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Journal of the Royal Society of New Zealand logoLink to Journal of the Royal Society of New Zealand
. 2025 May 6;55(6):2042–2059. doi: 10.1080/03036758.2025.2491510

Tooth wear and dental calculus in a group of orca (Orcinus orca) stranded on the New Zealand southern coast

Carolina Loch a,CONTACT, Ingrid N Visser b, Kali Stratford a, Sophie White c, Joe Wakefield d, Stewart Bull d, Lydia Matenga d, Muriel Johnstone d, Ramari Oliphant-Stewart e
PMCID: PMC12315212  PMID: 40756860

ABSTRACT

In 2014, a group of nine orca stranded in Te Waewae Bay, Western Southland, and their remains were collected, processed and studied in partnership with Ōraka Aparima Rūnaka. A total of 436 teeth were analysed to determine the location, type and intensity scores of tooth wear. The presence of dental calculus and percentage surface coverage was assessed on 1744 surfaces. Tooth wear and calculus were investigated in relation to sex and total body length (as a proxy for age). All orca had some degree of tooth wear; however, most teeth were worn superficially. Only two individuals had severe wear as the most common wear type; nonetheless, no exposure of the pulp cavity was observed. There were slightly lower wear scores in the upper dentition and wear scores were associated with body length. Calculus varied within and between orca, with eight of the nine having calculus deposits present. More calculus was found on the upper dentition on mesial and buccal aspects. Despite the limited number of individuals analysed here, the opportunity to investigate patterns of tooth wear and calculus in a group of nine wild orca was enabled by the research partnership with significant support of the local community.

KEYWORDS: Attrition, abrasion, culture, ecotype, teeth, tartar, killer whales

Introduction

Orca (killer whales; Orcinus orca) are the largest of the Delphinidae, with adults measuring 5–9 m in length (Heyning and Dahlheim 1988; Ford 2009). Orca have a characteristically robust body, pectoral fins which have an overall oval shape and species-specific pigmentation that is typically black or grey over most of their body, along with white areas such as flank patches, eye patches and most of the ventral surfaces (Visser 1999a). They are sexually dimorphic, with adult males having larger pectoral fins, taller dorsal fins and tail flukes curled inwards. Orca have been observed in oceans throughout the globe (Heyning and Dahlheim 1988), including New Zealand (NZ) (Visser 2000a). Orca are considered data deficient by the IUCN red list of threatened species and population trends are unknown (Reeves et al. 2017); however, the NZ orca have been designated ‘Nationally Critical’, the highest threat ranking given by the NZ Government (Visser 2000a).

In 2014, a group of orca stranded at Kutuawa (Wai-Koko River mouth) in remote western Te Waewae Bay on the boundary of the Fiordland National Park. Their remains were collected by the authors working alongside members of the Ōraka-Aparima Rūnaka, the indigenous governing body of the region within Ngāi Tahu whānui, following traditional protocols and methods guided by a senior practitioner and tohunga. As part of this mahi (work) the orca were photographed and measured in situ. Samples were collected, including skulls from 9 animals and 4 full skeletons.

Orca have 10–14 teeth per quadrant (Heyning and Dahlheim 1988), and although they are homodont, differences in shape and size can occur within each quadrant (Caldwell et al. 1956). The crowns are somewhat conical, but flattened mesio-distally, resulting in a generally elliptic shape, but often the distal surface is slightly flatter and the mesial more bulbous. The teeth of orca are curved lingually and the tooth crown makes up approximately a third of the tooth length (Graham and Dow 1990). Similar to other odontocetes, orca teeth do not occlude but most of the upper and lower teeth interlock when the mouth is closed (Tomes 1873; Graham and Dow 1990; Loch and Simões-Lopes 2013). The crowns of orca teeth are covered by prismatic enamel (Loch et al. 2013a) and the long roots are housed in deep alveoli which are also mesio-distally compressed (Heyning and Dahlheim 1988). Similar to humans and other mammals, orca teeth are subject to physiological tooth wear and other pathological processes.

Dental wear in mammals involves the combination of attrition (tooth-to-tooth contact), abrasion (wear against foreign substances), abfraction (micro-structural loss in stress areas) and erosion (chemical wear) (Grippo et al. 2004). Tooth wear is a cumulative process and progresses with age (Grippo et al. 2004). Different animals display diverse dental wear patterns, mostly related to differences in tooth morphology and structure, biomechanics of mastication, dietary consistency, physiology and behaviour, including parafunctional uses of teeth (Hillson 2005).

