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The Cranial Morphology of Large Wild versus Captive Felids

Bryttin Boyde & Hannah Selvey



Studies have shown that carnivores in captivity that are fed soft diets have more periodontal problems and that the morphology of their skeletons, skulls specifically, responds to environmental and behavior factors. Considering that captive diet affects teeth and gums, we have conducted research on Dr. Adam Hartstone-Rose’s hypothesis that variation will also be discernible through the examination of cranial morphology. We compared captive and wild lions Panthera leo and tigers Panthera tigris to see if differences were present, then analyzed data collected by a microscribe using a 3D morphometric analysis program. Next, Principal Component Analysis was completed to account for variation across eighty-one specimens, each with forty-three comparable landmarks. We found that captivity status is evident in felid cranial morphology and is even more pronounced than features of sexual dimorphism. The implications of this research could serve as possible bases for the reformation of captive diet. Thus, if a correlation between mechanical diet and detrimental effects on captive felids can be shown through research, measures to fix these may be taken.



Captive carnivores experience health disparities due to inadequate diets (Haberstroh et al. 1984). The genus Panthera (“big cats”) is a prime example of a carnivoran group that, when residing in captivity, is morphologically impacted by environmental factors such as mechanical diet. Although zoos and rescue centers attempt to provide food that mimics the nutrition these animals would acquire in the wild (Glatt et al. 2008), nutrition is not the only criteria a captive diet must meet in order to fully satisfy a wild animal’s physiological and psychological requirements: the consistency and texture of an animal’s diet, particularly those lacking the mechanical properties of a wild diet, such as bones and other elements that require chewing, can have adverse long-term effects on zoo animals, specifically carnivores (Glatt et al. 2008)
Enrichment beyond nutrition is important for zoo animals and much evidence supports the claim that captive animals’ diet affects them adversely. Skibiel et al. (2007) considered the lack of stimulation captive felids experience as a result of the absence of predator-prey relationships in their environments. Their study was based on the premise that captive animals are under-stimulated, both cognitively and physically, which creates negative repercussions on the animals that is then evident in their behavior. The animals’ temperaments and overall health were tested by supplementing their normal (mechanically and psychologically unstimulating) diets with bones and frozen fish. Results of this study thus confirmed that a diet closer to that which would be found in nature could possibly prevent negative changes due to under stimulation, both psychological and physiological, in captive animals.
Research has shown that, in addition to being under-stimulated, certain captive carnivores have experienced visible health disparities due to inadequate diets (Haberstroh et al. 1984). Captive Amur tigers, (P tigris altaica) have experienced oral health problems that researchers attribute to behavioral and environmental factors, including being fed soft, commercial, meat-based diets (Haberstroh et al. 1984). Haberstroh and colleagues suggests that factors beyond genetics influence dental pathologies. Despite a sufficient amount of nutrients, carnivores in zoos typically have more dental problems than those in the wild due to the lack of abrasive action that usually accompanies chewing on bones (Haberstroh et al. 1984). A few documented deformities include periodontal disease (Haberstroh et al. 1984), plaque buildup (Glatt et al. 2008) and hypoplasia (O’Regan et al. 2005).
