Two pus one makes thirteen: Senary numerals in the Morehead-Maro region

Southern New Guinea languages possess unusual senary systems, used for yam-counting. This article demonstrates the common presence of cognate base-6 numeral systems with monomorphemic power terms up to 6⁶, with attestation across the major branches of the Morehead-Upper Maro group. Kanum, related to the Morehead-Upper Maro family, has cognate forms but with ambiguity between the readings 6n and 'numeral in nth senary cycle', while Agöb in the neighbouring Pahoturi family has recently borrowed the senary power numerals. Evidence is presented for convergent cultural factors that would have selected for the emergence of a base-6 numeral system, including the six-petal arrangement in which they are laid out in piles and other aspects of ceremonial counting routines.