Sharks are cartilaginous fish. Rats have relatively small amounts of cartilage. Cartilage needs the movement of fluids for diffusion of nutrients. Fluid is the major component of the hydrotatic skeleton. Muscles act against the fluid.
Fluid is part of the cartilage matrix.
Fluid fills the synovial joint. Answers will vary but should include reduced movement. Answers will vary but should include ease of movement. Induced hypertrophy is not genetic. One could also argue that the father is setting an example and that could influence the children to work on their own hypertrophy.