Indirect “domestication” of animals exists and is impacting their morphology and creating further occurrences of dental pathologies. O’Regan et al. (2005) describes this phenomenon, discussing captivity’s unintended consequences, such as preventing the preservation of animals as they exist in the wild. Although recent measures to ameliorate the negative consequences of inadequate mechanical diets are currently under review, North American collections mostly still feed their captive felines a diet with physical properties that hardly require chewing to digest (O’Regan et al. 2005). Zoological morphologists have confirmed that differences in skull shape, including cranial thickness in captive lions (P. leo) have also been severely impacted by captive diets (O’Regan et al. 2005).
Hartstone-Rose et al. (2012) found support for the idea that what carnivores eat will ultimately affect their muscular and osteological masticatory architecture. Through analysis of masticatory muscles in captive felines, this research confirmed that a stronger bite force is correlated directly to the amount and size of obdurate, or hard, foods that the animals have consumed, thus supporting the notion that geometrical properties of food are indeed correlated to the feeding architecture of the studied carnivorous consumers (Hartstone-Rose et al. 2012). An additional study concerns reptilian consumers of obdurate foods and makes connections between durophagy, the practice of eating hard foods, and head shape (Schaerlaeken et al. 2012). Durophagous lizards were found to have significantly different head shapes and sizes from lizards that preyed on softer organisms. This study’s findings support the notion that studying resource use can aid in making deductions about a carnivore’s functional capacity. Because specialized diets have been correlated with the mechanisms required for digestion in carnivorous lizards, we can expect to find similar correlations between carnivorous felids and their diets in captivity and in the wild. A study conducted by Hartstone-Rose et al. (2013) supported that studying morphology of durophagous carnivores can tell us about diet. Using Principal Component Analysis (PCA), we can explore the morphological similarities and differences across a population of mixed captivity statuses and make inferences about the impact of captivity based on our observations.
One important factor driving the differences within this population is likely to be sex. Sexual dimorphism is evident in both lions and tigers (Mazak 2004 & Naples et al. 2012). At times, the sexes of tigers can even cause them to appear to be two difference species; in fact, an experiment using PCA yielded results showing that sex accounted for over seventy-seven percent of intraspecific variance (Mazak 2004). According to Mazak (2004), a large portion of the factors influencing the differences across sexes is related to morphology that is strongly correlated with predatory function, such as the rostra and zygomatic arches. This is significant when considering that lions are the most sexually dimorphic of the big cats (Naples et al.2012). A study of male and female lion specimens yielded results with substantial differences in the porosity of the skulls of the males as opposed to the females, which the authors deduced to be correlated with the lifestyle behavior of the felids (Naples et al.2012).
If indeed morphology is related to diet, we hypothesize that differences in mechanical diet across captive and wild populations of lions and tigers will be statistically discernible through PCA and with three-dimensional examination of the felids’ skulls. With knowledge of the diet of our carnivoran population, we will work backwards to see how morphology is specifically affected by that diet and eventually discover the ideal provisions for captive felids. We expect that, in addition to species, sex and captivity status will be responsible for the differences between the individuals in the sample population. Studying the impact of captivity on carnivores is vital to ensure the future health and safety of captive animals (Haberstroh et al. 1984). Therefore, in order to provide the best possible environments for carnivores in captivity, we must understand the impact captivity has on those carnivores.



