought was Odonteus obesus , since that is the only horned beetle that I had previously found. But I knew something was not right about that, so as soon as I got back to my van I busted out “Pacific Northwest Insects” and immediately confirmed it was not Odonteus obesus. But one flip of the page and there it was- Sinodendron rugosum (rugose stag beetle). I was actually quite disappointed in myself for not immediately recognizing it, since every time I see that entry I think ‘I need to find one of these!’ But, even though it clearly states that their size range is from 10-18mm, and my specimen was right in the middle of that at 14mm (including horn), I think I expected them to be far bigger. Regardless of my previous errors, I gave a fist pump and a ‘woohoo’ once I knew what it was.
These very cool looking beetles are in the family Lucanidae, and they are called stag beetles not because of the prominent horns (which other stag beetles in our region lack anyway), but because their elaborate mandibles are said to resemble the antlers of stags. Larvae feed on decaying rather than live wood, so this species is not considered to be a forest pest.
As I was sharing this profile to various social media I accidentally realized that I had found one of these before, the less spectacular, but still very cool female. I wish I could remember the location or circumstances of that find, but all I know for sure is that I photographed her in the field on October 25, 2021, and then identified and photographed her at home the next day.
In “Beetle horns and horned beetles: emerging models in developmental evolution and ecology” Kijimoto, et al state that “Despite the tremendous diversity in horn placement, shape, number, and size, all horns—when present—appear to be used for the same purpose: as a weapon to monopolize females against rival males. In each species studied thus far males were found to use their horns to push, block, prod, stab, lift, dislodge or otherwise impede rival males from accessing females.1 Fights are often intense, appear energetically expensive, and take time, yet are rarely injurious. Horns are effective, and males with larger horns typically enjoy an advantage in fights.” Beetle horns and horned beetles: emerging models in developmental evolution and ecology – PMC
It is also interesting to note that in some stag beetles “…sex is not always easy to determine without dissection of the genitals because there are males that mimick females, taking the opportunity to mate while their male rivals are fighting over the female…”, ONH Insects – Stag Beetle, though I cannot verify that this occurs in Sinodendron rugosum. But it seems likely, since “The ‘hornlessness’ of small males, however, is entirely mediated by larval feeding conditions: male larvae with access to good feeding conditions exceed a certain threshold size prior to pupation and initiate horn development, whereas male larvae with access to poor conditions develop into smaller pupae and adults without initiating horn growth…In species with pronounced male dimorphisms, smaller males lack horns, invest heavily into testes development and sperm competition and employ non-aggressive sneaking behaviors to circumvent physically superior rival males. ‘Hornlessness’ improves agility of sneaker males and is thought to represent an alternative morphological adaptation to the particular behavior niche inhabited by small males otherwise unable to successfully compete in direct male combat” Beetle horns and horned beetles: emerging models in developmental evolution and ecology – PMC
Description– “The adults of this species exhibit sexual dimorphism, a common trait in Lucanidae, as well as unopposable clubbed antennae. Both sexes are 11–18 millimetres (0.43–0.71 in) in length, black in coloration, and have small mandibles. The pronotum and elytra are covered in many small pits, leading to an overall rugose appearance. The head of the male is much narrower than the pronotum, and has a short, median rhinoceros-like horn. The female either lacks a horn or has a small median tubercle.” Rugose Stag Beetle articles – Encyclopedia of Life ; Males have a deeply dished out pronotum, and the mandibles do not project forward in either males or females.
Similar species– Male Odonteus obesus have a longer horn, and the pronotum is not dished; Bolbelasmus hornii is much lighter in color and lacks pitted rugosity; Ceruchus spp. have mandibles that project forward.
Habitat– Moist to mesic forests and woodlands containing deciduous trees.
Range– Endemic to the PNW; found region wide, except absent from shrub steppe; most common west of the east slope of the Cascades, and in sw Oregon/nw California.
Eats– The larvae “…have been associated with oak, alder, willow and cherry.[4] Adults may feed on plant juices or aphid honeydew.” Rugose Stag Beetle articles – Encyclopedia of Life ; larvae probaly also feed on the wood of other dead and dying deciduous trees.
Eaten by– “Sinodendron rugosum has been identified as one of many species eaten by the Northern Spotted Owl Strix occidentalis” Rugose Stag Beetle articles – Encyclopedia of Life; the larvae are probably consumed by various woodpeckers, and I would guess that adults are preyed upon by insectivores of all classes.
Adults active– BugGuide records indicate they can be found nearly year around, but they are most commonly seen from May through August.
Life cycle– “The white, C-shaped larvae hatch on bark and tunnel inward, creating pupal chambers in decaying wood.” Rugose Stag Beetle articles – Encyclopedia of Life
Etymology of names– Sinodendron is from the Greek words for ‘hurt/damage’, and ‘tree’, and may refer to the large horns borne by most of the males in this genus, though it could simply refer to the larvae eating wood, but I cannot find any definitive information on the subject. The specific epithet rugosum is from the Latin word for ‘wrinkled’, referring to the multiplicity of shallow pits and punctures that give this beetle a wrinkled appearance.
Rugose Stag Beetle articles – Encyclopedia of Life
Beetle horns and horned beetles: emerging models in developmental evolution and ecology – PMC
Species Sinodendron rugosum – Rugose Stag Beetle – BugGuide.Net
https://en.wikipedia.org/wiki/Sinodendron_rugosum
I was so excited to finally find one of these yesterday. By “I” I mean my son found 4 males under a log, but still im super excited to have finally seen this amazing miniature rhino looking beetle. Your info was very helpful! Thank you!
Very cool! I’ll bet your son was excited too. Glad to hear this profile was useful for you!