*3.2. Predicting Rarity*

Latitude but not body size predicted rarity in NW pitvipers. Bayesian phylogenetic models suggested an effect of latitude on rarity, i.e., the probability of a lineage being rare decreases with increasing latitude (Table S6).

#### *3.3. Rarity and Other Assessments of Extinction Risk or Prioritization*

When rarity patterns are compared to categorizations of extinction risk using IUCN categories and criteria (Table 1; Table S4), 20 species (91.0%) of the 22 threatened species (vulnerable, VU; endangered, EN; and critically endangered, CR) are in the higher rarity categories (high intermediate and rarest) and two (9.1%) are in the low intermediate category (specifically in the category E). For near threatened

(NT) species, two (33.3%) are in low intermediate, three (50.0%) are in high intermediate, and one (16.7%) in the rarest category. Just 22 (23.7%) of least concern (LC) species are in not rare, 38 (40.9%) are in low intermediate, 29 (31.2%) are in high intermediate, and four (4.3%) are in rarest. Finally, one (16.7%) data deficient (DD) species is in low intermediate, three (50%) species are in high intermediate and two (33.3%) in rarest. Additionally, there is a relatively high congruence in the distribution of the species in the rarest category (RT; Figure 2A) and threatened species (Figure 2C).

**Table 1.** Categories of rarity (based on Rabinowitz [10]) of New World pitvipers in relation to the categories of the IUCN. CR = Critically endangered; DD = data deficient; EX = extinct; EW = extinct in the wild; EN = endangered; LC = least concern; NT = near threatened; VU = vulnerable. Numbers for threatened categories (CR, EN, and VU) are in bold.


Furthermore, among our 15 rarest species, four are listed as endangered or threatened in the Mexican red list of threatened species [74], two were assessed as having a high threat index in Maritz et al. [43] prioritization, and six and three species were already suggested to show high vulnerability to extinction through EVS in Mexico [77] and in Central America [76], respectively (Table 2).

**Table 2.** Comparison of the categorization of New World pitvipers of this study (only the rarest, RT category) with those of extinction risk assessments by IUCN ([42] for the World), Secretaría de Medio Ambiente y Recursos Naturales ([78] for Mexico), Johnson et al. ([76] for Central America), Wilson et al. ([77] for Mexico), and the Threat Index of Maritz et al. ([43] for the World). Categories in the Mexican red list are: A = Amenazada (threatened) and P = En Peligro de Extinción (endangered). Category H (high vulnerability species) in the studies using the Environmental Vulnerability Score (EVS) system indicates a score ≥ 14 (see [75]). An empty cell indicates that the species was not assessed.

