Willing to work? Motivation for contrafreeloading in laboratory mice

Ragen Trudelle-Schwarz McGowan & Ruth C. Newberry Center for the Study of Animal Well-being, Dept. of Animal Sciences and Integrative Physiology and Neuroscience (IPN), Washington State University, Pullman, WA 99164-6520

Contrafreeloading occurs when animals choose food that requires effort to exploit when identical food is freely available.  Putative explanations for contrafreeloading have invoked the idea of ‘behavioral needs,’ whereby animals are motivated to perform specific behavior patterns even if the physiological endpoint achieved by the behavior is already fulfilled.  We hypothesized that mice are motivated to contrafreeload, predicting that mice would work for access to a contrafreeloading opportunity.  We utilized a consumer demand approach, requiring mice (female C57BL/6J, n = 28) fed ad libitum to pay a ‘cost’ (climbing tubes) to gain access to foraging resources.  We constructed a test arena containing two climbing tubes, each leading to a foraging compartment (C1=‘Free’-shelled seeds, C2=‘Contrafreeloading’-shelled & intact sunflower seeds).  The incline of the tubes was systematically increased to manipulate the effort required to gain access to the two compartments (0°, 22.5°, 45°, 67.5° and 90°).  Two consecutive trials were conducted whereby individual mice were observed continuously for 10 min/tube angle, with one angle/day, for five days.  The number of climbs and time spent in each compartment were used to construct demand curves.  The slopes of the demand functions indicate that, despite the increasing cost of gaining access, motivation remained high and equal for accessing both the ‘free’ and ‘contrafreeloading’ compartments (t=0.44, P=0.67).  Though mice were equally motivated to access both foraging compartments, mice entered the ‘contrafreeloading’ compartment more frequently than the ‘free’ food compartment (S=2792, P<0.01) and remained in it for longer durations (S=2617, P<0.05). These results suggest that mice actively seek opportunities to engage in contrafreeloading behavior.