While several studies have been conducted on tooth wear in humans and wild mammals, less is known on dental wear patterns in marine mammals. Scattered studies have assessed dental wear in pinnipeds (Stirling 1969; Drehmer and Ferigolo 1996; Labrada-Martagón et al. 2007; Marx et al. 2023), sirenians (Lanyon and Sanson 2006; Domning and Beatty 2007) and odontocete cetaceans (Caldwell and Brown 1964; Ramos et al. 2000; Loch and Simões-Lopes 2013; Loch et al. 2013b; Marx et al. 2023). In their study of tooth wear in odontocetes, Loch and Simões-Lopes (2013) reported high tooth wear frequencies in ten species. Despite high frequencies of wear, most teeth were worn superficially and facets in both the apex and lateral faces were more common.

Some publications have discussed orca tooth wear. It has been described from the 1800s in the framework of age and disease (Tomes 1873; van Beneden 1880), as well as investigated in light of masticatory movements and feeding behaviour (Caldwell and Brown 1964; Graham and Dow 1990; Tang et al. 2019; Visser et al. 2023). More recent studies have suggested that dietary specialisations and dental wear are indicative of genetic and ecological divergence among orca populations in the North Atlantic and Northeastern Pacific, including extreme wear in a population termed the ‘Offshore’ orca, that consume sharks (Foote et al. 2009; Ford et al. 2011; Tang et al. 2019). High incidence of extreme wear levels, including exposure of the pulp cavity, were reported as a result of oral stereotypes in captive orcas (Visser and Lisker 2016; Jett et al. 2017).

In mammals, dental calculus (tartar) is a result of dental plaque calcification due to the precipitation of salivary minerals and gingival crevicular fluid (Akcalı and Lang 2018). Calculus deposits are a risk factor for periodontal disease, a chronic inflammatory condition that can lead to the recession of gums, bone loss, and eventually edentulism (Nazir 2017). Once formed on teeth, calculus can only be removed via mechanical means. Dental calculus has been reported in odontocetes, including orca (Tomes 1873; Loch et al. 2011). In odontocetes, yellowish to black dental calculus is frequently found on the tooth cervix in a ring-like formation, often more prominent on the buccal surfaces (Loch et al. 2011).

The study of tooth wear and dental pathology such as calculus deposits in orca can contribute to our understand of their biology, including diet and conditions of health or disease. As such, knowledge gained about their periodontal conditions including from deceased stranded specimens, can elucidate the impact of disease and morbidity in mortality, but also shed light on how these animals lived. Due to extensive dental issues in captive orcas, understanding patterns of tooth wear and dental disease in wild orca populations is paramount to differentiate common conditions from those that might be exacerbated by captivity.

This paper evaluates and describes patterns of dental wear and calculus from the orca group, including assessments of the location, overall characteristics and intensity of macroscopic tooth wear and calculus deposits. To the best of our knowledge, this is the first study which has examined every tooth of all individuals in a group of stranded orca.

Materials and methods

On 11 February 2014, on the coast at approximately 46° 9’ S 167°23’ E, western Te Waewae Bay, Southland, NZ, a group of nine orca (six females and three males) stranded and subsequently died. Data on length, sex, and tooth counts were gathered for each orca (Table 1). Measurements of total body length were used as a proxy for age as absolute age was not known. In keeping with Māori customary practices, each recovered whale was identified with an individual name that was used alongside their stranding catalogue ID. The skulls were processed, the dentition was extracted from the upper and lower jaws and the soft tissues decomposed in beach sand (Figure 1b). Following decomposition, the teeth were stored in a wet 0.5% sodium bicarbonate solution prepared with rainwater. Solutions were changed monthly until the analysis was complete. Each tooth was numbered sequentially from the anterior to the posterior using a soft pencil and those from each orca (separated into quadrants) were kept in the same container during storage. The use of the wet solution was to prevent cracking of teeth and spalling of tartar, which is common in orca teeth exposed to air and desiccation (Robovský et al. 2009; Visser 2020; Mills et al. 2022). When removed for assessment, the teeth were kept damp in moss (Sphagnum sp.) (Figure 1c) or under damp towels. Each tooth was weighed, measured with digital callipers and photographed.

