Uhhh...I thought it was a type of wood !!
Some of both. Keep in mind that human babies take many years to grow up and gain the ability to live independently. They need stability more than most animals, who are able to hunt for food and fend for themselves often within their first year.
More of a social stricture than anything else, although there are some creatures that ar monogamous till death.
Social Structure.<br />
Most animals do not mate for life.