For many animals summer is a time of activity, but not for Inuit sled dogs. According to Nadine Gerth from the University of Munich LMU, it is when they are most sedentary. Chained to rocks by law, the dogs spend their summers inactive and are fed once every few days as they wait for the sea ice, and hunting season, to return. During their lazy summers, the dogs lose muscle as they lie around and fast, but as soon as they can return to the ice, the dogs build up muscle ready for months dragging sleds on the ice for their Inuit masters. Gerth explains that this is a classic example of muscle phenotypic plasticity, as the animals' muscles waste during the summer and rebuild for the winter. Curious to find out how the dogs' muscles fair during the active winter and idle summer, Gerth and her colleagues, Steffen Sum, Sue Jackson and Matthias Starck, headed north to the animals' Greenland homes to measure their muscles ().
But it soon became clear that the team could not test the animals in the way they had hoped. Gerth explains that the team had wanted to compare the inactive animals with working animals during the same season, but `it was completely impossible to get them to work in the summer,' she says; `there was no sea ice for them to work on and they overheat'. It was also impossible to compare inactive and active animals during the winter because the dogs must drag sleds for the hunters. Fortunately the team realised that the dogs'lifestyles varied sufficiently in different parts of the country for them to test environmental effects on the dogs' muscles; the Western Greenland dogs are no longer involved in winter hunting.
Heading to Qaanaaq in Northern Greenland and Qeqertarsuaq in Western Greenland, the team took muscle biopsies, for analysis back in the lab, and measured the thickness of the dogs' leg and shoulder muscles by ultrasound in the summer. They returned to repeat the biopsies and ultrasound on the same dogs in the winter.
Analysing the data back in Munich, Gerth could see that the muscles of dogs from both regions were relatively withered in the summer, but the dogs from Western Greenland suffered most muscle wastage as they were on a poorer diet. Looking at the microscopic muscle samples, the team could also see that the muscle fibres were much thinner in the summer, when the dogs were inactive and fasting, than in the winter, when the animals were well fed and working. The summer dogs also had lower levels of fat in their muscle tissue and fewer energy generating mitochondria than they had during the winter. The animals'muscle fibres are extremely flexible and are able to respond to the different physical demands and diets.
Most surprisingly, the network of blood capillaries in the muscles didn't alter from season to season, so the dogs had a higher density of capillaries in the summer than they did in the winter. Gerth explains that this was unexpected, but suspects that instead of scaling back their capillary system during their inactive summer, they maintain it in preparation for winter,which is only round the corner.