Table 1.

Te Reo Māori customary name, catalogue number, total body length, sex, total number of teeth, number and percentage (in brackets) of teeth with calculus and average percentage calculus coverage of the teeth from each orca.

Orca customary name Catalogue number Total body length (cm) Sex Total # teeth Teeth with calculus (% in brackets) Calculus % coverage
Hine Kōtea NZUE-001 570 Female 49 47 (96) 13.6
Hine Tetē NZUE-002 550 Female 48 48 (100) 23.7
Hine Mākehu NZUE-003 550 Female 50 50 (100) 21.2
Hine Kōrito NZUE-004 475 Female 48 48 (100) 5.8
Maripi e Mako NZUE-005 385 Female 49 49 (100) 11.6
Koropepe NZUE-006 328 Female 46 0 (0) 0
Kahukura ā Toka NZUE-007 630 Male 49 49 (100) 24.3
Rarakau NZUE-008 520 Male 48 44 (92) 3.5
Kutuawa NZUE-009 430 Male 49 49 (100) 9.9
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Figure 1.

Figure 1.

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Recovery and preparation of the stranded orca. (a) Stranded orca in Te Waewae Bay, Western Southland. (b) Orca teeth (separated in quadrants) laid out prior to covering with beach sand. (c) Wet moss (Sphagnum sp.) being used to stop teeth drying out during analysis. (d) Sophie White, Joe Wakefield and Ramari Oliphant-Stewart retrieving skeletal remains after controlled decomposition in the ground. Photo credits: a: Dale Green; b: Ramari Oliphant-Stewart; c, d: Ingrid Visser.

Dental wear

After preparation, each tooth was analysed and visually inspected to identify wear facets. Wear facets were categorised into location (cusp tip, lateral or cusp tip/lateral combined, Figure 2) and intensity based on visual estimates of hard tissue loss (no wear, superficial, moderate or severe) adapted from Loch and Simões-Lopes (2013). Each tooth was also assessed for wear down to or below the gum line, and for the presence of fractures, holes and missing teeth following Jett et al. (2017). Average prevalence frequencies were calculated for each category. Average wear scores were calculated by dividing the sum of wear scores per tooth by the number of teeth of each animal. Differences in tooth wear scores between the sexes were calculated and the association between tooth wear scores and body length (age) was tested using a linear regression.

Figure 2.

Figure 2.

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Wear location and intensity in orca (Hine Mākehu (NZUE-003) in buccal view). Wear facets can be found in the cusp tip, lateral or simultaneously on the tip and lateral faces. Wear intensity ranging from superficial, moderate to severe. Photo credits: Ingrid Visser.

Calculus

Teeth were visually inspected for the presence of dental calculus. All adherent mineral deposits firmly attached to the tooth surface and whose colour varied from yellowish to black, were classified as calculus deposits (following Loch et al. (2011). Each tooth was placed on a glass plate under lighting. Photographs were obtained using a tripod-mounted Canon EOS 5D Mark II camera with 50 mm lens. The shutter speed was set at ½ second, the ISO at 100 and the native RAW format was used. The mesial, distal, buccal and palatal/lingual views of each tooth were photographed. RAW images were converted to high-resolution ‘jpg’ format photographs and then processed using ImageJ (Ferreira and Rasband 2012). After using the ‘calibrate image’ function in ImageJ, areas with calculus deposits were selected and highlighted for each of the four views (Figure 3(a)). The total area of each view was also highlighted (Figure 3(b)). The percentage of calculus coverage on each view of the tooth was then calculated in relation to the total view area. Average percentages of calculus coverage were calculated for each tooth, quadrant, upper or lower dentition and each animal, and comparisons were made among categories. The prevalence of calculus was also assessed between sexes using an unpaired t-test and its association with body length was tested using a correlation matrix followed by linear regression.

Figure 3.

Figure 3.

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The mesial surface of a tooth from Hine Tetē (NZUE-002) is used as an example where ImageJ was used to select the calculus area (yellow lines) (a) and the corresponding total surface (b). The software computed the percentage of calculus coverage for each surface. Photo credits Jean Claude Stahl.

