As a cross-check, their density is very close to that of water. That gives 142 liters -- which has to fit in the 90 L fatcat.
But my 50 kJ/day was for RESTING metabolism. They only have to do it for 4 hours.
What's the ACTIVE metabolism? 100 kJ/hr ? So only 3015 are needed, and they fit in 70 litres .. with plenty of room to spare.
http://books.google.com/books?id=afVo_Oi6mesC&pg=PA141&lpg=PA141&dq=mouse+%22active+metabolic+rate%22&source=bl&ots=4bW4oi8ETE&sig=3_vf7ZOSMH2Z8cUslnSazgRdXE4&hl=en&ei=IEKOTo3uHsWesQKPtISnAQ&sa=X&oi=book_result&ct=result&resnum=7&ved=0CE8Q6AEwBjgK#v=onepage&q=mouse%20%22active%20metabolic%20rate%22&f=false
gives a ratio of 8X for resting-to-active -- so only 753 mice are required ... or 19 litres.
They might have to work in shifts, though ... I need to check with the International Brotherhood of mice ...
