From an anthropogenic perspective, rats are pests that consume much-needed resources , and are often the main targets of  large scale extermination projects. However, biologically speaking, many rat species  are considered to be human commensals as they benefit from associating with humans. Amruta Varudkar, a research scholar at the Department of Ecology and Evolution  at the National Centre for Biological Sciences, has provided experimental evidence of just how helpful we, human beings, have been to our furry long tailed nemesis over the years.

In this recent study, published in the journal "Heredity" from the Nature publishing group, Ms. Varudkar and her guide, Dr Uma Ramakrishnan studied rat populations from various regions of the Western Ghats. The Ghats proved to be an ideal sampling location, since it provides a discontinuous, montane landscape that creates ecologically isolated populations in its many villages and surrounding forest lands.  In the first phase of the study, the team collected two distinct species of rats - Rattus rattus and Rattus satarae. They were able to establish that R. rattus were almost exclusively found in villages, while the other, R. satarae, mostly inhabited the forested areas and were seldom found in villages.  "We know that R. rattus are commensals", she said, "however, such a distinct segregation of habitat preference between the two species was an interesting observation". In her paper, she proposes that although previous studies suggest that R. rattus and R. satarae are sympatric and syntopic in the Western Ghats, the two are allopatric at the microhabitat scale.

The study used genetic analyses to compare various populations of the 2 species, spread across the Ghats. Ms Varudkar hypothesised that a human commensal species (especially a small mammal such as a rat), would exhibit lesser genetic differentiation stemming from higher gene flow when compared to its non-commensal counterpart.  "We observed a complete lack of population structure with high admixture in R. rattus when compared to R. satarae" she explained animatedly. The results have revealed that while the different populations of R. satarae exhibited distinct and population-specific genetic signatures, the populations of R. rattus showed no such unique genetic identities. She also observed higher rates of long-distance genetic migration in the commensal species (R. rattus),

Genetic diversity is extremely beneficial to a population; populations with higher genetic diversity have better chances of surviving ecological and environmental catastrophes. The ability to move genes freely between R. rattus populations, due to its association with humans, results in higher genetic diversity: making these populations less prone to extinction than their forest-dwelling, wild counterparts. The study not only looks at the population genetic consequences of association with humans , but also hypothesises on the possible impacts of such patterns  on the evolution of the species.  For instance, commensals could be released from the effects of natural selection as they find abundant food, nesting resources and shelter from inclement weather or predators. Alternatively, they could face entirely new selection pressures such as a host of rodenticides. A high gene flow could result in rapid spread of adaptability to these pressures."There is much that we can learn about evolution from human commensal species, especially when compared to their sister non-commensal species."

About the authors:

Amruta Varudkar is a PhD student, working under the guidance of Prof Uma Ramakrishnan in the department of Ecology and Evolution at the National centre for Biological Sciences, Bangalore.

The paper first appeared online on 6 May 2015 in Heredity. http://www.nature.com/hdy/journal/vaop/ncurrent/full/hdy201534a.html

Savitha Sekhar Nair is a writer with the Research Media Services Division of Gubbi Labs.