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Bats are one of the only other mammals besides humans that have problems with calcium deficiency, and reproduction by female bats may be limited by their intake of calcium. Bat pups are considered “expensive” to female bats because pups nurse until they are 95% of adult skeletal size (Kurta and Kunz 1987, Barclay 1995) and during that time the female provides their total nutrition through extended lactation. To meet the demands of extended lactation, female bats donate their own skeletal calcium to build the skeletons of their pups. The heavy demands of raising several pups can result in female's bones becoming less dense, similar to osteoporosis in humans, which results in an increased risk of wing-bone fractures. Wing-bone fractures and tooth loss can impede the ability to forage and may affect longevity, fitness, and overall health of the bat (Barclay 1995). Lubee Bat Conservancy funded PhD researcher Suzanne Nelson to test the “calcium-constraint hypothesis” which proposes that reproductive females were more constrained by calcium than energy in their diet (Barclay 1995). This hypothesis was tested using four different species of fruit bats, representing all sizes of bats and inhabiting three distinct locations. In all of the experiments, we examined whether bats attempted to increase their consumption of deficient nutrients by their dietary choices.
For most of our research we utilized the Tongan fruit bat ( Pteropus tonganus ) in Tutuila , American Samoa . The Tongan fruit bat is an excellent species to use in the study of bat nutrition because it has a wide distribution throughout the South Pacific Ocean , it is a midsize fruit bat (300-600g), and it prefers to eat in both native and agricultural habitats (Banack 1996, Miller and Wilson 1997). Therefore, this species is considered highly adaptable and an excellent general model for animal nutrition, with results that can be applied to other fruit bat species.
Our research utilized wild-caught Tongan fruit bats and several different methods to determine if bats sought out calcium in their dietary choices. To examine this at the landscape level, we fitted bats with radio-collars and monitored their activities several times per week to determine if they were feeding in calcium-rich habitat or calcium-poor habitats across the island. All bats highly preferred (p< 0.001, 70% of all radio-collar locations) the mixed agricultural habitat, which is intermediate in its calcium availability.
Our research also included feeding trials on wild-caught captive bats, where food choices were recorded by an infrared video camera. Bats were given choices of foods that were either low or high in calcium content, and their choices were recorded nightly. Overall, bats preferred (p< 0.001, 76% of trials) the low calcium, high-sugar fruits vs. the high calcium, low-sugar fruits. It was determined that the sugar content of the fruits was the basis for preference rather than the calcium content of the fruits.
Bats were also given a concentrated calcium block to use while in the food trials (the white block in the picture). The blocks were used by both of the reproductive females, and were used much more frequently by reproductive females than by males or non-reproductive males. This may indicate that females were preferentially using the calcium-rich blocks to relieve calcium deficiencies.
This research also documented that bats frequently consumed leaves. Leaf-eating, or folivory, was previously described as practiced infrequently and only by male bats. This research found that 83% of the captive bats engaged in leaf-eating, and that 70% of female bats ate leaves. When bats consumed leaves on a regular basis, leaves had the potential to increase their daily calcium consumption by 46%. This may be particularly important calcium source for rapidly growing young bats and reproductive female bats. The picture shows the unique way that Tongan bats eat leaves
Mineral calculations using apparent absorption indicate that Tongan fruit bats may be in mineral stress under the current dietary conditions in American Samoa . Their food choices seem to indicate a preference for high sugar fruits over high calcium foods. Bats may prefer high sugar, succulent fruits to relieve hypoglycemia and dehydration when first coming off the roost to forage. As a result of their preference for sugar-rich, calcium-poor fruits, bats may either be mineral stressed or have compensatory physiological mechanisms that allow their body to conserve ingested calcium. The method of eating used by fruit bats, by ingesting mostly juices and rejecting the fibrous portion of fruits, may result in higher bioavailability of the calcium that is consumed.
Despite potential mineral deficiencies, the population of Tongan fruit bats has increased three-fold in the last decade (Brooke 1998) and pregnant and lactating bats are seen throughout the year in American Samoa (Banack 1996). A very recent (2004) hurricane has struck the island and fruit bat population numbers have been highly reduced, which will place further stress on the population. If and how bats are able to rebuild their skeletons following pregnancy and lactation is a fascinating question that requires further testing and analysis.
Banack , S.A. , 1996. Diet selection and resource use by flying foxes, genus Pteropus, in the Samoan Islands : interactions with forest communities. PhD thesis, University of California , Berkeley .
Barclay, R.M.R., 1995. Does energy or calcium availability constrain reproduction in bats? Symposium of the Zoological Society of London . 67: 245-258.
Brooke, A.P., 1998. Biology of the flying foxes in American Samoa : Pteropus samoensis and Pteropus tonganus ; a report to the National Park of American Samoa . Pago Pago , American Samoa .
Kurta, A., Kunz, T.H., 1987. Size of bats at birth and maternal investment during pregnancy. Symposium of the Zoological Society of London . 57: 79-106.
Miller, C.A. , Wilson , D.E., 1997. Pteropus tonganus. Mammalian Species 552: 1-6.
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