okay so i'll give you a few hints on how i achieved some of the effects here.
all the animals were cut using different techniques, basically depending on the background color or simply how furry they were. i used the pen toon, lasso, color range and channels. not once i erased a part of an image, everything had been masked for further adjustments.
the shadowing has all been done the same way basically. just copy the layer you want shadowed/ highlighted, and add 2 adjustment layers to the newly created layer - exposure and curves. don't forget to check the 'create clipping mask' box. the idea is to drasticly lower brightness using those adjustments. once you've done that, add a mask to the copied layer (not the adjustments) and simply mask the parts you want highlighted again using a soft brush with very low opacity (around 10% is fine).
the snake was painted by moi - i have the same one at home which i used for quick reference and i painted the snake using my wacom tablet. it's quite simple really, 90% of the realism comes from shadows and highlights which i explained earlier in this text.
finally, i created a new layer, painted the center orange and dark blue around it, and set the layer to soft light with reduced opacity. that's how i got the split toning effect (that black donkey on the right is a great example - notice how parts of him are blue and the area around the nostrils is yellowish).
to top it all off i added the image to lightroom 4 and did some final nitpicking.