To determine if differences exist across the two captivity statuses, captive and wild, of large felids, we implemented statistical analysis on three-dimensional renderings of the sample population of lion and tiger specimens (See Table 1). The sample population of specimens was sorted according to species, captivity status, and sex.

For the sake of this paper, “wild” refers to mature individuals who did not live in captivity and “captive” refers to mature individuals who resided in zoos or rescues. Samples were obtained from collections at the American Museum of Natural History (AMNH) the Smithsonial (USNM) and the research collection of Dr. Hartstone-Rose (University of South Carolina School of Medicine). These specimens originated at the Bronx Zoo, Central Park Zoo, New York Zoo, New York Zoo Society, New York Zoo Gardens, New York Park Commission, National Zoological Park (Smithsonian), Toledo Zoological Society, Academy of Natural Science, Barnum and Bailey, Prospect Park Zoo, and the Carolina Tiger Rescue.
Forty-three landmarks on the skulls of each specimen were recorded with a microscribe (Solution Technologies, Inc,). Microscribes can record common landmarks across objects, such as skulls, providing the shape of those objects relative to each other, using (x, y, and z) coordinates. Coordinates are used to code for three-dimensional renderings, which can then be entered into a Microsoft Excel document and viewed with various computer programs. Forty-three landmarks are as follows: 1 Foramen Magnum inferior, 2 Foramen Magnum superior, 3 Inion, 4 Vertex, 5 Nasion, 6 Rhinion, 7 Alveolare, 8 Infradentale, 9 Antero-lateral nasal corner L, 10 Buccal edge of maxilla at Canine L, 11 Distal P4* L, 12 Orbitale L, 13 Lateral orbit L, 14 Superior orbit L, 15 Medial orbit L, 16 Coronion (Coronoid tip )L, 17 Zygion L, 18 Porion L, 19 Tip of mandibular angle L, 20Antero-lateral nasal corner R, 21 Buccal edge of maxilla at Canine R, 22 Distal P4 R, 23 Orbitale R, 24 Lateral orbit R, 25 Superior orbit L, 26 Medial orbit L, 27 Coronion (Coronoid tip )R, 28 Zygion R, 29 Porion R, 30 Tip of mandibular angle R, 31 Anterior edge of Canine at premax/max sutures L, 32 Posterior edge of Canine L, 33 Anterior edge of lower p3 L, 34 Anterior edge of P4 L, 35 Anterior edge of masseter origin L, 36 Posterior edge of masseter origin L, 37 Superior edge of zygomatic arch at suture L, 38 Superior edge of masseter origin at thickest L, 39 Inferior edge of masseter origin at thickest L, 40 Anterio-superior corner of temporalis origin L, 41 Posterio-superior corner of temporalis origin L 42 Posterio-inferior corner of temporalis origin L, 43 Anterior-inferior corner of temporalis origin L.
The landmarks for each specimen were entered into an Excel Spreadsheet with columns respectively labeled: x, y, and z. The points for each specimen were entered into a text-only document, which we coded for upload into the three-dimensional geometric morphometric analysis program, Morphologika, (version 2.5). Morphologika translates the landmarks as recorded by the microscribe and plots them into an object that can then be viewed, moved, and analyzed (See wireframes in Figure 1 & Figure 2).
Renderings for each specimen were uploaded into Morphologika and we used Procrustes analysis to superimpose the landmarks across specimens so that they could be compared to each other in aligned space (O’Higgins and Jones 2006). Procrustes analysis accounts for changes that exist in coordinates due to the relative space where the specimen was located during the data collection. Procrustes analysis aligns object by “minimizing the sum of the squared distances between corresponding landmarks” (Von Cramon-Taubadel 2007). This prepares the three-dimensional population for Principal Component Analysis (PCA).
We then ran a PCA in Morphologika, comparing all the specimens to determine what the principle driving components were that accounted for the differences across specimens. The PCA output was graphed in Morphologika after all the principle components were calculated. We plotted PC1 and PC2 against each other (Figure 1), and PC2 and PC3 against each other on graphs (Figure 2). Because PCA only accounts for quantitative data, we examined the graphs as well as the differences in the wireframes of the specimens, generated by connecting some of the key landmarks, across the x-axis and y-axis to determine the principle components, or strongest factors driving variation across the sample population. Interpreting the markers for each type of specimen, we could easily interpret the driving factors behind the first three principle components (See Figure 1 & Figure 2).


Figure 1: The first two principle components of the lion and tiger 3D data. Individuals in the sample population are represented as they vary according to the two components. A clear distinction between species is visible along the x-axis and a clear distinction between captivity statuses is visible on the y-axis. Morphological variation is (marked) at each extreme. The right and left extremes of the x-axis represent P. leo and P. tigris specimens respectively. The upper range of the y-axis represents morphology characteristic of the captive felids and the lower range represents wild. A) Differences in rostrum length as it varies across (1) P. tigris and (2) P. leo. B) Variance of mandibular angle across (1) P. tigris, and (2) P. leo. C) Biangular-anterior mandibular angle, or dome shape is described in (1) captive and (2) wild specimens. D) Skull width across (1) captive and (2) wild specimens.


Figure 2: The second and third principle components of the lion and tiger 3D data. Individuals in the sample population are represented as they vary according to the two components. A clear distinction between captivity statuses is visible along the x-axis and a clear distinction between sexes is visible on the y-axis. Morphological variation is (marked) at each extreme. The right and left extremes of the x-axis represent captive and wild specimens respectively. The upper range of the y-axis represents morphology characteristic of males and the lower range represents females. A) Differences in the shape of the maxillary area as it varies across (1) males and (2) females. B) Variance of mandibular angle across (1) males and (2) females.


Table 2: PCA output from Morphologika 2.5 with forty-three points of comparison: eighty factors influencing variance across population were found. The Eigen value, percentage of total variance, and cumulative variance are described in this table.



