The best evidence so far that social supernumeraries are physically capable of bettering their lot concerns not rodents but birds.

[…]and also by all accounts of the surplus which shall from time to time be settled between the new company and the liquidators of the old company[…]

The data got lost, so I'll have to perform another run of the experiment.

The process of evolution of the specialised heterodont dentition of the Mammalia, or of the Theriodontia, from the simple homodont and polyphyodont dentition of the lower Reptilia would, I think, necessarily cause a reduction in number of the successional sets of teeth, due to an enlargement of one set and a consequent abstraction of growth, energy and material from the underlying sets.