Hammer-headed Fruit Bat
Social Spacing and Organization
photo courtsey of: NotTooHappy (member of www.AboveTopSecret.com)
Roosting
Hammer-headed fruit bats usually roost in foliage but have been found to roost in caves and among rocks. It has been studied that daytime roost are inhabited typically by a single bat (Bradbury, 1977). Wherever they roost, the bat has been observed to roost in the daytime at heights of 20-30 meters and found to forage up to 10 kilometers from the roost at night (Nowak, 1994). During the daytime they are inactive and have been found to roost in groups up to 25 individuals, but estimated to roost in an assembly of 4-5 individuals (Boland, 2003). It has been recorded that a roosting group of 15 individuals contained at least one male, a few females, and several younger bats (Bradbury, 1977). High in the forest canopy they roost on exposed branches which are usually in areas with dense vegetation beneath (Boland, 2003). Hammer-headed fruit bats that roost together each have an individual space about 10-15 cm a part from its neighbor. These bats are solitary, the males and females only spend time together during the breeding season.The only significant hammer-headed bat social interaction is the bond between a mother and her offspring, who will roost closer than 10-15 cm. These large bats aggregate in mixed age and sex gropus (Boland, 2003). Early studies found that individuals may change their roost daily and other studies showed that individuals would use a roost for an extended period if it was not disturbed (Barclay, 1990). These bats show patterns of continuous usage of nocturnal roost sites in the foliage, but display frequent changes in diurnal roost areas (Lewis, 1995). Hammer-headed fruit bats either roost individually or in groups but never in contact while roosting. No sexual behavior or calling by males have been observed at daytime roosts (Bradbury, 1977).
photo courtsey of: NotTooHappy (member of www.AboveTopSecret.com)
Mating System
Breeding for the hammer-headed fruit bat occurs during the dry season. Early studies have shown that mating occurs twice a year, usually during the months, June to August and from December to February (Barclay, 1990). Other studies have reported that they breed in the middle of the rainy season and at the end of it, which are the months from August to September and from October to December respectively (Boland, 2003). Due to the female hammer-head having a postpartum estrus, they are able to give birth to offspring twice a year (Barclay, 1990). The hammer-headed fruit bat is cited to participate in a lek mating system.Traditionally, males visit and assemble during the night time along streams or even river beds. Males are assembled in a linear fashion, from 25 to 132 bats on a narrow tree branch (400-1600 m) (Barclay, 1990). The males demonstrate agonistic behavior in establishing their territory at these sites. Territories are defended by males and spaces are obtained about 10 m intervals a part from each other in the arena (Boland, 2003). Since the males have unique characteristics such as the enlarged larynx and a long wing span, these features allow them to compete and advertise to attract females (Griffin, 1986). The hammer-head male attracts females by loud repetitive honks which are accompanied by wing flapping (Fenton, 2003). These loud honks have been reported to be produced at a rate as many as 50-120 honks per minute (Smith, 1984). Females visit the nocturnal assemblies and when they approach a male, he increases his directed mating call (Fenton, 2003). Once a female chooses a mate, she positions beside of him and the male emits a high buzz call when chosen. Shortly after selection, a very brief copulation occurs (30-60 sec.) and the pair departs from each other independently to forage. Reports suggest that two mating sessions occur during the night and early morning, one approximately 9:00 PM and the other beginning at around 4:00 AM. This bats' lek mating system represent lek promiscuity because only a few males due most of the mating. Several reports have stated that only 6 percent of males participate in 79 percent of actual mating. Males that are successful at attracting a mate sometimes are clumped together in the assembly, which signifies the value of territory in the arena (Boland, 2003). The bigger males with a larger voice box and wing span are able to outcompete most males for mates. Females have their preference to which mate they will select and usually the clustered males are favored. The female preference hypothesis stats that females choose clustered males because seemingly those males facilitate efficient comparable phenotypes (Beechler, 1998). Lek promiscuity evolved in the hammer-headed fruit bat because females are widely dispersed resources and males can not defend resources or females. Distribution of food supply is a reason related to this type of lek mating system as well (Kolata, 1975).
Parental Care
Female hammer-heads produce mainly one offspring, but some studies have shown twins. Studies have reported that the one offspring is taken care of solely by the female, until it is able to forage on its own. Females invest essentials such as milk, protection, and grooming for their young (Boland, 2003). Due to the low amount of offspring produced by the female, the male hammer-head is not needed for parental care.
Why Solitary?
The solitary nature of the hammer-headed fruit bat is most likely a result of their feeding strategies. Males and female hammer-headed fruit bats have different foraging strategies, with males usually foraging at higer elevations for food supply (Boland, 2003). In contrast, females forage at mid-range levels for a medicore-quality food supply, this foraging strategy is known as the trap-line strategy. Often females maintain a patterned route where they feed on food patches that are dependable but not the highest quality of food. Males tend to forage for richer food patches, flying up to elevations of 10 kilometers (Barclay, 1990). Their different forageing strategies may demonstrate their different metabolic requirements, due to their marked sexual dimorphism (Boland, 2003). These large bats are able to forage independently because of their size and different necessity for certain fruits. There are no interactions between the male and female before and after breeding seasons. These bats are found to live in aggreagations and have no social interactions besides mother and offspring. Vampire Bats tend to have identifiable social units because males in this species tend to demonstrate a dominant heirarchy for mating, function, and territorial purposes. Vampire Bats defend mating territories where females might inhabit (McCracken, 2000). Hammer-headed females do not group together, therefore males can only participate in lek mating (Kolata, 1975). No dominant heirarchy has been reported in males.
Social Spacing
The hammer-headed fruit bat is a home range species. This bat focues their feeding and foraging strategies on specific fruits. Their main diet, figs, attract these bats because they offer huge crops of clumped fruits. Fig trees in the canopy produce hundreds to several thousands of fruits over an extended period of time. In their home range, these fruits are often clumped in their distribution and overall locally abundant but widely dispersed. Due to the distribution of their food and different foraging strategies, they have a sizeable home range (Dumont, 2003).
This page was created by Lamar Hull for an undergraduate Animal Behavior 323 course at 
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Please contact Dr. Case via email if you have and questions or comments: vecase@davidson.edu
