Morphometric analysis of the skull in Indian wild boar using three-dimensional reconstructive imaging

Introduction

No single skeletal element reveals more about an animal's feeding ecology, sensory capacity, and social signalling than the skull. In wild boar, the cranium houses the powerful masseter and temporalis musculature that drives root-grubbing behaviour, supports the elongated nasal apparatus responsible for olfactory foraging, and bears the upper canine tusks used in malemale combat [1] . Quantifying skull dimensions therefore provides a window into both the functional biology and population-level variation of this highly adaptable species [2] . Traditional craniometric research in Suidae has relied on sliding and spreading callipers applied directly to dry skeletal specimens. While effective for simple linear distances, this approach suffers from inter-observer variability, difficulty reaching deep landmarks such as the basion, and an inability to capture three-dimensional spatial relationships among landmarks [3] . The advent of multi-detector computed tomography (CT) and surface-rendering software has transformed skeletal morphometry by producing high-resolution digital models that can be measured repeatedly without physical contact, rotated to any viewing angle, and archived indefinitely [4] . Among the four recognised wild boar subspecies, S. s. cristatus the Indian wild boar is the least well characterised morphometrically. Published data are limited to a handful of field reports based on small sample sizes and inconsistent landmark definitions [5,6] . This gap matters for several reasons. Wildlife forensic laboratories need validated reference datasets to distinguish wild boar remains from those of domestic pigs and feral hybrids in poaching cases [7] . Archaeological excavations across South and Central Asia regularly uncover suid remains whose taxonomic assignment depends on reliable craniometric norms [8] . And conservation managers planning population control strategies benefit from understanding body-size variation and sexual dimorphism patterns within the subspecies [9] . Sexual dimorphism in wild boar skulls is well documented for European populations, where males typically exceed females by 8-15% in total skull length and 10-18% in zygomatic width [10] . Whether S. s. cristatus shows a comparable degree of dimorphism has not been tested with adequate sample sizes or standardised methodology. Addressing this question requires both sexes to be measured using identical landmarks on calibrated 3D reconstructions. This research aimed to establish a set of 14 linear craniometric measurements from 3D-CT reconstructions of adult Indian wild boar skulls, to test for sexual dimorphism in each parameter, and to generate a correlation matrix and dimorphism ranking that can serve as a reference for future comparative, forensic, and archaeological work.

Material and Methods

Study area description

Skull specimens were accessed from the comparative osteology collections of the Madrid Research Institute of Comparative Anatomy and the National Museum of Natural Sciences (MNCN), Madrid, Spain. The specimens originated from Indian wild boar (S. s. cristatus) collected across various regions of the Indian subcontinent between 2008 and 2022, subsequently donated to or exchanged with Spanish institutions under CITES-compliant protocols. All specimens were from confirmed adult animals (≥ 2 years), as verified by third-molar eruption and cranial suture closure [1] .

Material

Thirty-four dry skulls (18 male, 16 female) with intact neurocrania and viscerocrania were selected. Specimens with pathological lesions, post-mortem fractures, or missing zygomatic arches were excluded. Each skull was labelled with a unique accession number and its sex, approximate age, and geographic origin were recorded from accompanying catalogue data. CT scanning was performed between March and November 2022 using a Siemens SOMATOM Definition AS 64-slice scanner (Siemens Healthineers, Erlangen, Germany) at the Veterinary Teaching Hospital, Madrid. Scanning parameters were: 120 kV, 200 mAs, slice thickness 0.6 mm, reconstruction interval 0.3 mm, bone kernel [11] .

Methods

DICOM image stacks were imported into OsiriX MD v.13.0 (Pixmeo, Geneva, Switzerland) for 3D surface rendering. Fourteen linear measurements were recorded on each reconstruction following standardised osteometric landmark definitions [3,12]: total skull length (Akrokranion-Prosthion), cranial length (Akrokranion-Nasion), facial length (Nasion-Prosthion), zygomatic width (Zygion-Zygion), cranial width (Euryon-Euryon), nasal length, nasal width, palatal length, palatal width, orbital height, orbital width, foramen magnum height, foramen magnum width, and mandibular length. Each measurement was taken three times on separate days by a single observer; the mean of the three readings was used for analysis. Intra-observer repeatability was assessed by the coefficient of variation (CV) and intraclass correlation coefficient (ICC). Sex differences were tested using independent-samples t-tests (or Mann-Whitney U for non-normal distributions, assessed by Shapiro-Wilk test). Pearson correlation coefficients were computed among all 14 measurements. A sexual dimorphism index (SDI = [male mean -female mean] / pooled SD) was calculated for each parameter and used to rank measurements by degree of dimorphism. Statistical analyses were conducted in R v.4.3.1 (R Foundation for Statistical Computing, Vienna, Austria); significance was set at P<0.05 [13] .

Ethical considerations

This research used dry skeletal specimens from existing museum collections; no animals were killed or captured for the purpose of this investigation. 0.89** 0.81** 0.83** **P<0.01. All pairwise correlations were positive and highly significant, reflecting the integrated growth pattern of cranial and mandibular structures. The strongest correlation was between total skull length and facial length (r = 0.91) Intra-observer repeatability was high across all measurements, with ICC values ranging from 0.94 to 0.99 and CVs below 2.3% for every parameter. This confirms the suitability of 3D-CT reconstruction for repeatable craniometric data collection.

