Using a large number of specific behavioral characteristics (position in the cage, position of ears, position of tail, etc) observed in the standard qualitative test for assessing silver fox behavior, we have developed a principal component matrix of orthogonal measures of behavior. The first component (PC1) provides a quantitative heritable phenotype that distinguishes between the aggressive and tame fox populations, and among populations and individuals derived from crosses between these two parental types ( and Supplementary Figure 1
). Thus PC1 scores for F1 foxes cluster about halfway between those characteristic of the two parental types, as do scores in the intercross F2 populations. Backcross-to-tame populations have PC1 scores segregating in a range midway between those in the F1 and tame populations, and scores for a backcross-to-aggressive population lie between those in the F1 and the aggressive populations. Using PC1 as a phenotype we have associated tame versus aggressive behavior with loci on VVU12, in a region orthologous to one recently identified in dogs and wolves as a locus for canine domestication (vonHoldt, et al., 2010
). Rat studies have identified two significant loci for domestication (Albert et al., 2009
). However, these rat loci do not correspond to the fox locus on VVU12. Fox chromosome 12 corresponds to a fusion of three canine chromosomes: 5, 35 and 12 (Supplementary Figure 5
). Because canine SSR markers were used to construct the fox genetic map, the fox map can be compared and aligned to the dog genome. The conserved synteny between VVU12 and CFA5 starts around 27 cM on the fox map and continues to the telomere of both chromosomes. Thus, the independent domestication of the fox (farm-fox experiment, reviewed in Trut, 1999
; Trut et al., 2004
) validates one of the major loci believed to be involved in the domestication of the dog.
A second behavioral component, PC2, corresponding to passive versus active behavior also maps to this chromosome. Although independent, by definition, the phenotypes measured by PC1 and PC2 are not entirely unrelated, in that activity can enhance differences in behavior that otherwise might be difficult to distinguish (e.g. an aggressive fox that attacks is more obviously aggressive than one that is passive; and a fox that greets the investigator and wags its tail is more obviously “tame” than one that is merely permissive). However, there are specific behavioral characteristics that contribute to the PC2 gestalt but not to PC1 and vice versa. Thus, in Supplementary Figure 6
“fox moved forward” is a major activity characteristic whether aggressive or tame and “ability to touch a fox in zone 5 or 6” is indicative of great passivity in either a tame or an aggressive fox. In PC1, pinned ears and tame ears readily distinguish aggressive and tame foxes, but these two ear conformations have little to do with distinguishing contrasts between the active/passive gestalt of PC2.
It seems evident that PC2 can enhance the expression of PC1. That is, if an animal is aggressive, passive behavior will reduce the expression of that trait (animal is wary but lies still) whereas active behavior will enhance the expression (attack, or avoid the investigator). In backcross populations the distribution of behavior is skewed toward the extreme of the recurrent parent, reducing the contrast between tame and aggressive behaviors. Under these circumstances PC2 will increase that contrast. We would therefore expect that whereas these are distinct principal components in a matrix composed of all populations, PC1 and PC2 could be correlated in particular backcross populations. This is in fact the case for the backcross-to-tame populations in which PC1 and PC2 are correlated (r = 0.75–0.8). In contrast, in F2 populations where the behaviors are more normally distributed, this is not the case (r = −0.06). As a consequence of this relationship between PC1 and PC2, the mapping of PC2 to VVU12 needs to be regarded with some caution - it could be argued that this may simply reflect enhanced expression of PC1.
PC2 described in this study has parallels to the “shyness-boldness” factor proposed as a fundamental axis of behavioral variation in humans and other species (Wilson et al., 1994
) and subsequently identified in studies of canine personality (Svartberg and Forkman, 2002
; Svartberg, 2005
; Saetre, 2006
) and found to be related to performance level in working dogs (Svartberg, 2002
). The relationship between fox PC1 and PC2 indicates that passive/active behavior is not context independent and can be influenced by overall animal motivation (e.g. driven by PC1). These results suggest that this “shyness-boldness” factor should be considered with caution because animal motivation in performing certain tasks can influence the evaluation of this personality dimension.
Although the multiple character/trait groupings using principal component analysis has been very useful in defining behavior, GWAS using this phenotype has been very challenging, with different outcomes in different segregating populations. The data in , , and make it clear that the same loci may determine different trait outcomes in different populations. The overall frequency with which each trait outcome associated with “tameness” rather than “aggressiveness” is observed is broadly consistent with the percentage of the tame genome in each population: around 75% in the backcross-to-tame, 25% in the backcross-to-aggressive and 50% in the F2 segregating populations (see Supplementary Table I
). It is not surprising, therefore, that we find differences in trait mapping (e.g. – ) between the backcross-to-tame, the backcross-to-aggressive and the F2 populations. It is surprising, however, that we find differences between the two backcross-to-tame populations. Although the frequencies of individual trait phenotypes remain very similar between these two populations (see and Supplementary Table I
), the VVU12 mapping profiles ( – ) are significantly different. This suggests that the loci on VVU12 may be expressed differently in different genomic contexts, depending on alleles elsewhere in the genome. This finding is consistent with the results from rats (Albert et al., 2009
) that demonstrated the existence of a five locus epistatic network influencing tameness. The size of our F2 populations is currently too small to evaluate epistatic interaction between loci on VVU12 and other less significant loci identified in fox pedigrees.
Tameness as a defining characteristic of domesticated animals comprises a very complex phenotype. In the informative fox populations described herein we have dissected apart multiple distinct traits that in different combinations produce a tame gestalt. Perhaps the most obvious example of this multiplicity is the combination of passive/active (PC2) with tame/aggressive (PC1) behaviors, which interact to create an impression of greater or lesser affinity/acceptance or aversion/fear in the interaction between fox and human.
Hare (Hare et al., 2002
) has shown that domesticated dogs can detect human intent (theory of mind), an ability which can provide the much needed mutual trust that is required for domestication. In the absence of language, communication must rely heavily on signals conveyed by motions or body language. These signals are provided by actions such as the positioning of the animal relative to the human interrogator, expressions of body language (ear position, tail wagging), and vocalizations. The suite of traits that combine to provide variations of the tame gestalt appear from the farm-fox experiment to be quite complex. It seems reasonable that a similar path was followed in the wolf/dog transition. The homology of loci described on dog CFA 5 and fox VVU12 attests to this similarity.
The data presented here will be important for studies of behavioral traits in mixed data sets that are often used in behavioral analysis of dogs and other species including humans.