Investigating the Domestication Syndrome Hypothesis: To What Extent Does Morphology Predict the Behavior and Physiological Stress Response of Dogs and Cattle?

The Domestication Syndrome Hypothesis posits that domesticated animal species exhibit a shared set of traits encompassing behavioral, physiological, and morphological characteristics. While this hypothesis has garnered significant attention, the evidence supporting its universality remains limited, particularly in species such as dogs and cattle. In this dissertation, I examined the Domestication Syndrome Hypothesis in mixed breed pet and shelter dogs as well as Holstein dairy heifers. This investigation focused on behavioral responses, physiological reactivity, and Domestication Syndrome Hypothesis-associated morphological variables in relation to human interaction. First, I explored the behavioral responses of Holstein dairy heifers representing a range of white spotting phenotypes towards people and handling, and their physiological response towards handling during an industry-standard palpation procedure. Additionally, I explored three pigmentation variables (coat color/pattern, eumelanin pigmentation, and white spotting) in dogs residing in a known stressful environment, an animal shelter. I assessed their baseline in-shelter cortisol and their cortisol response to an intervention in which they left the shelter to spend time with people. Lastly, I investigated citizen science behavioral surveys of owned mixed breed dogs to determine potential correlations between white spotting or ear set and various behavioral factors, including human sociability and aggression. Altogether, I found partial support for the Domestication Syndrome Hypothesis: some results aligned with the expectations of the Domestication Syndrome Hypothesis and some did not. Notably, in Holstein dairy heifers, I observed that less white spotting predicted a greater increase in cortisol levels and a higher rate of stepping in response to palpation, results that support the premises of the Domestication Syndrome Hypothesis. Moreover, in owned mixed breed pet dogs, I found that human sociability positively correlated with white spotting and floppier ears and that decreased arousal, decreased aggression, and increased proximity-seeking correlated with floppier ears, further supporting the Hypothesis. However, many of the correlations predicted by the Hypothesis were not statistically significant or exhibited an opposite direction to what was expected. For instance, more extensive white spotting in Holsteins predicted a higher rate of vocalization during palpation, pigmentation did not significantly predict cortisol levels in shelter dogs, and in owned mixed breed dogs playfulness and biddability negatively correlated with white spotting while biddability negatively correlated with floppier ears. Prior research on the Domestication Syndrome Hypothesis in dogs did not find covariation between domestication-associated behavioral and morphological traits, which the researchers suggest may have been driven by modern breeding practices; this confound could have also impacted my studies. Together, the results – both supportive and contradictory – included in this dissertation contribute to our understanding of the Domestication Syndrome Hypothesis. Further research is needed to reconcile these inconsistencies and explore the underlying mechanisms behind the observed relationships. By expanding sample sizes, using more standardized study designs, and investigating causative mechanisms, future studies can enhance our knowledge of the Domestication Syndrome Hypothesis and its implications in the domestication process of animals.