Dentition and Age Determination of the Warthog Phacochoerus aethiopicus in Zululand, South Africa

1. What they did

Knowing how old a warthog is helps researchers understand their breeding, growth and population changes. Warthogs in Zululand are born almost entirely in October, November and December, so animals of each year class are clearly separated in age. The author used this to test several ways of telling a warthog’s age.

• Teeth from culled animals. During population control culls in the Hluhluwe-Corridor-Umfolozi Game Reserve complex between June 1973 and June 1975, he collected the skulls of 381 warthogs (183 males and 198 females), plus 30 skulls and one found infant skull from Mkuzi Game Reserve. He recorded which baby and adult teeth were present, how far they had erupted and how worn they were, and measured the third molars.
• Growth rings in the teeth. He cut thin sections of the first lower incisor (and tried the upper third molar), softened them in acid, stained them and counted the dark “rest lines” in the cementum that coats the root. Teeth from seven Zimbabwean warthogs of known age (23 months to over eight years) showed whether one line forms each year.
• Eye lens mass. He dried the eye lenses of 289 warthogs and weighed them.
• Tusk length to snout width. He measured upper tusk length as a percentage of snout width in 362 warthogs, following an earlier photographic method from Kenya.
• Age in the field. He photographed young warthogs of known birth month, whose mothers could be recognised, at intervals up to two years old to record how body size, tusks and warts develop.

2. What they found

The teeth

• Young warthogs had 20 to 22 baby teeth. Adults most often had 28 to 30 permanent teeth, but totals ranged from 26 to 32, mainly because the number of lower incisors (four, five or six) and upper premolars varies.
• Replacement and eruption followed a clear timetable up to about two years: permanent canines start at about 5 to 6 months, second molars at 9 to 11 months, incisors from about 12 months, premolars at 12 to 14 months, and the third molars at about 17 to 20 months. All incisors were in by 16 to 20 months and all premolars by 20 to 22 months.
• The third molar is a huge, complex grinding tooth up to more than 65 mm long. It keeps growing into the fourth year and reaches full size by about four to five years. In four to five year olds it was significantly longer and wider in males than in females.
• As the third molars push forward, the teeth in front of them are worn down and shed: first molars mostly in the fourth year, second molars usually by about seven to eight years (never before six to seven), and premolars by about ten years. Very old warthogs keep only their third molars, tusks and worn lower incisors.
• Losing incisors did not seem to matter much: old warthogs in the field grazed short and tall grasses with no apparent difficulty, using their lips.

Growth rings

• Incisor cementum showed a broad pale band laid down in the wet season and a narrow dark rest line in the dry season, one pair a year. In known-age Zimbabwean teeth, the number of lines matched age where the lines were clear.
• About 33% of sections from 181 incisors could not be read, mainly because lines were faint, split, merged or unevenly stained. Some older animals were probably underestimated by one or two years.
• The oldest warthog that could be aged this way was a male of about 14 years. From the wear left on his third molars, the author estimated a maximum lifespan of about 16 to 17 years in Zululand.

Eye lens mass

Lens mass rose steadily with age up to 36 months, but from 24 months it overlapped more and more with adults, becoming increasingly unreliable for predicting age beyond 24 months and of little use beyond 36 months.

Tusk to snout ratios

Males had higher ratios than females. Ratios separated yearlings, subadults and three to four year olds fairly well within each sex, but overlapped too much in older animals. The Zululand ratios were clearly lower than those from Kenya.

Age in the field

• Four age classes were recognised: juveniles (up to 12 months), yearlings (12 to 24 months), subadults (24 to 36 months) and adults (over 36 months).
• Juveniles are small and their tusks are generally not visible. Tusks barely show at 12 months (1 to 3 mm), and by 24 months average about 2 to 3 cm in males and 1.5 to 2.3 cm in females. By 36 months they average about 4.5 cm in males and 3.0 cm in females.
• Many yearlings are nearly as big as adult females by about 17 months, so tusks, warts and head shape matter more than body size. Males have large infra-orbital warts below the eyes (6 to 12 cm in adults, compared with 1.5 to 2.5 cm in females) and a second pair of warts near the mouth that females lack.
• Subadult females can only be told from adults for two or three months after the birth season, so in practice only three classes can be recognised all year: juveniles, yearlings and animals over 24 months.

3. What they concluded

• Tooth replacement and eruption give a reasonably accurate age up to 24 months. After that, the growth of the third molars separates two to three year olds from older warthogs.
• Stages of tooth wear and loss only point to broad age classes, and may differ between populations with different diets. Counting cementum rings in incisors gives a more precise age for adults, especially beyond nine or ten years, but must be used with caution and some judgement; better stains could improve it.
• Eye lens mass is not recommended, because teeth are an easier way to separate the first two year classes.
• Tusk to snout ratios are reasonably distinct only for the first three year classes. The author suggests measuring the volume of the tusk pulp cavity as a possible new age index.
• An earlier formula (Spinage and Jolly 1974) that predicts age from the third molar’s length and height appears invalid for Zululand warthogs, because the tooth stops growing in length by four to five years.
• Maximum lifespan in the wild is probably about 17 years, close to zoo records, though few wild warthogs are likely to reach it.

6. Other technologies, skills or innovations

• Decalcifying teeth with formic acid and sodium citrate (which worked better than EDTA), wax embedding, and staining with haematoxylin and chromotrope 2R, with full laboratory schedules given.
• A cryostat (freezing microtome) cut satisfactory sections from a few incisors at -30 °C, which could speed up work on large samples.
• Repeated photographs of free-ranging young warthogs whose birth month was known, compared against measurements from culled animals of the same age.

7. Implications for conservation (from the paper)

• A reliable way to estimate the age of free-ranging warthogs is essential for assessing population parameters.
• Age estimates from tooth wear may not transfer between populations, because wear can vary with diet or genetic factors; the range of ages for first and second molar loss needs checking against known-age warthogs.