Sturnira lilium
little yellow-shouldered bat

Little yellow-shouldered bats are found in the neotropics, ranging from North Mexico (including Sonora and Tamaulipas) through Central America and tropical and subtropical South America to Chile. This includes northern Argentina, eastern and southern Brazil, and the Lesser Antilles. (Gannon, et al., 1989; Mello, et al., 2008; Stoner, 2001; Vieira and Carvalho-Okano, 1994; Wilson and Reeder, 2005)

Little yellow-shouldered bats occupy many different types of forest habitats, including mountainous forests (Mello, Kalko, and Silva, 2008), semi-deciduous tropical rainforests, and humid and semi-arid forests. They are also found in tropical lowlands and open areas, such as fields or farmland. They are common near streams or other bodies of water. They are not found at elevations over 1,000 m (Gannon et al., 1989). Little yellow-shouldered bats commonly roost in tree cavities in lower canopy levels, especially in mature trees with diameters 50% larger than surrounding trees. They prefer to roost in the tree species: Pimenta dioica, Metopium brownei, Vitex gaumeri, and Pseudobombax ellipticum, most likely because the heartwood of these trees easily decays, resulting in hollows (Evelyn and Stiles, 2003). They also roost in manmade structures and caves (Gannon et al., 1989). (Evelyn and Stiles, 2003; Gannon, et al., 1989; Mello, et al., 2008)

Little yellow-shouldered bats are medium-sized bats with total lengths of 62 to 65 mm, forearm lengths of 36.6 to 45.0 mm, and average wingspan of 30 cm (Vieira and Carvalho-Okano, 1994). Males tend to have slightly larger total length and cranial measurements than females (Gannon et al., 1989). Typical mass is 13 to 18g (Evelyn and Stiles, 2003). They have short, broad ears with a tragus that is one-third the size of the ear. They also have distinct nose leaves. No tail is present and calcars are extremely small or absent. Coloring varies by gender, age, and geography. Dorsal fur can be dark gray to reddish-brown and the ventral fur is always lighter than the dorsal fur. The head, neck, and shoulder hairs have a yellow tint due to yellow hair shafts and with dark-brown tips. Males tend to have yellow to red stained shoulder hairs that look like straps on a soldier’s uniform due to an excretion from their shoulder glands. It is this coloring that gives S. lilium the common name "little yellow-shouldered bat." The patagia is solid brown (Gannon et al., 1989). (Evelyn and Stiles, 2003; Gannon, et al., 1989; Nowak, 1991; Vieira and Carvalho-Okano, 1994)

Little yellow-shouldered bats are homoiothermic, with an average body temperature of 36.4 degrees Celsius and a range from 34 to 38 degrees Celsius. They have been known to survive with a body temperature of 41 degrees Celsius (Gannon et al., 1989). (Gannon, et al., 1989)

The mating systems of Sturnira lilium have not been extensively studied. Other phyllostomids do not tend to be promiscuous (Wilkinson and McCracken, 2006). Little yellow-shouldered bats tend to roost individually and the ratio of male testes size to body mass is relatively low, suggesting that sperm competition is not very high between males. Both of these factors decrease the likelihood of promiscuity in S. lilium, but the possibility cannot be ruled out. More research on the mating systems of S. lilium is needed. Reproduction of males corresponds with receptivity in females (Fleming, Hooper, and Wilson, 1972) and adult males secrete a strong scent from their shoulder glands attract a mate. This scent is what gives their fur the yellow coloration on their shoulders (Gannon et al., 1989). (Grzimek, 1990; Fleming, et al., 1972; Gannon, et al., 1989; Wilkinson and McCracken, 2006)

Little yellow-shouldered bats exhibit seasonal, bimodal polyestry (Fleming, Hooper, and Wilson, 1972), reproducing twice a year, once in the dry season and once in the rainy season. There is also a pattern of bimodal peaks in pregnancy and lactation in females bats, but pregnancy has been recorded in every month of the year (Nowak, 1991). Reproductive activity peaks in three periods throughout the year, generally February to June, October, and December (Fleming, Hooper, and Wilson, 1972). The exact length of gestation is unknown, gestation length in other phyllostomids is from four to seven months (Grzimek, 1990). Females give birth to one pup, but births occur twice a year, once towards the end of the dry season and once in the middle-to-late rainy season (Stoner, 2001). Biologists debate whether this seasonal birthing pattern is triggered by food availability or temperature cues (Mello, Kalko, and Silva, 2008). Information birth mass is unavailablebut, phyllostomids have the largest young to maternal mass ratio of any other bat family, with young weighing up to 14% of adult weight at birth (Grzimek, 1990). This larger birth mass is most likely the result of a longer gestation period, allowing for faster post-natal growth of young. Young phyllostomids are born with well developed hind limbs, open eyes, and fur when born. It takes from 5 to 20 weeks to wean young (Kurta and Kunz, 1987). Young phyllostomids tend to become independent 1 month after birth. Juveniles reach sexual or reproductive maturity at 8 to 11 months of age (Fleming, Hooper, and Wilson, 1972). (Fleming, et al., 1972; Gannon, et al., 1989; Grzimek, 1990; Kurta and Kunz, 1987; Mello, et al., 2008; Nowak, 1991; Stoner, 2001)

It is unclear whether females use maternity colonies to raise young. Though the young are well developed when born, they must be nursed by their mother for several weeks after birth. Male parental investment has not been documented. Frugivorous bats are more likely than carnivorous or insectivorous bats to carry young while foraging, so it is possible that females carry their young when looking for food (Jones, 2000). (Jones, 2000)

