Many people that love bighorn sheep like I do know that they can age a ram by counting the horn rings but there are a couple tricks and concerns if you want to do it correctly. On the young desert ram above you can see each horn ring all the way out to the end of the horn. But you are only going to be able to see all the rings on a young ram, as the older ones will "broom" off the edges when the horns get long enough they affect their vision. (NOTE: I realized these smaller images in the text are more difficult to see so I've put them in the Gallery below as well. You can click on those images for a larger size).
If you look at the ram above, you can see the ragged edges of the end of the horn. He's broken those ends off, most likely by banging them against a rock. I believe I can see the first year's ring just a few inches from the ragged edge but on larger rams they could have the second, and on occasion even the 3rd year broken off.
Before I go too much further I should explain why bighorn have these rings. The horns on a bighorn ram, and other horned species (antlers are shed each year, horns are permanent), grow until the animal dies. The core beneath the hard sheath is alive with blood running through it. Although the growth will slow as the animal gets older, it will continue and later in life the horns will get more "mass" or thickness and a large ram becomes more obvious. Bighorn, like almost all wild animals, go through a rough period each year as well. For the Rocky Mountain Bighorn above it would be the winter, for the Desert Ram at the top it would be mid to late summer. During this period the animal is nutritionally stressed and the horn growth really slows, leaving a ring which you can see later. Ewes (females) do the same but their horns are so short it's impossible to really distinguish a horn ring from the normal texture you can note above.
What has been studied and is used as a general guide is the deepest ring is the 4th year. If you use that method on mid to old rams you don't have to worry about how many years have been broomed off. Just start with 4 and count to the base as I did on the 2 below.
I've labeled each year, the one on the left is about as old as most sheep get, and you can fairly easily see the rings all the way to the base. And it's fairly easy to see the rings on the out of focus ram on the right as well. I didn't even bother to try and determine if or how much either ram broomed off and went straight to what I thought was the deepest ring. The study that looked at this determined that the deepest ring is the 4th year close to 90% of the time. We don't have laws in Biology like Chemistry or Physics, just "most of the times". The problem still is which ring is deepest is in the eye of the beholder.
I've labeled what I think is each year's ring, but I would not bet my next paycheck on it. I think he's broomed his first year off, but that's more based on the thickness of the horn where I see that ring. As I say above, it's not easy. Several times I've aged a hunter killed ram with another biologist and we disagreed on how old the bighorn was. That's more common than we would like to admit but it was rare that we were more than one year different. I tell my students on exams as long as they get within one year of what I think it is they're good. Rarely do any of them miss it with that amount of leeway.
The lighting can not only affect your image quality, but also your ability to see the rings as with this nice desert ram above. The problem with photographing Desert Bighorn is the easiest way to get close to them with all that large photo gear is to wait until the summer when they come to water. Unfortunately they don't need to drink in that sweet early morning or Dusk light that all photographers love. They drink during mid day and the AZ sun is brutal. Even on a photo it's hard to see the rings.
This Desert ram is really tough to age, and I used my 10 years of field experience with horn diameter (mass) to help me pick out the 4th year ring. But I'm not taking any bets I'm correct. Any one who thinks they are right all the time is worth walking away from. The gesture you can see the ram is giving the ewe is known as a lip curl in bighorn, but it's not uncommon in other species and it's also known as a Flehmen response. The theory is it helps a male smell and taste if the female is receptive. It's also interesting to note that he has lost all of his lower front teeth (incisors and canines). He's not that old so I'm assuming he fell or lost them when horn butting with another ram. That will make things difficult for him.
I've put a series below in the gallery and see if you agree with me. Get some practice before you go out shooting bighorn again.
The best way to age a bighorn, or any other mammal. Pull a tooth. Mammal teeth continue to grow, although barely. And rings are laid down during that stressful period just like the rings on the horns. After the tooth has been pulled, it can be sent to a lab where they acid melt the enamel off, freeze the tooth then take very narrow shavings. Then stain those shavings on a microscope slide. That's the most accurate way for any animal I've ever photographed, BUT, except for a few species like bears it can leave the animal in poor condition. Therefore it's only done with animals that have already died, or in extreme research situations.