What does "animal warmth" mean?

Solution 1:

It simply describes, rather graphically, the comfort to be derived from the presence of other people, particularly people you are close to.

Solution 2:

Animal warmth is literally a warmth generated from within an animal or person, as opposed to external sources of heat. It is used figuratively to mean a coziness and closeness with another being. Here are a few examples of its use.

The first excerpt is from an old medical journal, just showing that it's an old term and that animal warmth is different than artificial heat.

Baltimore monthly journal of medicine & surgery, Volume 1, 1830

Besides there is something we know not what in animal warmth which cherishes far more than artificial heat. The knowledge of the fact seems to be as old as the days of King David but the rationale we cannot explain until we shall have learnt something more of the constitution of heat.

This excerpt from a textbook shows the connotation of closeness and nurturing.

Anglo-Irish Autobiography: Class, Gender, and the Forms of Narrative By Elizabeth Grubgeld, 2004

As a small child I often longed for the animal warmth of simple maternal love. I longed for some one to take me in her arms, to kiss me and to hug me, to rock me to sleep in her lap.

As does this example from the journal of poet Sylvia Plath.

The Unabridged Journals of Sylvia Plath, 1950-1962

So there it was, two hours of driving through the dark, the warmth of the people on either side of me -- animal warmth penetrates regardless of sensibilities and arbitrary barricades.