These were completely new to me.  I was amazed at the size of them.  There was a whole field of them, all in little groups like this, and each group had a lookout.  Fascinating!  The information below was taken from the Marwell Zoo site........but there is more information on them on the website if you are interested. (:o)

The mara (Patagonian cavy) is a large rodent from the open plains of central and southern Argentina. Its habitat consists of open scrub desert and grassland. The harsh environment means that the animal has had to adapt to survive there. Mara have excellent sight and hearing, and long legs enabling them to run over 1 km at 45 km/h. The mara has several different gaits - from walking to hopping, galloping and 'spronking' (bouncing on all fours). It has large ears (like a rabbit's) and its body and legs are similar to those of a small deer. There are a number of carnivores that prey on the Mara, including species of fox and eagle and the jaguar, so its fugitive lifestyle is comparable to that of antelope and deer in other areas.

http://www.marwell.org.uk/default.asp?css=1