When individual data points were plotted, males and females formed largely distinct clusters along the total-skull-length axis, with modest overlap in the 285-295 mm range. Foramen magnum height 0.65 0.08 Low SDI categories: High (> 2.0), Moderate (1.0-2.0), Low (< 1.0). Zygomatic width emerged as the single most dimorphic parameter, likely driven by the enlarged masseter muscle mass in adult males

Comprehensive interpretation

The craniometric dataset confirms that adult male Indian wild boar possess significantly larger skulls than females across nearly all measured dimensions, with the degree of dimorphism greatest in structures associated with masticatory force and tusk support. The strong inter-measurement correlations and high ICC values validate 3D-CT reconstruction as a reliable, repeatable platform for suid skull morphometry. These reference values now provide a quantitative baseline for forensic, archaeological, and conservation applications involving S. s. cristatus.

Discussion

Zygomatic width ranked as the most sexually dimorphic measurement (SDI = 2.15), a finding that parallels reports on European wild boar (S. s. scrofa) where zygomatic breadth differences between sexes range from 10 to 18% [10] . The zygomatic arch anchors the masseter muscle, and its greater lateral projection in males likely reflects the need for powerful jaw closure during intra-specific combat and tusksharpening behaviour [1,2] . This functional interpretation is supported by the correspondingly high dimorphism seen in mandibular length (SDI = 2.09), since a longer mandible provides a greater moment arm for the masticatory musculature. Total skull length showed a sex difference of approximately 12%, consistent with the 8-15% range documented for European populations by Genov et al. [10] . However, the absolute values recorded here (males 311.8 mm, females 278.3 mm) are larger than those reported for S. s. scrofa in the Iberian Peninsula, where adult male means cluster around 290-300 mm [6] . This size advantage may reflect the generally larger body frame of S. s. cristatus relative to its European counterpart, a pattern consistent with Bergmann's rule applied in reverse to a tropical subspecies whose skeletal proportions may prioritise surface-area-tovolume efficiency differently [5,8] . The near-absence of dimorphism in foramen magnum height (SDI = 0.65; P = 0.08) is noteworthy. Because the foramen magnum transmits the brainstem and vertebral arteries, its dimensions are constrained by neural and vascular requirements that are sex-independent, making it one of the least dimorphic cranial features across mammalian taxa [3,4] . This finding confirms that foramen magnum measurements are poor candidates for sex determination in forensic casework involving wild boar remains. From a practical standpoint, the correlation matrix (Table 2) offers forensic analysts a tool for estimating missing dimensions from fragmentary skulls. For example, recovering only the zygomatic arch would allow a reasonably accurate prediction of total skull length (r = 0.84) and, by extension, an approximate sex assignment based on the dimorphism thresholds reported in Table 3.

Limitations

A number of limitations should be acknowledged. The sample size (34 skulls), while adequate for detecting large effect sizes, restricts the generalisability of the reference norms to the full geographic and genetic range of S. s. cristatus. Specimens came from multiple collection sources with varying documentation quality; age estimates relied on dental eruption patterns rather than cementum annuli, introducing some uncertainty. Geographic provenance data were incomplete for several skulls, preventing analysis of regional within-subspecies variation.

Finally, only linear measurements were examined; geometric morphometric approaches capturing shape differences independent of size could reveal subtler patterns of sexual dimorphism that linear metrics miss [14] .

Conclusion

The Indian wild boar (S. s. cristatus) represents the least morphometrically documented subspecies within the Sus scrofa complex, and the absence of standardised craniometric reference data has limited progress in forensic identification, archaeological taxonomy, and population management. This research addressed that gap by applying three-dimensional computed tomography reconstruction to 34 adult skulls and measuring 14 osteometric parameters under controlled, repeatable conditions. Several findings stand out. Males exceeded females significantly in 13 of the 14 measurements, with zygomatic width, mandibular length, and total skull length showing the highest dimorphism indices (SDI > 2.0). These three parameters are recommended as first-choice discriminators for sex determination when intact or near-intact skulls are available. The strong positive correlations among skull dimensions (r = 0.76-0.91 for principal measurements) support the use of regression equations to estimate missing values from fragmentary specimens a common scenario in both forensic and archaeological contexts. For forensic wildlife laboratories, we recommend incorporating the mean and standard deviation values reported in Table 1 into reference databases used for species and subspecies identification of confiscated suid remains. For archaeological researchers working with South and Central Asian excavation material, the data can help distinguish S. s. cristatus from domesticated pig (S. s. domesticus) remains, provided that the known size overlap between large domestic breeds and small wild boar is taken into account. Looking ahead, expanding the sample to include juvenile specimens would allow construction of growth curves for each craniometric parameter, enabling age estimation alongside sex determination. Incorporating geometric morphometric techniques particularly Procrustes-based shape analysis would capture allometric and non-allometric shape variation that linear distances cannot detect. Comparing the present dataset with equivalent 3D-CT measurements from European, East Asian, and Southeast Asian wild boar subspecies would clarify subspecific boundaries and contribute to ongoing taxonomic revisions within the Sus scrofa species complex. Such a multi-subspecies morphometric atlas, built on the standardised 3D methodology validated here, would serve wildlife forensics, comparative anatomy, and conservation biology alike.

Acknowledgement

Acknowledgements Funding sourcesThis research was supported by an institutional grant from the Madrid Research Institute of Comparative Anatomy. No industry funding was received. Institutional supportThe authors thank the National Museum of Natural Sciences (MNCN), Madrid, for providing access to their comparative osteology collections, and the Veterinary Teaching Hospital for CT scanning facilities. Contributions not qualifying for authorshipThe authors acknowledge Dr. Javier Herrero Cortés for assistance with specimen catalogue verification and Ms. Lucía Navarro for help with 3D image processing.

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