Smart rats hitched a ride

Did humans inadvertently give a sort of piggy ride to rats, helping them cross geographical distances and barriers? New research from National Centre for Biological Sciences (NCBS), Bangalore, says so. As humans traveled with their  their crop products in a variety of vehicles, they seem to have inadvertently also taken a some rats along, helping the rodents disperse, swap genes and become less prone to extinction.

Association with humans helped rats migrate long distances, and in their evolutionary trajectories, NCBS researchers Amruta Varudkar and Uma Ramakrishna report in Heredity.  The duo studied two ‘sympatric’ species that live in the same geographical area — Rattus rattus and R. satarae. The first species, R. rattus, is found in human habitations and is a ‘commensal’ species that lives in mutually beneficial relationship with other species, in this case, scouring around for food in human dwellings. The second species, R. satarae, lives in forests and is ‘non-commensal’.

The scientists sampled rats from villages and  forest areas in seven locations in the Western Ghats in southern India for their studies. the locations were 20-640 kms apart  — large by rats’ standards.  The premise was that the valleys and peaks in mountainous regions offered both distinct ecological niches and natural barriers to migration.

The scientists conducted detailed studies on the genetic material present in tiny microscopic structures called mitochondria that are powerhouses of cells; and found that populations of forest rat species was genetically more distinct, or unique, and showed lesser gene mixing. The village rats, on the other hand, showed higher rates of gene mixing and were, hence, more genetically diverse, with no unique genetic identity in the populations. In the long run, higher genetic diversity helps species adapt to different ecological areas and escape extinction.

The research also showed  higher rates of long-distance genetic migration in the commensal species (R. rattus).

The association with humans not only impacted the village rats’ ability to disperse to greater distances, but also the pattern of their migration, the researchers conclude.

The study also hypothesizes on the possible impacts of such patterns  on the evolution of the species, an NCBS release says.  “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.”

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