This photo was taken in the evening, after 8 pm, when they were clearly having a bit of fun. Perhaps there was a female nearby, or this is a good ovipositing site, who knows?
Unfortunatly six days later they seemed to have been discovered by a predator. Rae Ann wrote: "I went out last night to check on the beetles by the tree, and it looked like there was a war! There were headless bodies, and body-less heads! I know I've seen ants on dead beetles, but not sure if the ants would actually attack, or if the beetles fight amongst themselves. By evening there were plenty of them still crawling around, but there sure were a lot of dead ones (missing parts). It has been very hot here, 90-95 degrees Fahrenheit."
The stag beetles fight, yes, but I have never seen any casualties that way. The ants moved in to clean the dead bodies, they are really good at that.
If you want to know a bit more about their predators in England, where I live, click here in my website.