Chemosensory similarities among oils: does viscosity play a role?

It is widely thought that viscosity plays an important role in the perception of fats. Rats were conditioned to avoid oil suspensions by treating them with lithium chloride after they had ingested the oil suspension. Control rats received the same lithium chloride injection after they drank vehicle. Three, chemically different, oils were examined, triglyceride oil, silicone oil and mineral oil. Rats trained to avoid a 4% aqueous suspension of one of these oils, reliably avoided suspensions containing any of the other oils, although the rats could discriminate between the oils. The viscosity of the oil suspension was slightly greater than the viscosity of the vehicle alone. However, rats trained to avoid an oil suspension, did not avoid a fluid having a viscosity similar to that of the oil suspension. In order to assess the possibility that rats could sense the viscosity of the oil separately from that of the vehicle, rats were tested for their aversion to oils having viscosities much higher and much lower than that of the oil they were trained to avoid. Rats trained to avoid triolein having a viscosity of 67 cp, reliably avoided silicone oils having viscosities of 5 and 203 cp. However, rats trained to avoid oil did not avoid an oil-free fluid having a viscosity of 22-29 cp. A final experiment examined whether the use of viscous or non-viscous vehicles influenced the conditioned aversion. No significant effect of vehicle viscosity appeared. Thus, chemically-diverse oils are perceived, by rats, to have some perceptible attribute in common. It is proposed that this common physical attribute is boundary lubrication rather than viscosity.