Fig. 3. Representation of the four, non-mutually exclusive, candidate mechanisms impacting the relationship between per-birth twinning probability and total births.

Circles represent the three life-history events we considered: parity progression (PP), per-birth probability of twinning (T) and interbirth interval (IBI). The rectangles represent the variables potentially shaping these life-history events—maternal age and parity at a given birth (referred to as Age + Parity) and whether the last birth was a twin birth or not (Twin)—as well as a random effect capturing other sources of heterogeneity between mothers (Maternal identity). Black arrows represent relationships assumed in all simulation scenarios. Another random effect capturing differences between populations was also considered for all life-history events and all mechanisms (not shown). Red arrows represent relationships used to activate each mechanism.