Obviously you have never been in a medical research laboratory and know absolutely nothing about dosing of animals for drug research purposes.
Lets take the RAT for example.
the dosage at which 50% of the rats die within a designated period of time becomes the LD50 rating for a specified substance being tested, and is designated in mg per kg of body weight of the animal species being tested.
Very accurate records are kept of the weight of each animal..rats are generally weighed daily on long term studies..(so I assume you step on the scale each morning) and also detailed records are kept on food in and food out.