It is not clearly proven that yawning is contagious in other species than humans. All humans yawn, but no one knows why. There are many theories, but no one knows for sure why we yawn.

Thanks for the answers! Geir Bjorklund there is one theory, lack of oxygen. Thx again for the answers, it s maybe a silly question but it s good to know the answer!

http://www.sciencedirect.com/science/article/pii/S037663571100252X

Sir

yawning is a sign of physiological fatigue indicate body needs rest in the form of sleep,rest,or other form. pcg,cli

We do not now, why anybody yawns... But we do know, that - probably because of mirrow-neurons - we can feel the same things as our counterpart does. So, if I see s.o. eat, I´d like to eat myself. If I see s. o. laugh, at least I smile, too. Mirrow-neurons excite the same regions in the brain that are excited in the person in front of us. So we yawn, if s. o. yawns, even if it´s a dog. This is called the "Carpenter-effect". See:

http://en.wikipedia.org/wiki/Ideomotor_phenomenon

See also: http://en.wikipedia.org/wiki/Yawn

In my animal behaviour class taught by a well known ethologist, Jane Brockmann at the University of Florida, we were taught that yawning is a learned behaviour and that it could be learned across species! This was in 2003 so things may have changed but I always thought that this was interesting.

It is a good question really, It is depend on lack of oxygen.

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0091773