Results

We examined 436 teeth (Table 1), which we believe is the largest dataset of teeth from free-ranging, wild orca that has been examined for tooth wear and calculus. The presence of dental calculus and percentage surface coverage was assessed on 1744 surfaces.

Tooth wear prevalence and characteristics

All the orca had some degree of wear to their teeth, ranging from 61% of teeth worn in Koropepe (NZUE-006) the smallest female, to 100% in Hine Mākehu (NZUE-003) one of the middle-sized females, and Kahukura ā Toka (NZUE-007) the largest of the males (Table 2; Figure 4). For seven of the nine individuals, most of their teeth were worn superficially, affecting only the enamel and superficial layers of dentine, rather than severe levels of wear down to the gum line. The prevalence of superficial wear (enamel to outer layers of dentine) ranged from 13% of the worn teeth in Hine Tetē (NZUE-002) one of the medium-sized females, to 100% in Koropepe. Moderate wear (compromising 10-50% of the crown area) affected 20.4% of the teeth analysed, with prevalence ranging from 0% in Koropepe to 38% in Hine Mākehu. Severe wear, in which more than 50% of the crown area is compromised and a tooth may at times be worn down to the gum line, was registered in 21.3% of the teeth and ranged from 0% in Koropepe to 65% in Hine Tetē. Only two individuals (Hine Tetē and Hine Kōtea (NZUE-001), the largest female), had severe wear as the most common wear type. Despite many teeth being worn down to the gum line in these individuals, no exposure to the pulp cavity was observed (Figure 4(a) and (c)). The average wear score for all animals was 1.6 and ranged from 0.6 in Koropepe to 2.42 in Hine Tetē. Average wear scores were slightly lower in the upper than in the lower dentition (1.47 vs 1.69) (Figure 5(a)).

Table 2.

Te Reo Māori customary name, total number of teeth, number of teeth worn and percentage (in brackets), wear intensity (number and percentage (in brackets) of teeth in each category), wear score and wear location (number and percentage (in brackets) in each orca.

Orca Total n teeth Teeth worn Wear Intensity Wear score Wear location
Index 0 Index 1 Index 2 Index 3 Cusp tip Lateral CT/L
Hine Kōtea 49 48 (98) 1 (2) 15 (31) 14 (29) 19 (40) 1.96 11 (23) 5 (10) 31 (65)
Hine Tetē 48 46 (96) 2 (4) 6 (13) 10 (22) 30 (65) 2.42 15 (33) 1 (2) 30 (65)
Hine Mākehu 50 50 (100) 0 (0) 21 (42) 19 (38) 10 (20) 1.78 9 (18) 5 (10) 36 (72)
Hine Kōrito 48 46 (96) 2 (4) 32 (70) 10 (22) 4 (9) 1.33 12 (26) 0 (0) 34 (74)
Maripi e Mako 49 47 (96) 2 (4) 33 (70) 11 (23) 3 (6) 1.3 4 (9) 9 (19) 34 (72)
Koropepe 46 28 (61) 18 (64) 28 (100) 0 (0) 0 (0) 0.6 12 (43) 12 (43) 4 (14)
Kahukura ā Toka 49 49 (100) 0 (0) 23 (47) 11 (22) 15 (31) 1.84 9 (18) 4 (8) 36 (73)
Rarakau 49 47 (96) 2 (4) 32 (68) 8 (17) 7 (15) 1.41 3 (6) 9 (19) 35 (74)
Kutuawa 48 47 (98) 1 (2) 36 (77) 6 (13) 5 (11) 1.31 3 (6) 11 (23) 33 (70)
Average 48.4 45.3 (93.5) 3.1 (6.4) 25.1 (51.9) 9.9 (20.4) 10.3 (21.3) 1.6 8.7 (20) 6.2 (15) 30.3 (64)
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Figure 4.

Figure 4.

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Tooth wear in some of the individuals from a group of nine NZ orca. (a) Severe wear in the first three anterior teeth in the mandible of Kahukura ā Toka (NZUE-007), male, while his mid and posterior teeth were either moderately or superficially worn. (b) Superficial wear in the teeth of Maripi e Mako (NZUE-005), female. Note the upper and lower teeth interdigitating in occlusion visible due to the upper lip having been removed by sharks, post-mortem. (c) Severe wear in Hine Tetē (NZUE-002) with most of the teeth in the mandible worn down to the gum line. Note that despite the severe wear, no pulp cavity exposure was observed. (d) Prepared teeth of Rarakau (NZUE-008), male. Superficial crown wear was observed, alongside prominent calculus deposits on the cingulum and root (dark staining). Photo credits: a, b, c: Ingrid Visser; d: Carolina Loch.

