Reproductive biology

In part, roosting group size reflects the mating systems of American leaf-nosed bats. At one extreme is the carnivore Vampyrum spectrum, which roosts in small family groups consisting of a pair of adults and up to three of their recent offspring. This species has a monogamous mating system, the only known example so far in this family, and adults share the prey they capture with each other and their offspring. It would not be surprising to learn that other carnivorous species (e.g., Chrotopterus auritus) are also monogamous. At the other extreme are colonially roosting species with harem polygynous mating systems, the most common mating system in this family. In these systems, single males aggressively defend groups of females against the intrusions of other males. Only a fraction (about 20% in Carollia perspicillata) of adult males possesses a harem at any given time; the majority of adult males are bachelors. Harems either form seasonally or remain stable in their female composition year-round. The most stable harems occur in the greater spear-nosed bat (Phyllostomus has-tatus), in which groups of unrelated females may spend their entire adult lives together. More ephemeral groups of females occur in two common fruit-eating species, Carollia perspicil-lata and Artibeus jamaicensis. Genetic studies indicate that harem males father most, but not all, of the babies born in their harem.

Another variation on the theme of polygyny in phyllosto-mids is the multiple-male/multiple-female group. An example of this mating system occurs in the common vampire, Desmodus rotundus. In this species, stable groups of eight to 12 adult females live together in roosts with several adult males, which form a dominance hierarchy regarding mating rights. When roosting in hollow trees, males fight with each other for access to the top of the roost, where most matings take place. In the phyllostomine (Macrotus californicus), seasonal aggregations of adult males and females form in which males defend preferred roosting sites against other males. They attract females for mating with wing flapping and vocalization displays.

Like all bats, American leaf-nosed bats are low-fecundity animals. Females almost always give birth to a single young once or twice a year. Insectivorous or carnivorous species and vampires tend to be monestrous and undergo a single pregnancy each year. Most plant-visiting species, in contrast, are polyestrous and undergo two pregnancies a year. In Central America, one birth occurs in the late dry season (March-April) and another occurs in the middle of the wet

The vampire bat (Desmodus rotundus) feeds exclusively on the blood of invertebrates. (Photo by © Michael & Patricia Fogden/Corbis. Reproduced by permission.)

season (July-August) in these species. Gestation periods are relatively long and last about four months in carolliinines and many stenodermatines. It lasts about seven months in the common vampire. In at least two species, Macrotus cali-fornicus and Artibeus jamaicensis, gestation is prolonged as a result of delayed embryonic development.

The timing of reproduction in the glossophagine (Lep-tonycteris curasoae) shows interesting intraspecific variation. Its general pattern is monestry, but the timing of births varies geographically. In northern South America and northern Mexico, births occur in May after a five-to-six-month gestation period. In southern Mexico, births occur in December after a similar gestation period. May births coincide with the flowering seasons of columnar cacti in the Sonoran Desert of northern Mexico and in the arid regions of northern South America. December births in southern Mexico coincide with the flowering seasons of tropical dry forest trees and shrubs. In Mexico, most of the mating activity that produces these babies occurs in caves located in south central Mexico, far from the maternity sites. After mating, females migrate north (in the spring) or south (in the fall) to form maternity colonies. Genetic studies indicate that populations belonging to these reproductive demes currently undergo substantial gene flow. Different reproductive schedules have not resulted in genetic isolation in this species in Mexico.

With a few notable exceptions, parental care is not extensive in this family. Most young phyllostomids are weaned about six weeks after birth, and few young have any further contact with their mothers (or fathers). Extended parent-offspring contact occurs in at least two species, the spectral (Vampyrum spectrum) and the common vampire (Desmodus rotundus). In the former species, young bats remain with their parents long after weaning and are provisioned with vertebrate prey while they are learning to hunt for them selves. Young vampires remain with their mothers for up to a year after birth. Their mothers sometimes feed them regurgitated blood after they reach three months of age, and they continue to forage with their mothers until they are one year old.

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