Considering biological measures of maturity, notably investigated by Katherine Ann Dettwyler, yields a range of ages from years to 7 years as the weaning age analogous to other primates – the "natural age of weaning". This depends on the measure, for example: weaning in non-human primates is often associated with eruption of permanent molars (humans: to 6 years); comparing duration of nursing to length of pregnancy (gestation time) yields a factor of about 6 in chimpanzees and gorillas (humans: months = 54 months = years); body weight may be compared to birth weight (quadrupling of birth weight yields about to years for humans; of adult weight yields 5 to 7 years for humans); and similarly for other measures.