In order to reveal early species-specific differences, we observed the behavior of dog puppies (n = 11) and wolf pups (n = 13) hand raised and intensively socialized in an identical way. The pups were studied in two object-preference tests at age 3, 4, and 5 weeks. After a short isolation, we observed the subjects' behavior in the presence of a pair of objects, one was always the subject's human foster parent (caregiver) and the other was varied; nursing bottle (3 weeks), unfamiliar adult dog (3 and 5 weeks), unfamiliar experimenter (4 and 5 weeks), and familiar conspecific age mate (4 weeks). Dogs and wolves did not differ in their general activity level during the tests. Wolf pups showed preference for the proximity of the caregiver in two of the tests; Bottle-Caregiver at the age of 3 weeks and Experimenter-Caregiver at the age of 5 weeks, while dogs showed preference to the caregiver in three tests; conspecific Pup-Caregiver and Experimenter-Caregiver at the age of 4 weeks and dog-caregiver at the age of 5. Compared to wolves, dogs tended to display more communicative signals that could potentially facilitate social interactions, such as distress vocalization, tail wagging, and gazing at the humans' face. In contrast to dog puppies, wolf pups showed aggressive behavior toward a familiar experimenter and also seemed to be more prone to avoidance. Our results demonstrate that already at this early age--despite unprecedented intensity of socialization and the comparable social (human) environment during early development--there are specific behavioral differences between wolves and dogs mostly with regard to their interactions with humans.
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