Figure 5.

Figure 5.

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Tooth wear patterns. (a) Average wear scores between the upper and lower dentition. (b) Prevalence (%) of wear location in the upper and lower dentition. (c) Average wear scores (± SD) between males and females. (d) Average wear scores vs total body length.

In relation to the location of wear facets, most worn teeth had wear facets simultaneously affecting the cusp tip and lateral sides of teeth (64%) (Table 2). This was the common wear type for most individuals except for one animal (Koropepe). The prevalence of simultaneous cusp tip and lateral teeth wear ranged from 14% in Koropepe to 71% in Rarakau (NZUE-008) the middle-sized male. Wear which was restricted to the cusp tip was observed in 20% of the teeth, with prevalence ranging from 6% in Kutuawa (NZUE-009) the smallest male and Rarakau, to 43% in Koropepe. In 15% of the teeth, only lateral wear was present. Koropepe had the most teeth with only lateral wear (43%), whilst Hine Kōrito (NZ004), one of the medium-sized females, had no teeth with lateral wear.

Figure 6.

Figure 6.

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Erosion/glossowear/hydrowear lesions (arrows) on the surfaces of upper right teeth 6 & 7 of Hine Kōtea (NZUE-001). Note the smooth/polished aspect of the tooth cervix and shallow grooving formed which circumscribed the whole tooth. Photo credits: Jean Claude Stahl.

When considering the upper and lower dentition separately, tooth wear which was restricted to the cusp tip or lateral surfaces of the crown was slightly more common in the lower than upper dentition. In contrast, simultaneous wear on the cusp tip/lateral surfaces was most common in the upper dentition. Unworn teeth were also slightly more common in the upper than lower dentition (Figure 5(b)).

Average wear scores were similar among males and females (1.6 vs 1.5) (Figure 5(c)); however, the limited number of animals analysed needs to be considered. Average tooth wear scores were strongly associated with body length (R2 =  0.96), suggesting intensity and severity progress as animals grow and age (Figure 5(d)).

Erosion/glossowear/hydrowear

In addition to tooth wear caused by attrition and/or abrasion, loss of dental hard tissue consistent with erosion/glossowear/hydrowear (Figure 6) was also observed in four of the nine orcas. Those correspond to areas of smooth and glazed tooth surfaces in which enamel and dentine have been removed forming grooving which ranged from shallow to significant. Those lesions concentrate around the cervix area, just above the gum line. Lesions were more prominent in lingual/palatal surfaces, although sometimes this type of wear circumscribed the whole tooth. Teeth worn via erosion/glossowear/hydrowear were more common in the upper dentition (three out of four orca), although Hine Kōtea (NZUE-001, the largest female) had such lesions in both upper and lower jaws. For the animals presenting erosion/glossowear/hydrowear, its prevalence ranged from 12.5% of the lower dentition in Hine Kōtea and Hine Tetē, to 44% of the upper dentition of Kahukura, the largest male.

Calculus prevalence and characteristics

Calculus was present in the dentition of eight of the nine orcas, with the exception of Koropepe who was a young animal, estimated at 2–3 years of age based on orca of similar sizes and known age (Myrick et al. 1988; Duffield et al. 1995). In all animals with calculus, either most or all teeth showed signs of calculus deposits. However, the average tooth surface area covered by calculus varied, ranging from 5.8% in Hine Kōrito (NZUE-004) a medium-sized female, to 24.3% in Kahukura ā Toka, the largest male. Three orca had over 20% of the total tooth surface area covered by calculus; Hine Mākehu (21.2%), Hine Tetē (23.3%) and Kahukura ā Toka (24.3%) (Table 1).

The percentage of tooth surface area covered with calculus varied between the upper and lower dentitions. On average, more surface area was covered by calculus in the upper compared to the lower dentition (17 vs. 11.7%; Figure 7(a)). When comparing quadrants, the highest surface area covered by calculus was seen on the upper right side (18%) and the lowest in the lower left side (12%) (Figure 7(b)). In relation to the tooth surfaces affected, more calculus was seen on mesial surfaces (16%) and less in palatal/lingual surfaces (12%) (Figure 7(c)).

