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Tenrecs - An Example of Evolution in Isolation
by Link Olson (Website)
When asked to name any endemic mammal from Madagascar, most people would be hard pressed to come up with a single example other than lemurs. In addition to its unique primates, Madagascar is also home to an equally distinctive family of small mammals called tenrecs. Relatives of the shrews, moles and hedgehogs (all members of a group called insectivores), the Malagasy tenrecs stand out as a classic example of island radiation and present evolutionary biologists with a natural experiment of evolution in isolation.
Madagascar, together with India, broke away from Africa more than 100 million years ago - well before the evolution of most modern groups of mammals. Thus, zoologists suspect that tenrecs, like lemurs, reached Madagascar by "rafting" on floating mats of vegetation across the Mozambique Channel that separates Madagascar from Africa. In Africa, tenrecs probably had to compete with other insectivores. Today, only three species of tenrecs are found in Africa, all of which have adapted to a semiaquatic lifestyle - a niche unoccupied by any other African insectivore. None of these other groups became established on Madagascar, opening the door for tenrecs to diversify into more than 25 species.
Biologists once regarded the Malagasy tenrecs as evolutionary relics whose persistence was attributable only to their protective isolation. This view has been toppled by the last two decades of research into tenrec diversity and life history. For a relatively small family of mammals, tenrecs have nonetheless evolved several impressive adaptations attesting to their evolutionary lability. Immediately obvious is the remarkable degree to which tenrecs have evolved to resemble other mammals like shrews, moles and hedgehogs - none of which are found on Madagascar. Several species are also capable of using echolocation, the sonar-like system found in most bats that involves using high-pitch sound to orient in the dark. Variation, the sine qua non of evolution, almost seems to have run amok in tenrecs, which vary more in body size than any other family of mammals and exhibit remarkable differences in metabolism. Making sense of this variation, however, is far from straightforward.
Problems Defining Species of Tenrec
One of the biggest hurdles to understanding the diversity of tenrecs has been the difficulty in delineating the species themselves. This is particularly true for the shrew-tenrecs in the genus Microgale. In the past decade, this genus has varied in the scientific literature as having between 10 and 20 species. There are three related reasons for this confusion.
The first is that Microgale species, despite their external differences, are surprisingly similar in adult dentition. Traditional mammalian taxonomy has relied heavily on the variation in dental patterns (both numbers of teeth and the morphology of individual teeth), a criterion that is poorly suited on its own for separating between every species of Microgale.
Complicating this fact is the second cause for confusion, which involves the unique pattern of tooth replacement in tenrecs that makes it difficult to distinguish adults from juveniles. Individuals with different combinations of milk and permanent teeth can appear dissimilar enough to warrant status as distinct species.
Finally, until recently, many species of Microgale were poorly represented in natural history museums. Consequently, the history of tenrec taxonomy has been fraught with new species being named based on a few museum specimens (and sometimes a single individual). As more specimens have been added to museums, giving taxonomists a better understanding of dental growth and variation within species, many of these "species" are now considered juveniles of previously named species at different stages of tooth replacement. Some taxonomists have taken this to the extreme, lumping many species together based on similarities in permanent dentition.
The consequences of this confusion extend well beyond the seemingly esoteric realm of taxonomy. Indeed, taxonomy is the foundation on which several other subdisciplines of biology (like conservation and evolutionary biology) depend. For example, the International Union for the Conservation of Nature's (IUCN) Red List of threatened animals includes six species of Microgale. One of the three species accorded vulnerable status, Microgale pulla, was described from a single individual. This specimen has since been placed within another species, Microgale parvula, which is one of the three species on IUCN's endangered list.
Tenrec Research at The Field
Fortunately, the situation is changing, largely because of the efforts of Field Museum biologist Steven Goodman who has spent the past decade in Madagascar conducting biological inventories. In addition to several new species (and even a new genus) of birds, Goodman and colleagues from the United States, Madagascar and England have discovered and described several new species of shrew-tenrecs. This involves not only collecting species never before captured, but also gathering additional specimens of previously described species used by researchers to confirm or refute older descriptions. Having more extensive collections from several localities throughout Madagascar makes it possible for Goodman and other taxonomists to gain a better understanding of variation within the same species (including geographic variation), as well as variation among different species. Both are crucial in determining the diversity of species.
My own graduate research builds on the work of Goodman and others. To address questions of biogeography and evolution, it is often necessary to know how different species of tenrecs are related (their phylogeny). In addition to comparing the anatomical differences used by zoologists in the field and in museums, I am looking at differences in chromosome structure and variation in individual genes. By analyzing these three separate types of data, a better understanding of tenrec relationships is beginning to emerge. Once this phylogenetic framework is known, I can test competing theories of whether body size or metabolism is more important in affecting how fast a species' DNA evolves. Interesting biogeographical applications include testing the theory that tenrecs have invaded Madagascar more than once.
Promising Results
I recently joined Goodman on two separate expeditions to tropical forests in Madagascar and collected material from other localities throughout the island. Thanks in large part to the ambitious pitfall trapping methods employed by Goodman - and borrowed by me on a separate excursion - we obtained chromosomes from 19 species of tenrecs. Analysis of this material in the museum's recently renovated biochemical laboratory uncovered yet another striking element of tenrec diversity. Tenrecs, it turns out, vary widely in the number of chromosomes found among different species. In fact, their variability in chromosome number is equivalent to that found in true shrews, a family with more than 300 species. We also found that several species of Microgale have very different chromosomal formulas, despite being similar externally, suggesting that there may be "hidden" species awaiting discovery.
Another interesting result of field work done last December involves the diminutive mouse-eared tenrec, Geogale aurita, sole member of its subfamily and one of the few tenrecs that have adapted to the island's arid southwest. Based on its overall peculiarity and fossil evidence from Africa, paleontologists think Geogale may represent a second, independent colonization of Madagascar. Together with a Malagasy and an English student, I captured Geogale and found it had far fewer chromosomes than any other tenrec species (and the lowest number known for any insectivore). While this by itself is inconclusive, it nonetheless highlights the distinctiveness of the species, as well as the incredible variation found in this poorly studied but diverse group.
Tenrecs are only one example of evolutionary radiation on an island ecosystem. However, no other groups possesses the same combination of features, or has experienced the same history. Studies like these improve our ability to generalize about the evolutionary process, while uncovering interesting patterns and processes unique to specific taxa. They also underscore the need for extensive, well-documented and well-managed museum collections - which are arguably the only scientifically valid way of documenting biodiversity.