1. What do male stag beetles use their big jaws to compete for at sap sites?
In male stag beetles, the jaws develop extremely, while in females, they hardly develop at all. It is widely known that these jaws are used in fights between males over females or feeding spots[1]. The stage for these battles is the "sap site," where tree sap flows. On the mainland, the saw-toothed stag beetle gathers at sap from trees like Konara oak and protects its spot with its large jaws[3]. A study of a stag beetle in Taiwan (*Cyclommatus mniszechi*) found that males defend territories at sap sites on broadleaf trees during the breeding season from May to August, and also showed behavior protecting females[2]. Tracing the body structure, these jaws are thought to be modified legs. Genes that build legs, such as *dachshund*, are also used to build the jaws, as reported in the Metalliferous stag beetle[1]. So, what kind of match occurs when two males with raised jaws meet?
2. How does a fight between male stag beetles go, from pinching to throwing?
A study observing *Cyclommatus mniszechi* in Taiwan indoors recorded the flow of matches between males. First, they combine their jaws and push against each other. Next, they pinch the opponent with their jaws, flip them over, and the match ends when the opponent retreats[2]. Within the range observed, no injuries were seen in any of the males[2]. The jaws seem to be used more for comparing strength by pushing than for injuring the opponent. However, this is a result from observing one species indoors. It cannot be said to be the same for all stag beetles. Still, it is a result that slightly changes the view that "big jaws = weapons to hurt opponents." When the match is decided, is the one with larger jaws at an advantage? It is reported that even males of the same species change their fighting style based on size.
The fights look rough, but in this study, no injuries were seen in the observed males. Within the range observed, no injuries were seen in any individual[2].
3. Do large and small male stag beetles fight in different ways?
Male *Cyclommatus mniszechi* in Taiwan are divided into large and small types based on a jaw length of 14.01 mm. Large males have relatively larger jaws compared to their body size. They use the same fighting actions, but large males are more likely to go directly to intense stages, while small males tend to stay in earlier stages, as reported[2]. In Japan’s saw-toothed stag beetle, there is a conference abstract by a graduate student at Hokkaido University. It states that small males always have a longer left jaw, while large males have a smaller difference between left and right lengths (as larvae, all males have a longer left jaw)[4]. The fight experiments in the same abstract were conducted with 38 pairs of large males and 39 pairs of small males. No relationship was found between jaw asymmetry and winning/losing. It was said that only small males showed strong losing habits after fights, and only large males showed more complex "tactics"[4]. This is a single student study and not a peer-reviewed paper. It should be read with the strength of "in one experiment, this happened." If fighting styles change with size, is it better to have large jaws? Is there no cost to making them big?
4. Does growing big jaws cause problems for the rest of a stag beetle's body?
In a study of the Metalliferous stag beetle, it was reported that jaws grow suddenly larger than the rest of the body (steep positive allometry). Larger males have stronger jaw strength, and jaw size can be called an "honest signal" of fighting ability[5]. On the other hand, the safety factor of the jaw structure, wings, and flight muscles were relatively smaller in larger individuals compared to their body size. From this, researchers suggest that growing large jaws may have a substantial cost[5]. As the word "suggest" implies, this is not a definite statement but a pointing out of possibility. Having large jaws is a sign of advantage in fights. However, individuals with larger jaws tended to have smaller flight muscles and wings relative to their body size. There may be such costs to continuing to send this signal. In places where the advantage of a big weapon is small, this cost may become heavier. So, do places where the "advantage is small" actually exist?
On the other hand, the safety factor of the jaw structure, wing size, and amount of flight muscle are relatively smaller in larger individuals (negative allometry). If there are few sap sites, the advantage of the weapon decreases, and the cost may remain.
5. Why did stag beetles on Hachijo Island end up with small jaws?
Hachijo Island has an endemic species called the Hachijo saw-toothed stag beetle, which is close to the mainland saw-toothed stag beetle. This species has notably small jaws and almost no curvature[3]. Comparing populations in the Izu Islands, the relative size of the jaws becomes smaller the further south from the mainland, shrinking the most on Hachijo Island, the southernmost point[3]. Why? Trees like Konara oak, where saw-toothed stag beetles gather, do not grow naturally in the Izu Islands. Because sap sites are scarce, the research team thinks the advantage of having large jaws becomes smaller. The shrinkage of jaws is explained as an adaptation to the island’s resource environment[3]. This is a hypothesis at the stage of "what is thought to be possible." Combining this with the cost of growing large jaws seen in the previous section, the logic is: if there are many sap sites, it is worth paying the cost to grow large jaws. If there are few sap sites, that value drops. However, this logic is a way of reading two sources together, not a conclusion made by the research team.
However, the story about fighting over beds is not written in the research team’s published materials, so it is safe to read it as how the newspaper reported it.
6. Has a gene that affects stag beetle jaw size been found?
A research team at Nagoya University focused on the insulin receptor gene *InR2*, which conveys nutritional status. In Hachijo Island populations, two amino acid changes were found in this *InR2*[3]. When the function of *InR2* was suppressed in mainland stag beetle larvae, the males’ jaws became smaller even at the same body size, approaching the Hachijo Island type (13 suppressed, 19 control)[3]. However, *InR2* is said to be "one" cause of the jaw shrinkage, not the only cause[3]. It is estimated that Hachijo Island populations have been isolated from populations on the mainland and intermediate islands in Izu for about 149,000 years, accumulating unique genetic variations. The "Kuroshio Barrier Hypothesis" proposes that changes in the Kuroshio Current may have acted as a barrier to this isolation. The 149,000 years is an estimate, and the Kuroshio story is at the hypothesis stage[3]. This research was published in the international journal *Molecular Ecology* on June 25, 2026[6]. The abundance of sap sites, the cost of growing large jaws, and the long time isolated on the island—each source illuminates a different aspect of one question. How can we verify this jaw story with our own eyes?
7. How can you compare stag beetle jaw shapes yourself in nearby forests?
Look for tree trunks with flowing sap in nearby mixed forests or parks. Whether you meet a stag beetle depends on the time and place. Even if you do not meet one, looking at the tree with the thought "this might be where males fight" will make it look different from usual. If you find a stag beetle, do not touch it and observe from a distance. Compare the shape of the male’s jaws with the female’s. Hornets may come near sap, so do not get too close and go with an adult. If you have a field guide or photos, comparing the jaw size and curvature of the saw-toothed stag beetle and the Hachijo saw-toothed stag beetle side by side is a good method. From the sources, you can read that jaw size differs between males of the same species and between islands. For those who want to know more, the Nagoya University press release (June 2026) and the paper on stag beetle fights in Taiwan (Chen et al., 2020) are starting points. You should notice that even for the same question, the sources look at different places: a lab in Taiwan, sap sites on the mainland, and genes on an island.
Since hornets also come to sap, do not get too close and go with an adult.