Figure 7.

Figure 7.

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Tooth calculus. (a) Calculus % area coverage in the upper and lower dentition. (b) Calculus % area coverage among quadrants. (c) Calculus % area coverage tooth aspect. (d) Calculus % area coverage vs total body length.

The percentage of calculus coverage was associated with total body length, meaning larger/older animals had more tooth surface area covered by calculus than smaller/younger animals (Figure 7(d)). The average percentage calculus coverage was similar between males and females at 13%.

Discussion

Tooth wear and calculus were prominent in the orca group investigated here. Although all animals presented with tooth wear, wear prevalence ranged from 60 to 100% of the teeth in each animal and most teeth were worn superficially. Only two individuals had severe wear as the most common wear type. Calculus deposits were observed in eight of the nine animals studied. Of those eight, the average tooth surface area covered by calculus ranged from 6 to 24% in any given orca examined. Differences in surface area covered by calculus among tooth surfaces and the upper and lower dentition were observed.

As tooth wear was common in all orca analysed here, it is plausible this group displayed similar behaviours and as such, were subject to similar physiological processes. This has been postulated for a population of orca off the coast of the Pacific Northwest (termed ‘the Offshores’), which feed on sharks (Ford et al. 2011). The differences between their teeth and those of the fish-eating and mammal-eating orca in the same region were compared. The distinction between ecotypes of orca, the state of their dentition and their feeding behaviour has been a reoccurring theme in a range of publications about the species in Mexican, Pacific Northwest and South African waters (Salinas and Urbán 2006; Dahlheim et al. 2008; Best et al. 2014).

Caldwell and Brown (1964) described tooth wear as a consequence of feeding behaviour and the wide jaw movements in orca, including vertical, lateral and palinal movements. In the current study, simultaneous wear to the cusp tips and lateral surfaces of teeth were the most common types, similar to what has been reported for other odontocetes, including small dolphins, orca and false killer whales (Loch and Simões-Lopes 2013). While lateral wear is the result of attrition due to tooth-to-tooth contact when the upper and lower teeth interlock in occlusion, cusp tip wear can be caused both by attrition during jaw movements (when the cusp tips of upper and lower teeth interact), or by abrasion due to contact with an external object when piercing, grasping and shearing food items (Caldwell et al. 1956; Loch and Simões-Lopes 2013). In particular, lateral wear affecting interproximal surfaces is a common feature of homodont teeth such as in orca, resulting from the interlocking of the dentition (Miles and Grigson 2003). Loch and Simões-Lopes (2013) reported higher wear frequencies in the Globicephalinae (orca and false killer whales), likely due to the larger size of their teeth and the fact they are always in contact when the jaw is closed, and the upper and lower teeth interdigitate.

Despite the high prevalence of wear, most teeth analysed here were only worn superficially, affecting the enamel and outer layers of dentine. The unworn teeth on the maxillae of each orca were potentially protected by the fibrous and effectively non-flexible upper lips. Severe wear was recorded in 21% of the teeth and was the most common wear type in two of the nine animals analysed. This is similar to the findings of Ford et al. (2011) in Resident (fish-eating) and Transient (mammal-eating) wild orcas from the northeastern Pacific, in which most wear was negligible to moderate. However, most Offshore (squid and shark-eating) orca had severe wear with teeth worn down to the gum line and sometimes exposing the pulp cavity (Ford et al. 2011). The severe tooth wear in the Offshore ecotype was linked to the abrasiveness of their diet. In NZ waters, there are at least two ecotypes which can be found regularly around the coastline; the Pelagic ecotype (cetacean-eating) (Visser et al. 2010) and the Coastal ecotype, which prey on rays (Duignan et al. 2000; Visser 1999b), sharks (Visser 2000a) and fin-fish (Visser 2000b), with elasmobranchs as the most common food type (Visser 2000a). It is possible that abrasive prey such as rays and sharks could contribute to some of the severe wear observed in the nine orca stranded on the southernmost coastline of the South Island of NZ. However, unlike northeastern Pacific offshore orca, which at times had teeth worn down to the gum line with exposure to pulp cavities (Ford et al. 2011), this was not observed in any of the orca analysed here.

