Mammalian coloration is far from a trivial trait. To date, camouflage appears to be the primary explanation for overall coloration, whereas localized patches of colored fur are generally associated with intraspecific signaling (Caro 2005). Background matching may account for the white pelage, while black-and-white combinations are often associated with aposematism and conspecific signaling. Contrasting coloration can play a significant role in crypsis, mainly through mechanisms such as pattern blending and disruptive coloration, among other functions (Caro 2009). Additionally, Gloger’s ecogeographical rule associates darker coloration with more humid environments. As Delhey (2019) points out, the two most plausible mechanisms underlying this pattern are camouflage and protection against parasites or pathogens, with the latter potentially involving pleiotropic effects on the immune system.
Chromatic disorders in mammals result from either excess (hyperpigmentation or melanism) or a deficiency (hypopigmentation) of melanin (Abreu et al. 2013). Hypopigmentation disorders include albinism, leucism, piebaldism, and hypomelanism (such as flavism, erythrism, and rufism). Albinism is an inherited condition characterized by a complete lack of melanin due to the absence of enzyme tyrosinase. Leucism is a condition characterized by a total or partial loss of pigmentation throughout the body, resulting in a white, whitish, or yellowish appearance. This condition is caused by a recessive gene that inhibits melanin synthesis, although it rarely affects hairless areas, such as the nose, feet, and other exposed skin, and it never alters the pigmentation of the iris (Miller, 2005). As a result, the eyes and/or extremities may retain their original coloration (Fertl and Rosel 2002; Miller 2005; Acevedo and Aguayo 2008). Piebaldism is characterized by localized areas of depigmentation resulting from the absence of melanocytes in affected skin and hair follicles, typically caused by mutations in several genes (Lamoreux et al. 2010). Hypomelanism results in hair varying shades of red or yellow, while eye coloration remains unchanged. This condition is linked to the synthesis of pheomelanin and may be caused by mutations that affect melanin biosynthesis, pigment granule trafficking, or membrane sorting (Lucati and López-Baucells, 2017).
These conditions are generally considered rare in wild populations (Abreu et al. 2013), as they are often associated with various pathologies, including visual (Pérez-Carpinell et al. 1992; Grant et al. 2001; Garipis and Hoffmann 2003) and immunological defects (Carretero et al. 2009; Summers 2009). Hypopigmentation may increase an individual’s conspicuousness, thereby raising susceptibility to predation (Krecsák 2008). Conversely, in some cases, it may reduce predation risk if predators exhibit neophobia, or fear novel stimuli (Mappes et al. 2005)
Hypopigmentation conditions have been documented across all major vertebrate groups in the Neotropics (Abreu et al. 2013), including fish (e.g., Wakida-Kusunoki et al. 2022; Paschoal et al. 2024), amphibians (e.g., Sanabria et al. 2010; Valdez-Villavicencio and Peralta-García 2014), reptiles (e.g., Silva et al. 2010; de Noronha et al. 2013; Sosa-Cornejo et al. 2022), birds (e.g., Ayala-Pérez et al. 2013; Reséndiz-Cruz and Caballero-Jiménez 2016; Palacios-Vázquez 2016; Salgado-Flores and Rodríguez-Ruiz 2022), and mammals (e.g., López-González 2011; Camargo et al. 2014; Arriaga-Flores et al. 2016; Lucati and López‐Baucells 2017; Ramos-Luna et al. 2022). In Mexico, several cases have been reported in bats (see Uieda 2000), including albinism in the common vampire bat (Desmodus rotundus, Uieda 2001; Ramírez et al. 2010), piebaldism in the common fruit bat (Artibeus jamaicensis, Sánchez-Hernández et al. 2010), and leucism in Waterhouse’s leaf-nosed bat (Macrotus waterhousii), the free-tailed bat (Tadarida brasiliensis), the common fruit bat (A. jamaicensis, Sánchez-Hernández et al. 2012), and the ghost-faced bat (Mormoops megalophylla, Hernández-Aguilar and Santos-Moreno 2018). Other documented cases in carnivores include the coyote (Canis latrans, López-González 2011) and the neotropical otter (Lontra longicaudis annectens, Arriaga-Flores et al. 2016), as well as in primates such as howler monkeys (Ramos-Luna et al. 2022) and marine mammals like dolphins (Ortega-Ortiz et al. 2022), to name a few.