Graphing the PCA showed that species was the first principle component, accounting for 21.28% of the variation, and visibly separating the groups into lions and tigers with almost zero overlap on the x-axis (See Figure 1 & Table 2). The second principle component clearly represented captivity status, accounting for 15.58% of the variance, and separating the wild and captive specimens across the y-axis. The third principle component represented the sex of the individuals, accounting for 7.97% of the variance. Figure 2 shows the second principle component, captivity status, plotted against the third, with females occupying mostly the lower extreme of the y-axis and males occupying the higher extreme.
Differences in morphology were evident at each extreme of the x-axis and y-axis of each PCA test (See Fig. 1 & 2). Among some of the differences were the length of the rostrum, mandibular angle, flexion of the mandibular angles relative to the mandibular symphysis, and the width of the skull. Rostral length differed across species, as tigers were shown to have shorter rostra than lions, which is a trait that has been described by Sunquist (2002) and Christiansen (2007). Christiansen also describes increased nasal height in tigers and differentiates between canine heights across lions and tigers. Mandibular angles also varied across the species, as tigers showed mandibles wider at the top (i.e., bi-coronal breadth) and lions showed the widest point at the base of the mandible (i.e., bi-angular breadth).
Different skull shapes and differences in width were also observable across captivity status. Mandibular angle and rostrum length varied across lion and tiger individuals (Figure 1). Captive individuals appeared to have flatter heads than the wild specimens against whom they were compared. The width of the skull across specimens was also noticeably different, as captives seemed to have wider skulls than wilds. Within both species, mandibular angle variation was also evident across males and females.



As expected, the first driving factor, PC1 (21.28%) separated the two species, as obvious phenotypic differences exist between tiger and lion individuals (Sunquist and Sunquist 2002). As you move further right on the x-axis of the graph shown on Figure 1, you can see that the frequency of tiger individuals declines and the frequency of lion individuals increase. Unexpectedly, the second most important source of variation (PC2) was most influenced by captive status (15.57%) and not sex (which emerged as the key factor in PC3). This means that, after species, captive status is the most discernable characteristic across this population. Previous studies on both captive tiger and lion individuals have yielded results supporting the idea that captivity status affects morphology. Geordie Duckler attributed malformations in the external occipital region of tiger skulls to phenotypic plasticity, judging that significant differences between the examined captive and wild specimens of the study were due to “reduced jaw activity” (Duckler 1998). A similar observation was made by O’Regan, examining the cranial thickness in captive lion individuals (O’Regan 2005).
Differences across species were evident upon examination of the wireframe renderings of the specimens in Morphologika. One difference is a shortened rostrum in the tiger specimens relative to the lion specimens, which is consistent with descriptions of tigers (Sunquist and Sunquist 2002). This is a possible topic for future research. Mandibular angles also varied across the species, as tigers exhibited wider bi-coronal breadths and lions exhibited wider bi-angular breadth. The results of the PCA output are encouraging, demonstrating the ability of statistical analysis to account for observable qualitative differences across specimens. Variation of skull width and in biangular-anterior mandibular angle, or dome shape was observed differences across captivity status. Wild specimens were found to have more robust domes, than captive (Figure 1). Relatedly, captive specimens had greater skull widths relative to length than wild.
The third principle component, observed to be sex, accounted for nearly eight percent of the variance across the sample population (7.97%, respectively.) The fact that sex was the third principle component, behind species and captivity status, suggests that it is easier to tell the difference between captive and wild felids than it is to tell the difference between the sexes of the two species. One explanation for this phenomenon is that behavioral differences between sexes that occur in the wild due to hunting do not occur in captivity and therefore do not contribute to sexual dimorphism in captivity. A further question comes up in the analysis of these data: are females and males affected by captivity to different extents?
The results of this study supported the captivity status hypothesis and statistically confirmed that observable differences in cranial morphology do exist across species. These results are especially significant because sexual dimorphism is a known characteristic of both lion and tiger species (Naples 2012, Mazaak 2004). Whether the differences that occur are actually due to mechanical diet is a persistent question. The strength of captivity status as a component of difference in the data also opens up a new line of inquiry about sexual dimorphism. There are two questions to consider: How much do the mechanical properties of food really affect felids? In what other ways captivity status affects morphology of captive felids?
A study to further investigate captivity’s impact on cranial morphology will include more species of large carnivores as well as a control group. A type of zoo animal such as Zalophus californicus, the California sea lion, with a diet primarily made up of fish in captivity and the wild should therefore exhibit little to no differences in cranial morphology across captivity status if the morphology is mostly influenced by the food’s mechanical properties. If there are differences in morphology across captivity status, regardless of diet, other possible factors contributing to the differences in cranial morphology across captivity statuses may include genetic issues, for example those stemming from inbreeding.
If further studies yield similar results to those of this analysis and continue to show a direct relationship between captive diet and changes in cranial morphology of carnivores, these studies could be a possible basis for handlers and animal dietitians to rethink their policies regarding the mechanical diets of captive carnivores. If a mechanical diet that requires further engagement of masticatory muscles, because nutrients alone are insufficient for the proper maturation and health of captive felids, zoos must take proper measures to assure the health of captive specimens.



