Nursing frequency and birth spacing in Kung hunter-gatherers

citation for long birth intervals in !Kung

Foraging and Menstruation in the Hadza of Tanzania

citation for shorter, but still 2.5 yr birth intervals for Hazda

Analysis of Factors Involved in Lactational Amenorrhea

citation for big variation in how long infertility lasts during nursing, depending on specific situation...in other words, you can't safely assume that the same thing applies to people with totally different nursing habits and totally different nutritional profiles.