We covered a simple lickometer circuit previously. Another useful interface is a capacitive touch sensor. In contrast to light gate sensors, there are no extra photons floating around that could disturb an imaging device, and the touch required gives gentle tactile feedback. In practice, they can be much lower noise as well, in case of a simultaneous electrical recording.
The popular Qprox QT113 (datasheet) makes implementing a capacitive touch sensor dirt simple. If you’d like something more than a simple go-nogo signal, then the AD7746 24-bit Capacitance-to-Digital Converter is for you (datasheet). Using this chip, you can use a capacitance measurement as a proxy for pressure, or some other parameter. For example, if you use a large plate sensor, you can estimate the portion of the animal on the sensor at any one time. For most users, the QT113 will suffice. And its datasheet has some helpful diagrams to get your circuit off the ground.