The hardware for field testing has been narrowed to the parts below:

• MAX30105 (Sparkfun Breakout) Particle/Heart rate sensor: Technically the MAX30101 is for optical heart rate measurement, but as far as I can tell the sensors are identical. ALL the registers are identical, just the names and descriptions in the datasheets differ in some cases. The idea is to measure the heart rate on the exposed skin inside the ear of an animal
• MPU9250 9-axis IMU (Sparkfun Breakout): Widely used IMU with integrated 3-axis accelerometer, 3-axis gyroscope and 3-axis magnetometer/compass. Used to detect movement and possibly do some intelligent activity recognition. Other lower costs sensors might be considered later but the Sparkfun unit is convenient for testing.
• LMT01 Temperature Sensor: Ideal for creating a temperature probe to insert into the ear canal of an animal to measure body temperature. The sensor comes in small TO-92 package with a digital pulse train interface.

GPS will not be in the basic unit, but rather an optional extra where the application requires it. This is mainly due to the additional cost and current consumption of a GPS module. The two units with the lowest current consumption and a small form factor I have found so far are:

• OriginGPS ORG1410 Hornet: A tiny 10x10x5.8mm GPS module with an integrated antenna. Low current consumption
• Quectel L70: Small GPS module. Does not include an antenna