A new study sheds light on why dogs can be tamed, but not wolves; even if they are genetically closer to each other.

Evolutionary biologist Kathryn Lord at the University of Massachusetts Amherst suggests that different behaviors of the two subspecies of Canis lupus are related to their earliest sensory experiences and the critical period for socialization.

For the study, Lord examined seven wolf pups and 43 dogs in order to understand their developmental differences. She exposed the dogs and the wolf pups to familiar and new smells, sounds and visual stimuli and tested them during a four-week developmental window called the critical period of socialization.

She found that both the dogs and wolves developed their senses at the same time. When the socialization window opens, the wolves and the dogs begin walking and exploring without fear. They also retain familiarity all through their lives with those things they come in contact. As the period progresses, fear increases and when the socialization window closes, new sounds and smells extract a fear response.

Based on observations, Lord found that the both wolf pups and dogs develop a sense of smell at the age of two weeks, hearing at the age of four weeks and vision at the age of six weeks. But the critical period of socialization differs between these two subspecies. While dogs begin their socialization period at the age of four weeks, wolf pups are quick enough to start socializing at the age of two weeks. Interestingly, the wolf pups will still be blind and deaf when they begin to explore their environment. They primarily depend on smell at this stage, Lord said.

"When wolf pups first start to hear, they are frightened of the new sounds initially, and when they first start to see they are also initially afraid of new visual stimuli. As each sense engages, wolf pups experience a new round of sensory shocks that dog puppies do not," she added.

The period when socialization begins among wolf pups and dogs is completely different, and it likely leads to different developmental paths. This could explain why a dog needs just 90 minutes between the ages of four and eight weeks to socialize with a human or a horse. But a wolf needs at least a 24-hour contact starting before the age of three weeks and even then, it is less likely to get the same attachment to humans, Lord said.

The findings of the study appear in the journal of Ethology.