I'm going to load the guinea pig into photoshop and outline where I think improvements can be made and I'll tell you why I think so.
Unexpectedly, I disagree with Black Spot about the gradients of grey. I think a dark value such as what you have now is fine. Its the same concept with portraits - many portrait artists put a darker value next to the face for contrast purposes. With a pure white animal with occassional brown/black (which color?) fur, I think the extra contrast is an asset, if not needed. You could make a light or medium value tonal on the paper and lift the animals shape out, but drawing its shape would be much easier. Also, push the dark further with the guinea pigs eye. I'm doing the photoshop thing now - but I left my tablet at my apartment so you'll have to excuse my poor workmanship with the mouse I have.
Doctors heal you, Artists immortalize you.
"Those who can, do. Those who can't, teach" - bullshit.
The usual staples for anatomy: