Methods, systems, and apparatuses for an ingestible sensor device are described. The ingestible sensor devices may be swallowed by an animal to diagnose one or more conditions of the animal. The ingestible sensor device may include a sensor configured to receive a stimulus inside the gastrointestinal tract of an animal, wherein the sensor is configured to output a signal having a characteristic proportional to the received stimulus. The ingestible sensor device may further include a communications module that transmits a signal modulated with the sensor output signal and a housing configured to have a size that is swallowable, wherein the housing substantially encloses the sensor and communications module.