The holotype of medius is BMNH 9.7.17.10, an adult male (skin and skull) from “Jimenez, mountains inland of Chocó, W. Colombia, 2400 feet” (Thomas 1909).

The holotype of orinomus is USNM 179157, an adult male (skin and skull), from “Cana (altitude 1,800 feet), in the mountains of eastern Panama” (Goldman 1912). Goldman (1912) figured the holotype skull.

Bassaricyon medius is a medium-sized olingo, smaller (on average) than Bassaricyon gabbii of Mesoamerica and larger than Bassaricyon neblina of the Andes. It requires closest comparison with the closely-related, allopatrically-distributed taxon Bassaricyon alleni, from which it differs especially in having (externally) less strikingly black-tipped dorsal pelage (which gives the pelage a slightly darker appearance in Bassaricyon alleni), (cranially) in its proportionally narrower and (on average) longer rostrum, and in having less inflated auditory bullae (Table 3), and (dentally) in its generally smaller p4 (Table 4). Bassaricyon medius medius is considerably smaller than Bassaricyon alleni (of South America east of the Andes), such that there is a clear body-size contrast between the two lowland olingo taxa of South America (Bassaricyon alleni east of Andes vs. B. medius medius west of Andes), but Bassaricyon medius orinomus (of eastern Panama and northwestern Colombia) is very similar in size to Bassaricyon alleni. Bassaricyon medius orinomus often has a reddish tail that contrasts somewhat with the less rufous head and body; Bassaricyon alleni tends to be more uniformly colored head to tail. In life, Bassaricyon alleni usually has a darkly pigmented nose, whereas in Bassaricyon medius the nose is often pink (Ivo Poglayen-Neuwall to C.O. Handley Jr., in litt., 9 February 1973; Figures 21–22). Sequence divergence in cytochrome b in these sister species (Bassaricyon medius, Bassaricyon alleni), separated by the Andes, is 6–7% (Table 2).

Bassaricyon medius occurs in forests from Central Panama to Colombia and Ecuador west of the Andes, where it is recorded from sea level up to about 1800 m elevation. We recognize two distinctive subspecies of Bassaricyon medius, distinguished especially by clear differences in size (Tables 6–7).

Bassaricyon medius medius (Figure 22) occurs in most of the South American portion of the range, where it is recorded west of the Andes in western Colombia (Thomas 1909) and western Ecuador (Lönnberg 1921, Parker and Carr 1992, Tirira 2008, Pinto and Tirira 2011a), in the Chocó region, on the western slopes of the Andes, and in outlying western ranges. It occurs in regional sympatry with Bassaricyon neblina at Otonga–San Francisco de las Pampas and probably elsewhere along the western versant of the Andes.

Bassaricyon medius orinomus (Figure 23) occurs primarily in the Central American portion of the range, where it is recorded from central and eastern Panama, from the vicinity of the Canal Zone in the west to the Darien Mountains in the east (Goldman 1912, 1920, Handley 1966, Mendez 1970, Kays 2000). In the Darien, records include Cerro Tacarcuna and Cerro Pirri (USNM series), and as these mountain blocks extend across the Colombian border, there can be no doubt this subspecies enters South America in northwestern Colombia. The two north-westernmost records of Bassaricyon medius in Colombia, from Villa Arteaga, Antioquia District (FMNH 69578) and from the Cauca Valley (AMNH 37797) have large anterior premolars relative to Bassaricyon medius medius and we tentatively attribute them to Bassaricyon medius orinomus, with Bassaricyon medius medius then recorded further south in forests of the Chocó and Western Andean slopes. The specimen from the Cauca Valley (from Puerta Valdivia, Antioquia District: Allen 1916) demonstrates that Bassaricyon medius also occurs in lowland forests in between the Western and Central Andes; another low elevation specimen from the Cauca Valley of Colombia reported by Saavedra-Rodríguez and Velandia-Perilla (2011) (UV-3774, Río Agua Sucia, Río Cajambre, 725 m) that we have not examined is also presumably Bassaricyon medius.

As noted above for Bassaricyon gabbii, the nature of interactions with the distribution of Bassaricyon gabbii on the western margin of the range of Bassaricyon medius in Panama (whether characterized by allopatry, parapatry, or sympatry) is unknown and worthy of study.

The first detailed description of representatives of this species was published in French by Huet (1883), based on two specimens from Chorrera, near Panama City (Thomas 1909, Goldman 1920), the first specimens to become available after the original discovery of Bassaricyon gabbii in Costa Rica in 1876 (Allen 1876). Thomas (1909) was first to name it (Bassaricyon medius), based on specimens from western Colombia, and Goldman (1912, 1920) described the larger Panamanian subspecies (orinomus) and provided brief notes on the species, noting its arboreal, nocturnal, and frugivorous habits, and its association in fruiting trees with Potos. More recent field studies in Panama (Kays 2000, Kays et al. 2012, Figure 23) are limited but likewise show Bassaricyon medius to be nocturnal, arboreal, frugivorous (and nectarivorous), and to feed in the same trees as kinkajous, which sometimes displace the smaller olingos. It is mostly solitary at night, spends its days in tree holes or other arboreal hide-outs, and usually has one young at a time. One olingo was found to have a home range of 37 hectares. Field workers have described this species’ vocalizations as “whey-chuck”, “wer-toll”, or “wake-up.”

Relevant field notes associated with Bassaricyon medius include: “shot at dusk in high tree in forest” (FMNH 29180); “shot at 8 pm, 40 feet up in large tree, active and agile, but curious, eyes shine brightly” (USNM 305748); “shot at 8:30 pm in avocado plantation” (USNM 305749); “shot near banana plantation (at night), stomach with banana” (USNM 305750); ” “shot at 8:30 pm in large tree in cafetal [coffee plantation], stomach with soft fruit with tomato-like seed” (USNM 305751); “shot at 8 pm in forest” (USNM 307037); “lactating” and pregnant with “1 embryo”, “stomach: fruit pulp” (USNM 310666); “shot in tree at night” (USNM 335767, 338348); “shot at night in tree in forest” (USNM 335769); “shot at night in tree in cocoa grove” (USNM 335770); “shot in small tree in plantain patch at night” (USNM 335771); “one embryo” in a pregnant female “shot in forest” (USNM 363342); “shot in banana tree” (USNM 363343)

Bassaricyon medius medius

Colombia: BMNH 9.7.17.10 (holotype of medius), 9.7.17.11, FMNH 29180, 86852, 90049, 90051, MVZ 124112, USNM 598997. Ecuador: AMNH 66752, BMNH 34.9.10.81, 34.9.10.82, EPN 841, 900, MECN DAP37, NMS A59-5081, A59-5082, QCAZ 8758, 8659.

Bassaricyon medius orinomus

Panama: USNM 171138, 179053, 179157 (holotype of orinomus), 179158, 179779, 179917, 206123, 284773, 284903, 284933, 284934, 284935, 305748, 305749, 305750, 305751, 305752, 305753, 305754, 307035, 307036, 307037, 310666, 310667, 310668, 324295, 324296, 335767, 335768, 335769, 335770, 335771, 338348, 338894, 363342, 363343, 363344. Colombia (tentatively attributed): AMNH 37797, FMNH 69578.