Cemental Deposition, Tooth Succession, and Horn Development as Criteria of Age in Dall Sheep

1. What they did

The author looked for reliable ways to tell the age of Dall sheep from their remains. He worked with the skulls, lower jaws and horns of 129 Dall sheep. Most came from Nunamiut hunters in the central Brooks Range of Arctic Alaska, who kept and labelled the remains of sheep they killed for food between 1950 and 1966. Others came from wildlife biologists in south central Alaska (including one sheep of known age) and the University of Alberta (the skull of a 5-day-old lamb). A birth date of 1 June was assumed, the middle of the published lambing dates (5 May to 28 June).

He tested three methods:

• Horn rings. He counted the annual rings on the horns of 89 sheep (37 rams and 52 ewes), and checked his counts on horns sawn in half lengthwise, where each year’s growth shows clearly.
• Tooth replacement. He examined the teeth and used X-rays (radiograms) of 50 lower jaws that showed teeth coming in, to work out when each baby (deciduous) tooth is replaced by a permanent one.
• Cementum layers. He cut thin sections of the first lower incisor and looked for yearly layers in the cementum, the bony layer that covers the tooth root. Standard staining did not work, so he developed a new method: washing, drying and mounting unstained sections and viewing them under a fluorescence microscope (fluoromicroscopy).

2. What they found

Horns

• Horns begin to grow when lambs are about 4 months old. Lambs cannot be sexed from their horns, but after the first year males have larger horn bases and females have more slender horns.
• Horns grow throughout life, but most of their length is added in the second and third summers.
• One ring forms each winter, starting in the first winter, when growth stops. Little or no growth was seen in sheep killed from October to January. Of 18 sheep killed in February, one had a new ring; by May, all horns showed new growth. Growth is fastest in early summer and has mostly stopped by the end of September.
• Because new growth starts up to 3 months before the 1 June birthday, a sheep not yet 5 years old may show 5 rings and 6 growth segments.
• A true annual ring is a deep, continuous groove with loosened edges; other ridges on the horn are smooth and rounded. Telling them apart takes some experience, especially in females and older males.
• Rings were hard or impossible to count on very old animals, and some ewes’ horns were broken off close to the base. Horn tips were rarely “broomed” but could be well worn after 5 years. The first ring was missing in 13 of 47 sheep aged 5 to 17 years, but its place was usually marked by a swelling; the second ring was never worn away.
• With the date of death known, horn rings gave an age to within about 1 month.

Teeth

• Lambs are born with nearly all their baby teeth. The permanent teeth come in on a regular schedule (Table 1): first incisor at 13 to 16 months, second incisor at 25 to 28 months, third incisor at 33 to 36 months, and the incisor-like canine at 45 to 48 months.
• So sheep killed in the fall at 1, 2 and 3 years old have one, two and three pairs of permanent incisors. This schedule was consistent across the years of collection and was the same for rams and ewes.
• The last baby premolar has 3 cusps and the permanent one has 2, so a sheep can be recognised as 2½ years old or younger at a glance.
• Tooth wear varied so much between individuals of the same age that it was of little use for aging.
• The order of tooth replacement differs from that in bighorn sheep: in Dall sheep the second premolar is the last premolar to come in, while in bighorn sheep it is the fourth.

Cementum layers

• Each year adds one bright (fluorescing, pale green) band and one dark band. The dark band starts forming in the fall, before the rut, and ends the following spring, matching the timing of the horn rings.
• Counting the dark bands, plus one year because the first incisor only comes in at about 13 months, gave the age in years; with a known date of death, to the nearest month.
• One sheep showed one fewer cementum layer than horn rings, possibly from injury to the tooth. Otherwise the layers were well defined, and 17 layers were counted in two sheep, close to the maximum age of wild sheep. The layers become thinner with age but could still be read in the oldest animals.

3. What they concluded

• Counting cementum layers in the first incisor with fluorescence microscopy can be used to estimate the age of all Dall sheep older than 1 year.
• During the first 4 years of life, age can be told from the order in which the incisors are replaced.
• Counting annual horn rings also gives age, but the rings can be hard to tell apart in older animals, especially females.

6. Other technologies, skills or innovations

• A new way of preparing tooth sections without staining or decalcifying them, viewed with a Zeiss fluorescence microscope; sections 50 microns thick could be made in under 10 minutes.
• Sawing horns lengthwise to show each year’s growth as a stack of cones.
• X-rays of the lower jaw to see permanent teeth developing below the gum line.
• Cleaning skulls with dermestid beetles.