Pursuing Parasites

Bats represent a spectacular radiation of mammals, diversified in both form and function. The more than 1200 living species live on all continents save Antarctica and feed on insects, fruit, nectar, fish, other small vertebrates, and even blood. Thanks to complementary studies in paleontology, systematics, and molecular biology, the major features of bat evolution are now understood.  However, we are only beginning to unravel the coevolution of a group of flies that have become parasitic on these mammals. The Field Museum's MacArthur Curator of Mammals Bruce Patterson and Research Associates Carl Dick (Western Kentucky U.) and Katharina Dittmar (SUNY Buffalo) are collaborating to understand this system more fully.

Although bats are diverse as mammals go and represent about 20% of all living species, they don't hold a candle to flies.  The insect Order Diptera contains 150,000 species, 10,000 genera, and nearly 200 families!!  And in all this variation, only 3 families are obligate ectoparasites; two of these--Streblidae and Nycteribiidae--infest bats (and the third is related to these and infests other mammals and birds). 

Bat flies live in the fur and on the wing membranes of bats, feeding periodically on blood. Some are flattened back-to-front so that they are hard to scratch off. Others are compressed side-to-side so they can deftly slip between hairs to escape bat grooming. Still others have long spider-like legs that enable them to scramble over their host's body. This is remarkable "aspect diversity" for such a closely related group of flies and seems to result from stringent selection pressures on the flies imposed by host grooming activities.

Now the extent to which parasites associate with a single class of hosts is termed host specificity.  Host specificity in most ectoparasites is strongest when:

• Parasites are small and have limited mobility
• All phases of the life-cycle are completed on the host’s body
• Hosts are solitary
• Hosts do not come into contact with other potential host species On these bases, we would expect bat flies to be weakly tied to any single bat species. After all, bats are highly social, sometimes roosting in groups as large as 12,000,000 individuals.  Bats roost in various structures and some of these--caves in particular—often support several to many species of bats, all roosting side-by-side and using the same cave entrance. And like other flies, bat flies must metamorphose in a pupa or cocoon, and this is deposited off of the bat (typically in the roost). This means that a newly emergent fly must locate a suitable host before it feeds. This task, difficult enough in itself, is complicated several times over by host specificity. Finally, although many bat flies are wingless and some have a single eye facet, others have highly complex eyes and fully functional wings, so they are clearly capable of colonizing greener pastures.  How and why are these flies so host-specific?! How is it possible for the bats to be unaware of these comparatively giant blood-sucking parasites clambering over their bodies?!

Bat diversity reaches its acme in the tropics.  During the last 9 months, our team has visited the epicenters of biodiversity in South America (Ecuador), Africa (Kenya) and Southeast Asia (Malaysia) to capture a diverse assemblage of bats, knowing that these are apt to support a diverse and equally endemic set of bat parasites. These samples are the final pieces needed to complete a comprehensive analysis of the phylogeny of bat flies of the world.  With this in hand, understanding trends in eye facet reduction or wing reduction, or body size variation will be greatly simplified. In addition, we will be able to assess whether the parasitic flies have speciated in lock-step with their hosts or whether "horizontal transfers" have been critical in the diversification of this group.

Some bats are thought to be reservoirs for human viruses (including rabies, Ebola, and Marburg). By feeding on multiple hosts, the flies are capable of transmitting these through the bat population. Thus, studies on the bat-bat fly system may have implications for public health. They have already enlarged our understanding of the host-parasite relationship.