493.

Santiago Galapagos Mouse

Nesoryzomys swarthi

French: Oryzomys de Santiago / German: Santiago-Galapagosratte / Spanish: Raton de Galapagos de Santiago

Other common names: Santiago Nesoryzomys, Swarth's Nesoryzomys

Taxonomy. Nesoryzomys swarthi Orr, 1938, Sullivan Bay, Santiago Island, Galapagos Islands, Ecuador.

This species is monotypic.

Distribution. Restricted to Santiago I, Galapagos Is.

Descriptive notes. Head-body 110-190 mm, tail 100-163 mm, ear 19-25 mm, hindfoot 29-40 mm; weight 31-181 g (mean male 106 g, mean female 82 g). Living species of Nesoryzomys —an endemic genus of Galapagos Islands—are small (c.30 g) to moderately large (c.100 g) oryzomyine rats, with medium-sized ears and well-haired tails shorter than headbody length. Fur is medium in length and color varies from medium brown or dark brown to black; belly is paler. White subauricular patch is usually present. There are four pairs of mammae. The Santiago Galapagos Mouse is the largest species of Nesoryzomys . Males are slightly larger and heavier than females. Pelage is relatively long and dense. Dorsum is brownish brown, with bicolored or tricolored hairs, always with gray bases. Venter is paler than dorsum. Tail is bicolored, covered with hairs, denser at base and with loosely visible scales; apical tuft is barely present. Base oftail is covered with dense fur similar in color to the back. Legs are whitish.

Habitat. Arid and shrub areas, specifically mature cactus thornscrub dominated by Opuntia galapageia ( Cactaceae), Bursera graveolens ( Burseraceae), and Croton scouler: ( Euphorbiaceae).

Food and Feeding. Spool-and-line to track visitation to plants and dietary trials showed that the Santiago Galapagos Mouse ate fruits and plant matter, with narrow dietary breadth in wet season that broadened in dry season. Female Santiago Galapagos Mice had more diverse diets than males. Although dietary overlap was high between native and invasive rodents (e.g. Roof Rat, Rattus rattus) that tended to select fruits yearround, the Santiago Galapagos Mouse selected Opuntia .

Breeding. Reproduction of the Santiago Galapagos Mouse seems limited to the rainy season (January-April), with peak in pregnancy and lactation in April. Females have c.2 litters/year, each with 2—4 young. Median survivorship (interval between captures) was 199 days for males and 203 days for females, with a record of 812 days for a female. Longevity is estimated at more than two years for females and less than one year for males.

Activity patterns. Santiago Galapagos Mice are usually nocturnal, with peak activities between 20:00 h and 03:00 h; crepuscular activity was recorded, apparently connected to areas with high densities of Roof Rats. Santiago Galapagos Mice are terrestrial and take refuge in cavities on the ground or between rocks.

Movements, Home range and Social organization. Sex ratio was skewed toward males (1:0-77). Male home ranges (2-8 ha) were larger than females (0-99 ha) and typically overlapped multiple females’ home ranges suggesting promiscuous or polygynous mating strategy.

Status and Conservation. Classified as Vulnerable on The IUCN Red List. The Santiago Galapagos Mouse is threatened by non-native rodents introduced to Santiago Island, including the Roof Rat and the House Mouse ( Mus musculus).

Bibliography. Clark (1980, 1984), Dowler (2015), Dowler et al. (2000), Harris & Macdonald (2007b), Harris et al. (2006), Johnson (2009), Musser & Carleton (2005), Orr (1938), Tirira (2017), Tirira, Dowler et al. (2008a), Weksler (2006).