In Nature, Plenty of Animals Pass the Paternity Test

A male black tufted-ear marmoset serves as the primary caregiver for his twin offspring. (J.E. Fite)

Only six more days until the holiday of, "I love you, even though . . ."

You've seen the cards. The ones that go like:

When diapers needed changing

You rarely left your rocker

You didn't make us breakfast

Or pick us up from soccer

You never do the laundry

Let's face it, you're a cad

But we will always love you

Because you are our dad.

These affectionately disparaging greetings reflect a myth as old as fatherhood itself. Dads, be they animal or human, are natural-born deadbeats -- big on courtship, short on childrearing and ever on the lookout for the next big fling.

Evolutionary theory even offers a scientific explanation for this presumed paucity of paternal instinct. Mom knows that the newborn she just bore is her own, but Dad cannot be absolutely sure who sired that screaming bundle. If the primary directive is to propagate your genes, it may be smart to minimize familial investment and use your free time to sow more seed.