Do links have default inertial values?

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My model has two links, connected by a revolute joint. I apply a torque via SetForce() and the second link accelerates relative to the first as one would expect. The model.sdf only provides collision and visual information for the links. Why does the link not have infinite acceleration if I've not specified inertial values? Does Gazebo use some default inertial values if none are specified?

After adding large <inertial><inertia> values to the model.sdf the link accelerates more slowly than before, which sort of adds weight (get it?) to the case that Gazebo has default values for these parameters. If so, do you know what they are and where I can find more information about topics like this?