Severe wear causing exposure of the pulp cavity has been described for many orca in captivity, and such dentition issues have been linked to morbidity and secondary infections (Heyning and Dahlheim 1988; Graham and Dow 1990; Visser and Lisker 2016; Jett et al. 2017; Marino et al. 2020). While severe wear in some wild orca is seen at the ecotype level as it is related to their feeding behaviour, in contrast, severe wear in captive orca is variable within facilities (Jett et al. 2017) and the causes are linked to being held in captivity (e.g. Graham and Dow 1990; Visser and Lisker 2016). Captive orca are never fed abrasive live prey (Jett et al. 2017), and dead thawed fish or squid are dropped into the back of the mouth, requiring little to no contact with the teeth, nor any manipulation (Marino et al. 2020). Oral stereotypic behaviours in captive orca include chewing, gnawing and biting tank surfaces, corners, gates, bars and grates as a potential stress response (Graham and Dow 1990; Jett et al. 2017; Marino et al. 2020), and manipulating the hard plastic and rope toys given to the orca by trainers (Visser 2019a). In captive animals, oral stereotypy is associated with sub-optimal living conditions, frustration, stress, fear, and lack of physical or mental stimulation (Jett et al. 2017). In wild orca, the rate of physiological wear seems to be matched by the rate of secondary dentine deposition through life. This means that while teeth might be worn, at times even down to the gum line, the pulp cavity will likely be obliterated and not exposed. Oral stereotypy accelerates the rate of wear in captive orcas, so pulp cavities become exposed to the external environment (Jett et al. 2017), even when the animals are young (Visser and Lisker 2016).

In this study, wear scores were similar among males and females; however, the small number of animals analysed needs to be taken into consideration. Wear scores were also strongly associated with total body length – here used as a proxy for age. This suggests that at least for free-ranging wild orca, tooth wear is a cumulative process that progresses as the animal's age (Caldwell and Brown 1964; Loch and Simões-Lopes 2013). Erosion/glossowear/hydrowear lesions were observed in four of the orca analysed, more commonly in the upper dentition. While erosion relates to chemical wear due to acid dissolution of dental hard tissues (Loch et al. 2013b), glossowear/hydrowear is proposed to be generated by tongue and water movements during suction feeding (Marx et al. 2023). Morphological features of erosion/glossowear/hydrowear in the orca studied here show smooth and glazed tooth surfaces with apparent shallow grooving due to the removal of enamel and dentine. However, the precise aetiology of such lesions is still unclear.

Calculus was present in eight of the nine orca analysed. Only Koropepe, the shortest/youngest orca, did not have calculus. For the animals with calculus deposits, they covered from 12 to 25% of the tooth surfaces. Calculus deposits ranged from light-grey ring-like mineral deposits around the cingulum, to more prominent black calculus covering considerable areas of the root surface. The teeth in the upper quadrants had more tooth surfaces covered by calculus, likely because the upper dentition is covered and sheltered by thick upper lips which are relatively inflexible. Mesial surfaces had the highest percentage of surface area covered by calculus, while palatal/lingual surfaces had the least. A strong positive correlation between the surface area covered by calculus and total body length suggests calculus deposition is cumulative over time.

The presence of calculus deposits in eight of nine orca, and the fact that six out of nine orca had calculus deposits in 100% of the teeth suggests this orca group was highly susceptible to calculus accumulation. The presence of calculus deposits is a risk factor for periodontal disease, a chronic inflammatory disease resulting in gum recession, alveolar bone loss, and potentially edentulism, both in humans and other mammals (Niemiec 2008; Oz and Puleo 2011; Nazir 2017). Periodontal disease starts with gum inflammation from the accumulation of plaque biofilms. Minerals from the saliva and crevicular fluids promote plaque mineralisation over time, forming dental calculus on tooth surfaces (White 1997; Oz and Puleo 2011; Akcalı and Lang 2018). The retention of plaque, increased saliva production and lack of calculus removal are known risk factors for periodontal disease in humans (White 1997; Struillou et al. 2010). While plaque retention and lack of calculus removal are plausible in orcas, increased saliva production seems less likely to occur due to the rudimentary salivary glands in cetaceans (Cozzi et al. 2016). In human populations with poor oral hygiene practices and low access to dental care, subgingival calculus can be extensive and is directly correlated with enhanced periodontal attachment loss (White 1997). The extensive calculus deposits observed in the orcas analysed here are consistent with a lack of calculus removal throughout life; however, their impact on periodontal attachment is less clear. No signs of antemortem tooth loss were evident in the orcas analysed and none of the teeth exhibited any root resorption, destruction of their structural integrity, or abscesses in the maxillae or mandibles, as has been described for other cases of severe wear in orca (Tomes 1873; Colyer 1938). Dark grey to black conspicuous calculus deposits were also observed in the cingulum region of orca and false killer whale specimens from Southern Brazil (Loch et al. 2011). All specimens analysed (three orca and four false killer whales) had calculus deposits which suggests this condition might be common in the Globicephalinae; however, caution is needed due to the low number of individuals examined. Loch et al. (2011) also observed calculus in other smaller odontocetes with smaller teeth, suggesting calculus accumulation is not related to tooth size.

