Monica Lopes-Ferreira and others at Brazil's Butantan Institute are studying the freshwater stingray species Potamotrygon rex to determine whether biological characteristics such as the rays' age and sex have _________ effect on the toxicity of their venom--that is, to see if differences in these traits are associated with considerable variations in venom potency.

Which choice completes the text with the most logical and precise word or phrase?