Published: July 28, 2011

Pursuing Parasites

Bruce Patterson, MacArthur Curator of Mammals, Integrative Research Center

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.

Bruce Patterson

I study several topics in evolutionary biology, focusing on the diversification, distribution and conservation of mammals. The breadth of my research is testimony to the facts that no interesting biological questions are ever fully answered and progress towards answering them invariably opens up a variety of others.  Curiosity, opportunity, and a bit of wanderlust have diversified my program and caused it to span two continents.


Density of terrestrial vertebrate species ( Wonder why I study tropical animals?!

For most of my career, I have used museum specimens to study the systematics and biogeography of Neotropical mammals.  Collaborating with scientists and students in Ecuador, Peru, Bolivia, Brazil, and Chile, I have worked throughout the Andes, Amazonia, as well as Atlantic, Valdivian, and Magellanic Forests. While documenting some of the world's richest and most highly endemic faunas, we regularly discover and describe new taxa of marsupials, rodents, and bats and use them in regional and continental reconstructions of phylogeny and biogeography. The program offers abundant training opportunities for American and Latin American students, both in the lab and in the field.  Beginning in 2011, I started a parallel project on the The Bats of Kenya with colleagues Paul Webala and Carl Dick.  This project is designed to document the distribution and status of more than 100 species of bats that occur in Kenya and to shed light on their ecological roles and current status.

Collecting parasites in the course of these systematic studies led to my interest in host-parasite coevolution.  Ectoparasites recovered from mammals and birds are used to reconstruct the radiation of parasite groups and to assess their distributions across hosts and geography.  These studies identify factors that govern the distribution, abundance, and host specificity of parasites.  Together with Carl Dick (until 2009 a post-doc here at the Museum, now at Western Kentucky University) and Katharina Dittmar (SUNY Buffalo), we have developed a broad range of studies on the ecology, coevolution, and phylogeny of these interesting flies.  Interest in the unstudied ectoparasite communities of African bats helped fuel our collaborations with Kenyan Paul Webala to survey the diverse bat communities of Kenya.        


A Hipposideros bat with an ectoparasitic Penicillidia bat fly

A second, derivative program focuses on host-parasite coevolution.  Ectoparasites recovered from mammal and bird specimens are used to reconstruct the evolutionary radiations of parasite groups and assess their current distributions across hosts and geography, factors governing their distribution, abundance, and host specificity.  Work on bat flies has been developed with Carl Dick (until 2009 a post-doc here at the Museum, but now at Western Kentucky University) and Katharina Dittmar (SUNY Buffalo) on their ecology and phylogeny. With NSF funding, we recently curated the world's largest collection of flies, which now guides our understanding of host associations and fuels the taxon-sampling in our phylogenetic work (also supported by NSF). Undergrad and grad students are involved in this work in Chicago, Buffalo, and Bowling Green. Interest in the mostly unexplored ectoparasite communities of African bats helped fuel my collaboration with Kenya Wildlife Service ecologist Paul Webala on surveying the diverse bat communities of Kenya (see above). 

Photo by B. A. Harney in Tsavo, Kenya (July 2007)

A research program that I am now concluding focused on the Tsavo lions, infamous as man-eaters a century ago but more remarkable because many of them lack manes. In a series of papers, I have explored the morphology, genetics, behavior, and ecology of lions in SE Kenya with Samuel Kasiki (Kenya Wildlife Service) and Alex Mwazo (Kenyatta University), Roland Kays (NY State Museum), Jean Dubach (Loyola University), Justin Yeakel (UC Santa Cruz), and others.  Our aim has been to understand this distinctive and environmentally-plastic trait (manelessness) at genetic, hormonal, histological, anatomical, and behavioral levels. Concurrently, we gathered information to mitigate the impacts of lion depredations on livestock to ensure their continued survival and the preservation of their habitats. Until 2009, this project had the help of volunteers from the Earthwatch Institute.

As detailed in Students, interactions with undergraduate and graduate students enrich, extend, and complement these studies. All four research arenas offer opportunities for student research projects and post-graduate collaborations alike.