Mouse burrowing 'in their genes'
The details of how mice burrow appear to be driven by genetics and not through learning, researchers report.
Biology has spent enormous effort in determining how genes affect physical traits, but little is known about how they affect structures animals make, such as bees' hives and beavers' dams.
Researchers reporting in Nature crossed mouse breeds and measured the burrows the resulting mice made.
The study has behaivoural implications of many animals, including humans.
"Modular" genetic regions even relate to specific burrow parts, it suggests.
The findings bear out an idea first put forward by British evolutionary biologist Richard Dawkins, called "the extended phenotype".
It suggests that our view of genes as controlling only proteins in an individual is tremendously limited, and that genes "express" themselves in a rich variety of behaviours - or in this case, homes.
The study's key subjects were more than 300 oilfield mice (Peromyscus polionotus), which are known to make burrows into the ground toward a nest, and then an "escape route" from the nest to just below the surface, which they can break through easily in the case of danger.
They were cross-bred with deer mice, a closely related species (P maniculatus) that is known to make shorter burrows, without the extra escape route.
Hopi Hoekstra of Harvard University, lead author on the research, said that one of the motivations for the study was "the idea that we could measure behaviour not by directly observing the mice themselves but by measuring this morphological representation of their behaviour - that is, their burrow".
The team did that by injecting expanding polyurethane foam into the burrows to make easily measured casts.
"One of the reasons that a lot of studies that try to find genes contributing to behaviour fail is just because it's so difficult to measure behaviour. Here, with this little trick, we're able to at least minimise measurement error," Prof Hoekstra told BBC News.
The two species of mice were successively bred, and those hybrids built long, two-tunnel burrows like oilfield mice - showing that the behaviour is a "dominant" trait.
The hybrids were bred again with deer mice, and the type of burrows those "backcross" offspring made show the inheritance patterns of the behaviours - and strong hints of how many genetic changes are required to modify it.
This showed that the propensity to build a long entrance tunnel and build an escape tunnel are genetically separate.
The team carried out a preliminary genetic analysis of the backcross mice, and showed that the propensity to dig long burrows is actually due to changes in three genetic regions, with a fourth region controlling the escape route behaviour.
"It was certainly a surprise that it seems most of the variation in this behaviour is controlled by just a few genetic regions," said the Harvard researcher.
But Prof Hoekstra added that the "modular" nature of the different behaviours that evolved separately in the two is the most suggestive finding: seemingly complex habits and actions may be built up from smaller, genetically-controlled behaviours.
And while the specific genes involved are yet to be identified, these findings have implications for understanding the behaviours of more complex creatures - such as humans.
"There's a lot of complex behaviours out there in the wild, and what this study has shown is that we have these different modules that have evolved, that are separable - but when you put those together you get this complex behaviour."