In opossums (order Didelphimorphia), hypopig-0mentation has been reported in Brazil (Didelphis marsupialis, Abreu et al. 2013), Colombia (D. marsupialis, Hoyos et al. 2020), Panama (Caluromys derbianus, Fuentes et al. 2024), and Mexico. In Mexico, cases have only been documented on the Yucatán Peninsula, including an albino D. virginiana (Cuxim-Koyoc et al. 2020) and a case of flavism (characterized by cinnamon-colored fur in Didelphis sp.; Tenorio-Rodríguez et al. 2024). This is particularly notable given that opossums (genus Didelphis, Linnaeus, 1758) are widespread and relatively common across Mexico, occurring in nearly all states except for the Baja California Peninsula and some arid regions bordering the United States and the Central Plateau (Ceballos et al. 2002; Gardner and Sunquist 2003). On the Yucatán Peninsula, Didelphis is represented by two sympatric species, D. marsupialis and D. virginiana, with populations of D. virginiana appearing to dominate the northern region over D. marsupialis (Jones et al. 1974; Ruiz-Piña and Cruz-Reyes 2002).
Populations of D. virginiana in the Peninsula may correspond to the subspecies D. v. yucatanensis (Gardner 1973; MacManus 1974), which is reportedly smaller than D. v. californica, although both subspecies share the same coloration pattern (Gardner 1973): dichromatic, with a common dark phase characterized by black legs, feet, and ears; extensive black pigmentation on the tail; dark body coloration extending forward over the top of the head forming a wedge between the eyes; darker sides of the head and neck; and a prominent ocular stripe extending from in front of the eye to a pale spot at the base of the ear, distinctly outlining the white cheek area. If local populations are small, isolated, and subject to inbreeding, we might expect an increased frequency of developmental abnormalities, including coloration disorders (MacManus 1974).
Here, we presented the first documented case of leucism in Didelphis virginiana and the third recorded case of hypopigmentation for this species in the Yucatán Peninsula.
We conducted a photo-trapping survey targeting medium- to large-sized mammals associated with the wildlife crossing structures (pipes, box culverts, and wildlife underpasses) along the Nuevo Xcan–Playa del Carmen Highway, located in the municipalities of Solidaridad and Lázaro Cárdenas, Quintana Roo, Mexico. The region is predominantly flat, with an elevation ranging from 5 to 10 meters above sea level. The climate is warm and sub-humid, with annual mean temperatures ranging from 26°C to ٣٣°C, and an average annual precipitation of approximately ١,٣٠٠ mm, concentrated primarily between June and October (INEGI 2017). The natural vegetation in the area consists of subperennial evergreen forest at various stages of succession (Rzedowski 2006).
We set up 28 monitoring stations that continuously operated from July 2016 to July 2017. Cameras were placed approximately 50cm above the ground in the middle section inside the crossing structure, perpendicular to the entrance aimed at photographing all passing animals (González-Gallina et al. 2018). We considered a camera night a 24-hour period during which the camera was operating. Sampling effort per station was obtained by counting the days the camera was active subtracting days when the camera was not functioning and total sampling effort as the added number of camera nights for each station (Ramesh and Downs 2015).
As part of the project, we also recorded opportunistic observations of road-killed animals encountered while traveling between photo-trapping stations by vehicle. When the condition of the carcasses permitted, individuals were identified to species level using specialized field guides (Reid 2009). To differentiate Didelphis virginiana from D. marsupialis, we relied on diagnostic features such as cheek coloration, whisker color on the muzzle and cheeks, tail length, and the proportion of the tail covered in dark pigmentation. For each observation, we recorded the GPS location and took reference photographs. In the case of photo-trapping data, individuals of the genus Didelphis (D. marsupialis and D. virginiana) were grouped into a single category, as distinguishing between the two species was not feasible in most photographs.
On March 11, 2017, at 15:44 h, we documented a road-killed adult male opossum (Didelphis virginiana) with leucistic coloration along the highway connecting the Mérida–Cancún freeway with the town of Juárez, between the villages of Agua Azul and Juárez (20°50’18.36” N / 87°19’30.75” W) (Figure 1). While traveling this route, we encountered the specimen and examined it for diagnostic characteristics to distinguish between Didelphis species. These included uniformly white whiskers and a tail shorter than the body length, with the dark portion of the tail longer than the white portion—though in this individual, the contrast was less conspicuous (Figure 2)—allowing for identification as D. virginiana. The specimen appeared relatively fresh and based on the degree of rigor mortis and overall condition, we estimated that it had likely died during the night of March 10 because of a vehicle collision. This was the only individual exhibiting abnormal coloration that we encountered during our entire photo-trapping survey in the area.
