Dissertation
Dental size reduction in the domesticated silver fox: implications for the domestication syndrome model
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Spring 2020
DOI: 10.17077/etd.005402
Abstract
Domestication theory has long held that selection for tameness (i.e., mitigation of aggressive and fearful behavior) toward humans is the underlying, causative factor in the inadvertent emergence of a suite of morphological and physiological characteristics common among domesticated species. This phenomenon is widely referred to as the ‘domestication syndrome.’ Long-term domestication experiments, such as the Russian silver fox experiment, have demonstrated that intentional selection for tame behavior does result in inadvertent soft tissue and hormonal changes. Over several generations, tame-selected foxes developed piebalding, curly tails, and floppy ears, among other traits (including purported hard-tissue skull traits) also seen in dogs. A domestication syndrome feature commonly attributed to domesticated animals is a reduction in tooth size relative to their wild progenitors. However, the dynamic of selection for tameness resulting in reduced dental sizes has not yet been metrically quantified in tame-selected foxes. The primary goal of this dissertation was therefore to assess dental size variation in the experimental foxes.
Data were collected on a recent cohort of all three experimental trial groups in the Russian fox study, including the control, or ‘unselected,’ strain (n = 50), the tame-selected strain (n = 49), and the aggressive-selected strain (n = 50). Each sample included approximately equal numbers of males and females. A total of ten linear dental measurements were taken on the maxillary canine, the mandibular and maxillary carnassials (M1 and P4), and an accessory maxillary carnassial (M1). Measurements included mesiodistal length and buccolingual breadth at the cemento-enamel junction of all sampled teeth, as well as additional maximum height and crown height of the maxillary canines. Univariate and multivariate comparisons were conducted on the linear tooth dimensions and associated measures of palatal length and width to assess patterns of size, variation, and sexual dimorphism across the experimental fox strains. In addition, micro-computed tomography (μCT) scans were collected on a subset of 18 maxillary canine teeth to assess enamel and dentin volumetric proportions across strains.
Aggregate dental size was significantly smaller and more variable in the tame-selected strain compared to the unselected and aggressive-selected strains. This was especially the case in tame females. These results are in line with expectations outlined by the domestication syndrome model. The tame-selected strain exhibited no difference in palatal dimensions in the case of females and increased palatal lengths in males. Moreover, tame foxes showed weak covariation of dental and palatal measures. Together, these results suggest that the dentition in the tame-selected foxes is smaller even in the absence of palatal size reduction. Contrary to expectations of the domestication syndrome model and findings among other domesticated taxa, tame-selected foxes show an average 3% increase in sexual dimorphism across dental measures. Internal structural analyses also revealed differential scaling of enamel volume to total crown volume of the canine tooth in females and males irrespective of trial group, suggesting a sexually dimorphic, non-isometric pattern of enamel compositional growth as dental size increases.
While several authors have cited dental size reduction as a product of the domestication process, this is the first study to explicitly calculate the degree of dental size reduction in a controlled experimental population under domestication. Furthermore, the findings presented here substantiate claims that dental size diminution is an unintended by-product of selection for non-aggressive behavior. The results of this thesis research also have important implications for the ‘self-domestication hypothesis’, which posits that morphologies like dental size reduction are predicted by increased human prosociality. However, this study reveals that the nuances of this dynamic are more complex than the model currently accounts for.
Details
- Title: Subtitle
- Dental size reduction in the domesticated silver fox: implications for the domestication syndrome model
- Creators
- Emma Kathryn Wood
- Contributors
- Robert G Franciscus (Advisor)Russell Ciochon (Committee Member)Andrew Kitchen (Committee Member)Darren Hoffmann (Committee Member)Scott Maddux (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Anthropology
- Date degree season
- Spring 2020
- DOI
- 10.17077/etd.005402
- Publisher
- University of Iowa
- Number of pages
- xx, 290 pages
- Copyright
- Copyright 2020 Emma Kathryn Wood
- Language
- English
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (pages 239-290).
- Public Abstract (ETD)
One of the hallmarks of domesticated animals is their docile behavior toward humans. Many scholars have argued that breeding specifically for tameness results in the unintentional emergence of physical traits common across domesticated species (i.e., the ‘domestication syndrome’). One oft-attributed characteristic is a reduction in dental size, yet dental size variation in domesticated animals has not been heretofore studied in detail. Using a sample of foxes under domestication from a long-term, controlled study, this dissertation is the first to quantify the degree of dental size diminution as a result of selection for non-aggressive behavior.
Dental measurements were collected on sex-balanced samples of foxes from three experimental strains: unselected (n=50), tame-selected (n=49), and aggressive-selected (n=50). Univariate and multivariate comparisons of tooth crown area were subsequently conducted on molars, premolars, and canines, as well as height and structural comparisons of maxillary canine teeth.
Results show that selection for tame behavior results in dental size reduction within foxes. Moreover, dentition in the tame fox strain was highly variable and, against expectations, showed greater sexual dimorphism than unselected counterparts. The proportion of enamel increases as teeth get larger in females more so than in males, indicating sexually dimorphic patterns in dental composition. A general pattern of dental size reduction in line with expectations of the domestication syndrome model was confirmed, with potential implications for the associated idea of human ‘self-domestication.’ However, this study reveals that the nuances of this dynamic are more complex than the model currently accounts for.
- Academic Unit
- Anthropology
- Record Identifier
- 9983949592702771
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