Friday, July 29, 2016

Lemur habitat was changing before humans even arrived

New research from the DNA of mouse lemurs (of the genus Microcebus) has lead to a surprising discovery about Malagasy forests and their ancient past. Madagascar, a large island off the west coast of Africa, is made up of many forests that are presently not connected and haven't been since around the time humans reached the island. The eastern rainforests (green in the image below) are separated from the western, deciduous forests (blue in the image below) by an expanse of grasslands (in yellow). There are multiple theories concerning what the landscape looked liked in the past and why the eastern and western forests are separated, and research from Yoder and colleagues (2016) uses the genetic history of lemurs to tell us more.

Habitats of Madagascar and species locations. 
From Yoder et al., 2016
Some believe that, rather than this disconnect we now see between western and eastern forests, Madagascar was covered in closed-canopy forests prior to the arrival of humans, with humans acting as the primary agents of change. This so-called forest hypothesis maintains that the large expanse of grasslands that experience frequent burning were once forests and their reduction was not natural.Others believe that the grasslands we see today are similar to ancient grasslands found in Madagascar. Thus, the landscape was not so radically different from the present. This is referred to as the grassland hypothesis by Yoder and colleagues (2016). Finally, another idea termed the mosaic hypothesis by Yoder and colleagues (2016) suggests that the central part of the country was covered with a mix of wooded savanna and closed-canopy forests.

Some of the world's top lemur experts have provided us with new information on the history of Madagascar's forests. Yoder and colleagues (2016) looked at the DNA of five different species of mouse lemurs, a genus dependent on the forest, to understand how the forests across this island nation changed. One of their research questions asks if the divide between the two forest habitats is natural or what remains of a transitional cline?

Results suggest that the five species studied started diverging from one another roughly five hundred and forty thousand years ago with the last node of divergence around fifty-five thousand years ago. The timing of mouse lemur evolution and divergence coincides with evidence suggesting great variation in climate.

M. murinus. Photo: Joachim S. Muller

Genetic analysis of M. myoxinus, a subspecies of mouse lemur that lives in continuous forests, and M. lehilahytsara, another subspecies that lives in a mosaic of forests, show significantly different patterns in regards to geographic patterns of divergence. Results suggest that M. lehilahytsara, the mouse lemur in a mosaic of forests, has lived in this type of habitat for a very long period of time, since before the arrival of humans. Thus, genetic evidence from mouse lemurs does not support the forest hypothesis.

Yoder and colleagues (2016) also discovered that  M. berthae and M. rufus are more closely related than would be expected, given their geographic differences. One species lives lives in the southeast in humid forests and the other in dry forests of the west. This close genetic relationship points towards the mosaic hypothesis of Madagascar's ancient geography. We see a genetic divergence that is tens of thousands of years old and not uniform across all Microcebus species.

Thus, using the genes of mouse lemurs, Yoder and colleagues (2016) were able to conclude that the central part of Madagascar was composed of a mosaic habitat composed of forests and grasslands. The arrival of humans does not appear to have caused a tremendous change from an entirely forested landscape to grasslands similar to what we see today.

Links of possible interest:

Ecosystems in Madagascar
Read the paper here
They may be cute, but you shouldn't have a primate as a pet

Works cited: 

Blome, M. W., Cohen, A. S., Tryon, C. A., Brooks, A. S., & Russell, J. (2012). The environmental context for the origins of modern human diversity: a synthesis of regional variability in African climate 150,000–30,000 years ago. Journal of Human Evolution, 62(5), 563-592.
Cannon, C. H., Morley, R. J., & Bush, A. B. (2009). The current refugial rainforests of Sundaland are unrepresentative of their biogeographic past and highly vulnerable to disturbance. Proceedings of the National Academy of Sciences, 106(27), 11188-11193.
Scholz, C. A., Cohen, A. S., Johnson, T. C., King, J., Talbot, M. R., & Brown, E. T. (2011). Scientific drilling in the Great Rift Valley: the 2005 Lake Malawi Scientific Drilling Project—an overview of the past 145,000 years of climate variability in Southern Hemisphere East Africa. Palaeogeography, Palaeoclimatology, Palaeoecology, 303(1), 3-19.
Yoder, A. D., Campbell, C. R., Blanco, M. B., dos Reis, M., Ganzhorn, J. U., Goodman, S. M., ... & Ralison, J. M. (2016). Geogenetic patterns in mouse lemurs (genus Microcebus) reveal the ghosts of Madagascar's forests past. Proceedings of the National Academy of Sciences, 201601081.


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