In this activity you should use your knowledge of forces to explain how animals move!
Remember that it is normally the reaction force on the body of an animal which gives rise to the resultant force. To obtain a reaction force, an action force must be applied in the opposite direction. There must be a resultant force on the body to change the velocity of the body.
More reading....
Animal Locomotion
http://en.wikipedia.org/wiki/Animal_locomotion
Dolphins
Snakes
http://animals.howstuffworks.com/snakes/snake3.htm
http://www.ucs.louisiana.edu/~brm2286/locomotn.htm
Penguins
http://www.livescience.com/41998-emperor-penguin-huddle-physics.html