Little yellow-shouldered bats live for approximately 20 years in the wild. (Grzimek, 1990)

Little yellow-shouldered bats often roost alone, but they can roost in groups as large as 10. It is unclear whether males and females roost together or separately (Evelyn and Stiles, 2003). Little yellow-shouldered bats are nocturnal and relatively inactive during daylight hours. At night they fly through the forest understory, canopy, and open areas. They can fly quickly in open areas but can also maneuver well in dense forests (Jennings et al., 2004). Individuals in high altitudes may migrate to warmer areas during colder months due to a lack of fat reserves and an inability to sustain long periods of torpor (Mello, Kalko, and Silva, 2008). (Evelyn and Stiles, 2003; Jennings, et al., 2004; Mello, et al., 2008)

Compared to other South American bats, little yellow-shouldered bats tend to have relatively small ranges. Banding studies show that they roost and forage in the same area for several years at a time (Fleming, Hooper, and Wilson, 1972). (Fleming, et al., 1972)

Little yellow-shouldered bats use echolocation to navigate. Because they are frugivorous, their echolocation is not as specialized as bats that feed on insects. They use echolocation frequencies with low duty frequency and four harmonics. Low duty frequencies are excellent for short-range detection (Jennings et al., 2004). Little yellow-shouldered bats, like other bats, use vocalizations in frequencies audible to humans to communicate as well. The reduced horseshoe and spear structures on the noseleaf may suggest that these bats use olfaction to find food (Arita, 1990). Although nocturnal, little yellow-shouldered bats use visual information to evaluate their surroundings. Phyllostomids, including little yellow-shouldered bats, have larger visual centers in their brain compared to other bats, suggesting that vision plays a more significant role in their lives. Their night vision exceeds that of humans, which is a benefit when foraging. The yellow shoulder color of males also suggests that vision is important in sexual selection (Altringham and Fenton, 2006). The spicy smelling pheromones secreted from the shoulders of males are also important forms of chemical communication between potential mates (Gannon et al., 1989; Altringham and Fenton, 2006). (Altringham and Fenton, 2006; Arita, 1990; Gannon, et al., 1989; Jennings, et al., 2004)

Little yellow-shouldered bats are frugivorous. They prefer to eat fruits of plants in the Solanaceae family (nightshade family). When they are unavailable or low in abundance, these bats supplement their diet with fruits of the Piperaceae (pepper) and Cecropiaceae (nettles or cecropias) families (Mello, Kalko, and Silva, 2008). They occasionally drink nectar. Little yellow-shouldered bats begins foraging at dusk and tend to forage in the forest understory (Evelyn and Stiles, 2003). They search for food in shrubs and low trees but also forage at canopy level. They handle understory fruit quickly and canopy fruit slowly. Little yellow-shouldered bats can consume several fruits in a 5 to 15 minute time period. After eating, they pause to digest food, usually excreting within 20 minutes of consumption. They forage for up to 4 hours nightly (Bonaccorso, 1987). (Bonaccorso, 1987; Evelyn and Stiles, 2003; Mello, et al., 2008)

Known predators of S. lilium include humans, snakes, opossums, raptors, monkeys, and spectral bats (Evelyn and Stiles, 2003). Many of these predators are avoided by night foraging. Their dark color helps conceal them during foraging periods and flight is an advantage in escaping non-volant predators. (Evelyn and Stiles, 2003)

Little yellow-shouldered bats are important in the dispersal of seeds of tropical plants, especially those of the Solanaceae family (Mello, Kalko, and Silva, 2008). They are the primary dispersers of seeds of Solanum riparium (Iudica and Bonaccorso, 1997). They are also important pollinators of Mabea fistulifera and other flowering plants (Vieira and Carvalho-Okano, 1994). Their seed dispersal and pollination roles make them important in the regeneration of forests. (Iudica and Bonaccorso, 1997; Mello, et al., 2008; Vieira and Carvalho-Okano, 1994)

Many parasite species use little yellow-shouldered bats as hosts. Endoparasites, such as nematodes, and ectoparasites, such as mites, bat flies, and ticks are all found on this species (Gannon et al., 1989). (Gannon, et al., 1989)

In addition to helping regenerate economically valuable forest habitats, little yellow-shouldered bats and other frugivorous bats have positive impacts on farming and agriculture. They help in the pollination of crops and shade and support trees. Citrus, cocoa, coffee, allspice, mixed vegetation, and live fence vegetation have all been reported to benefit from these bats. Despite their appetite for fruit, farmers report that frugivorous bats such as S. lilium are not destructive to their main crops (Estrada, Coates-Estrada, and Meritt, 1993). (Estrada, et al., 1993)

Little yellow-shouldered bats have few negative impacts on humans. The most notable would be their potential to carry disease and viruses such as rabies, but interactions with humans are rare. (Grzimek, 1990)

Little yellow-shouldered bats tolerate and adapt to deforestation well compared to other species. They are the most abundant bat species in disturbed forest areas. Deforestation is a concern, however, because of their preference for roosting in large diameter, mature trees that are often harvested first (Evelyn and Stiles, 2003). Neither the IUCN Red List, US Federal List, or CITES suggest S. lilium is a species to be concerned about at this time. (Evelyn and Stiles, 2003; "IUCN Red List of Threatened Species", 2009)