In addition to this observation, during the survey, we recorded other roadkill specimens, including two opossums (Didelphis marsupialis), two gray foxes (Urocyon cinereoargenteus), two coatis (Nasua narica), and one Yucatán squirrel (Sciurus yucatanensis).
Throughout 10,166 camera trap nights, we obtained 33 records of Didelphis spp. using underpass structures to cross the highway. Of these, nine were inside wildlife-specific crossing structures, 19 utilized box culverts, and five employed concrete pipes.
Pigmentation anomalies in natural populations of Neotropical mammals have been considered rare (Abreu et al. 2013). To provide local context, from November 2012 to July 2013, the consultancy SEGA S.A. de C.V. conducted biological monitoring along the Nuevo Xcan–Playa del Carmen highway (Hidalgo-Mihart et al. 2013). This effort involved 54 camera traps across 81 photo-trapping stations, resulting in a total of 7,937 trap-nights. Opossums (Didelphis spp., as the species are difficult to distinguish in photographs) emerged as a dominant species, with 325 independent records. Also, González-Gallina et al. (2018) reported an additional 36 records of Didelphis. Across both monitoring efforts—361 total records—only a single case of color abnormality was detected within the genus. This single record of a pigmentation anomaly through roadkill rather than photo-trapping shows that opportunistic highway surveys can yield important complementary records (González-Gallina et al. 2016).
Hypopigmented animals are generally more susceptible to fitness declines compared to hyperpigmented (melanic) individuals. The occurrence of hypopigmented wild mammals appears to be more frequent in areas with high levels of human activity, where fragmented populations are more prone to environmental stress and inbreeding (Guestalla et al. 2021; Cotts et al. 2024). This pattern may reflect underlying environmental conditions in the Yucatán Peninsula, as all reported cases of hypopigmentation in Didelphis from Mexico originate from this region. However, in our study area, low population, isolation and inbreeding are unlikely explanations, as Didelphis (including both virginiana and marsupialis) was the most abundant species recorded (Hidalgo-Mihart et al. 2013). Still, these genetic disorders are naturally occurring (sometimes by random mutations) but rare (Abreu et al. 2013) as in this case this record represents 0.28 of all records.
If white pelage is better explained by background matching (Caro 2009), we propose that the light-colored karstic substrate that characterizes much of the Yucatán Peninsula (Bautista et al. 2015) may play a role in the survival of these hypopigmented individuals. The local soil type, known as sascab (meaning “white dirt” in Maya), could favor them by reducing their detectability to predators, thus mitigating the fitness costs often associated with conspicuous coloration. In this context, the cryptic advantage provided by sascab may reduce the non-physiological (e.g., predation-related) adverse effects of hypopigmentation observed in other environments.
This could help explain how the albino opossum reported by Cuxim-Koyoc et al. (2020) reached adulthood despite likely visual impairments—limitations not typically present in leucistic individuals such as the one we report here. It might also account for our observation of a leucistic female great curassow (Crax rubra) captured during photo trapping in the same locality (Hidalgo-Mihart et al. 2013).
We suspect that color patterns in mammals may be influenced by both habitat characteristics and human-induced factors (Ausband and Krohner 2022). Further research is necessary to better understand how selective pressures related to habitat, life history, developmental stage, and season contribute to the evolutionary mechanisms underlying external coloration. To advance this understanding, it is necessary to compile enough records of chromatic disorders across a wide range of species and taxonomic groups worldwide. This would allow us to begin identifying potential environmental patterns and, importantly, to determine which variables may promote the occurrence of such chromatic anomalies in wild populations.
Acknowledgement
For funding this project, CONAHCYT for the doctoral grant 335814/232663 given to A. González Gallina together with Conservación Panthera México A.C. and the Rufford Foundation for the grant number 22395-1.
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Associated editor: Beatríz Bolívar Cimé.
Submitted: January 14, 2025; Reviewed: April 23, 2025.
Accepted: July 6, 2025; Published on line: August 12, 2025.
