The passage suggests that Jarvis’s work has called into question which of the following explanatory variables for naked mole rat behavior?

It is mentioned in the passage that, "Jarvis posits that growth rate may affect how long any individual rat performs a task, regardless of its age"

Then clearly explanatory variable should be rate of growth and not age, as passage clearly states 'Regardless of age'. So the variable that Jarvis' work is bringing forth is rate of growth.

And don't you think that the usage "Jarvis' " is better than "Jarvis's" as used in the passage to denote possession??

Please explain.