Which vital sign is usually the least invasive to collect?

Weight is considered the least invasive vital sign to collect because it typically involves simply placing the animal on a scale without any physical contact or discomfort beyond that. This process is non-intrusive, allowing for accurate measurements without the need for invasive procedures or manipulation of the animal.

In contrast, measuring temperature often requires the insertion of a thermometer, which can be uncomfortable for the animal. Heart rate and respiration rate assessments may require more intervention, such as palpating a pulse or observing breathing patterns more closely, which could cause stress to the animal or might necessitate restraint. Thus, weight collection stands out as the most straightforward and least invasive method among the vital signs listed.