Thanks to Dr. Adam Hartstone-Rose for giving us this study to work on and guiding the research process. Thanks to Dr. Erin Connolly for reviewing this paper and providing guidance in the approach to writing it. Thanks to Joseph Villari and Kristen MacNeill for collecting the data used in this study. Thanks to “Elvin” Boone for technical support and helping with the installation of Morphologika at the workstation.




About the Authors


Bryttin BoydeBryttin Boyde

My name is Bryttin Boyde and I am from The Woodlands, Tx. I am currently a junior at USC and in the spring of 2015 I will graduate with a degree in Anthropology and minors in Biology and Chemistry. As a member of the Honors College I have experienced the opportunity of being awarded a SURF grant in order to pursue biological anthropology research in Dr. Adam Hartstone-Rose’s comparative anatomy lab at the USC SOM and have grown so much as a student researcher. The motivation behind this project comes from my desire to expand my knowledge of anatomy at the osteological level in order to fully comprehend everything that goes into masticatory muscle architecture, motor development and the effects of diet on both animals and humans. Being able to assist in this research has impacted me by directly enhancing my understanding of morphometrics and the effects that diet has on both captive and wild animals, and also how much attention this issue deserves. Moreover, the process of generating a scholarly research article has confirmed in me my want to pursue further research and publish in a well-respected anthropology journal. Additionally, the opportunity to publish in Caravel will add merit and accomplishment to my resume and help shape me as a skilled and experienced applicant to both national fellowships and graduate schools. My future plans include attending graduate school to get a Ph.D. in biological anthropology with emphasis on comparative anatomy, osteology and functional morphology with the end goal of being a researcher and teaching at the university level. All of this would not be possible without the assistance and support of my mentor, Dr. Hartstone-Rose, my research partner, Hannah Selvey and the Honors College SURF grant.


Hannah SelveyHannah Selvey

In 2015, I will graduate with a degree in biology and anthropology and continue to a graduate program in evolutionary biology with an emphasis on comparative anatomy. Acceptance into the TRiO Ronald E. McNair Postbaccalaureate Achievement Program taught me how to conduct scholarly research and encouraged me to continue to a Ph.D. level in pursuit of learning about and teaching and biological anthropology. The program also enabled me to apply for Magellan Guarantee funding, with which I have been continuing my research. I have also taken advantage of the undergraduate research funding opportunities in the Honors College to continue my work with my mentor, Dr. Hartstone-Rose, with the Exploration Grant this spring. My first paper will be in review for publication in a scholarly journal by the spring. Initially, Dr. Hartstone-Rose helped me pick this topic, due to my interest in cranial comparative anatomy, and has since helped me develop it into the paper that it is today. The most memorable part of my research process thus far has been probably collecting morphometric data back in my hometown, at the American Museum of Natural History. This research is particularly exciting because it has shown me that I really do want to take the research route to a professional career in what I love to learn about plan to eventually teach. In addition to the personal realizations and new goals I now have as a result of conducting research as an undergraduate, I now also have experience presenting my research, as I conducted oral presentations at the 2013 SC McNair Research Symposium and the 2013 SAEOPP McNair/SSS Scholars Research Conference, thanks to Melissa Kupfer with TRiO at USC, Dr. Hartstone-Rose, and Joe Valleri who helped with data collection.




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Software/Analysis Programs

”Introduction to Microscribe and Morphologika”
2011 Harcourt-Smith, Will. 35min. Paleo-Tech Concepts, Inc. Youtube.

Microsoft (2010) Microsoft Excel [computer software]. Redmond, Washington: Microsoft.

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