Activity is an essential part of life for animals, whether it be to find food, shelter or a mate. And the key ingredient for such forays is energy, derived from the food they find. Animals need energy not only to be active, but also for a suite of basic life functions, including reproduction. In many mammals, males are often tasked with finding a mate, whereas females invest energy in their young during pregnancy and lactation. Consequently, there is often a mismatch in the energetic needs of the sexes, resulting in differences in activity across the year. On top of managing their energy expenditure during the reproductive period, animals also have to deal with general day-to-day energetic constraints, such as bad weather, which can be particularly demanding for mammals inhabiting freezing Arctic habitats.
Arctic ground squirrels hold the title of the northern-most small mammal hibernators. They have to successfully raise and prepare their young for the long winter hibernation period over the short spring/summer breeding season, which requires an enormous energy investment from the animals. This is why Cory Williams, from Northern Arizona University, USA, and his group were interested in the activity behaviour and energy expenditure of Arctic ground squirrels between April and August.
Braving the Alaskan wilderness, Williams and his team travelled to two field sites in Alaska, one near Atigun River and a second, colder site at Toolik Lake. There they secured collars to male and female ground squirrels to measure how much time they were inactive in the burrow and active above the surface (using light loggers), and then recorded what the animals got up to when they were out and about (using accelerometers). The team also collected a wealth of environmental data from weather stations at the two field sites, including ambient temperature, solar radiation, wind speed, rainfall and major snowfall events. Finally, they calculated an index of the animals’ energy expenditure from the accelerometer recordings to discover how their energetic needs correlated with the weather conditions and their reproductive state.
Arctic ground squirrels often encounter freezing conditions, even in the summer, and Williams and his group revealed that their activity patterns were indeed strongly influenced by the weather conditions. The ground squirrels preferred to be above ground and active when it was warmer and, not surprisingly, they also liked staying dry, displaying less activity on days with rain or snow. This meant that individuals at the cooler and wetter Toolik Lake site spent less time above ground than their counterparts at Atigun River. For Arctic ground squirrels, such flexible adjustments of activity behaviour are really important as they lose lots of body heat in cold and wet conditions, dramatically increasing the amount of energy they need.
The team also observed that the females’ maternal responsibilities ensured that they occupied their underground burrows more often than males, particularly during early lactation. However, while the males spent more time above ground, they expended just as much energy as the active females. It seems that the females employed more effort while foraging above ground in comparison to the males, which would help them to obtain the energy required for lactation while allowing for more time with their pups: newborn ground squirrels need their mothers for food and body warmth, especially as their burrows are enclosed by frozen soil. So, female Arctic ground squirrels really have it tough, trying to balance nursing their pups and foraging activity to successfully raise young in frosty Arctic summer conditions.