Despite the limited number of individuals analysed here, a strength of this study is the detailed dental evaluation of all teeth of all members of this group, which almost certainly experienced similar environmental influences, disease susceptibility and diet, albeit over different periods due to age ranges. The analysis of tooth wear and calculus deposit patterns in both sexes added another layer of detail which has rarely been conducted in other studies, especially for animals within a single mass stranding event. However, the lack of age-specific information and the limited number of males and females suggest that the age and sex differences may only be indicative and as such should be interpreted with caution.

Another strength of this study was the partnership with local communities for the recovery, preparation and study of dental and skeletal remains. In New Zealand, Māori have a long and historical relationship with marine mammals. They represent abundance, and richness and are regarded as chiefly animals (Rodgers 2017). Early Māori interpreted stranded dolphins and whales as a gift from Tangaroa (the God of the sea), using the meat, oil and bones for a range of uses. Māori used whale teeth to make rei niho (whale tooth pendants). Although the local Rūnaka involved with the stranding and recovery process recognised that the orca teeth may hold limited value for carving due to their susceptibility to cracking and fracturing, there was a strong desire to keep the dentition with the skulls, to preserve them as a ‘family unit’ in recognition of their links and respect to whakapapa (genealogy) and tupuna (ancestral) connections. The opportunity to study and document the teeth of these orca was only possible because of this important partnership with Ōraka-Aparima Rūnaka, alongside the assistance and guidance of senior expertise, support from local Western Southland communities, and the Department of Conservation.

This study investigated patterns of tooth wear and dental calculus in a group of nine orca stranded on the southern coast of New Zealand, increasing our knowledge of dental characteristics of wild orca. As dental problems are prevalent and ubiquitous in orca held captive in theme parks and aquariums worldwide, investigations such as this one can help elucidate dental conditions that are common in the wild and those which are exacerbated by captivity, such as extreme wear resulting in pulp exposure. Future studies on deceased-stranded animals or specimens deposited in museums will shed further light on the occurrence, aetiology and implications of calculus deposits in orca and other marine mammals.

Acknowledgements

The authors warmly thank Iain MacCallum, Riki Dallas, Dean Whaanga, Ōraka-Aparima Rūnaka and their marae Takutai o te Tītī, and Te Rūnanga o Ngāi Tahu for support during this work. Thanks are extended to Stuart Hunter (Massey University), Jean Claude Stahl of Te Papa for photography, Ros Cole and the Department of Conservation staff involved in managing the stranding. Thanks also to Natalie Barefoot, Shaun Wilson, Jeff & Debbie Drain, Alan Harnett, Andy & Fletcher, Steve Hathaway, Dale Green of Fiordland Helicopters, and the local Colac Bay, Riverton, and Western Southland communities for their assistance during the stranding event including recovery logistics and support of our teams. Ingrid Visser and the Orca Research Trust thank dodoland (Orca EUGY) and Uber Ultrafast Broadband. Ingrid also thanks her Patreon supporters for their financial assistance during the preparation of this publication and she extends a special thanks to the Avatar Alliance Foundation and OceanX for their philanthropic support of her work and that of the Orca Research Trust, through the Global Orca Charity. Part of this research was carried out under a permit #DOCDM-310447 issued by the Department of Conservation to Ingrid Visser and the Orca Research Trust.

Disclosure statement

No potential conflict of interest was reported by the author(s).

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