Fig 1.
Study area and origin of samples.
Geographic locations of muskox mandibles received from hunters in Nunavut and the Northwest Territories, Canada through a Community-based Wildlife Health Surveillance program (n = 260). Harvest community/region was unknown for three muskoxen (i.e., excluded from the map). Inset map depicts the region sampled in Canada. The map was created using country and province boundaries from naturalearthdata.com.
Fig 2.
Lateral radiographs of muskox mandibles illustrating the progression of molar 2 (M2) eruption, with the bone line indicated in red: (A) Eruption Stage 1–starting to erupt through the bone; (B) Eruption Stage 2—partially erupted through the bone; (C) Eruption Stage 3—complete eruption through both bone into oral cavity.
Table 1.
Mandibles collected between 1986 to 1992 from Banks Island, Nunavut. Listed by sex and confirmed estimated age according to tooth eruption pattern.
Fig 3.
Morphometrics recorded for muskox mandibles and a description of each measurement.
Adapted from a CircumArctic Rangifer Monitoring and Assessment (CARMA) protocol [52]. I = Incisor, P = Premolar, M = Molar.
Table 2.
Age categories assigned to adult muskoxen.
Fig 4.
Diagram of a muskox mandible and age when each permanent tooth has completely erupted.
Mandible outline adapted from a CircumArctic Rangifer Monitoring and Assessment (CARMA) protocol [52]. Abbreviations yo = years old, mo = months old, I = Incisor, P = Premolar, M = Molar.
Fig 5.
Timeline of tooth eruption shown in radiographs and photographs of muskox incisor arcades and mandibles of selected ages for each year of tooth (A–F).
Primary incisors and molars are denoted with a lowercase “i” and “p” and permanent incisors and premolars are denoted with capital “I” and “P”, respectively. Permanent molars are labeled with a capital “M”. Eruption stage 1–starting to erupt through the bone; stage 2—partially erupted through the bone; stage 3—complete eruption through both bone into oral cavity. The scale bar for the incisor arcade measures 3 cm and the scale bar for the lateral view of the molars measures 5 cm. The age is provided in years old (yo) and months old (mo). The season beside each age when the animal was harvested. Missing incisors are either pre-mortem fractures or were extracted for cementum annuli analysis.
Fig 6.
Radiographic images of muskox incisor arcades from Ulukhaktok, NU.
These were adult muskoxen (5 years or older) based on molar eruption and wear. (A) The permanent I4 are fully developed but have not erupted. (B) The left I4 has not developed while the right I4 has erupted. The first incisors (I1) were extracted for cementum annuli analysis before radiographing.
Fig 7.
The relationship between age in months and caudal mandible length for wild muskoxen.
Mandibles were collected from 88 females and 90 males harvested on Banks Island from 1986 to 1992. Age was determined from tooth eruption patterns, and its relationship with caudal mandible length was modeled using a regression that included linear and quadratic terms for caudal mandible length (cm), sex, and their interactions. Fitted regression lines (black for females and blue for males) and their 95% confidence intervals (shaded regions) are shown, with predictions and intervals truncated to a maximum age of 60 months. Observed ages from tooth eruption patterns are represented by individual data points.
Fig 8.
The relationship between cementum age and actual age or predicted age of muskoxen.
(A) Data from incisors of 14 captive muskoxen with known ages. Cementum age was the average between two sections of the same tooth. The regression equation and adjusted R2 value are displayed on the plot. The blue regression line indicates the trend, and the grey shaded area represents the 95% confidence interval. The dashed line indicates a perfect linear relationship with no deviation. (B) Data from 32 adult (5 yo+) muskoxen harvested through the Community-based Wildlife Health Surveillance (CBWHS) program in Nunavut and the Northwest Territories. Predicted values were derived using the average cementum age and the fitted linear model from panel A. Age categories were assigned based on tooth wear before cementum annuli analysis. Vertical bars indicate 95% confidence interval.
Fig 9.
Decision tree for selecting appropriate aging method for muskoxen based on available sample.
*Sex is required to estimate age since both total and caudal length differ for muskoxen 3 years and older.