Monitor arithmetic

Natural History, March, 2004 by Jeff Orleans

In "The Lizard Kings" [11/03], Samuel S. Sweet and Eric R. Pianka leave us hanging when they write that white-throated monitors "can count up to six." How do we know? And why only to six?

Jeff Orleans

Princeton, New Jersey

SAMUEL S. SWEET AND ERIC R. PIANKA REPLY: We based our statement about monitor counting on recent experiments on captive white-throated monitors (Varanus albigularis) by John Phillips, an investigator at the San Diego Zoo. A lizard was fed snails in groups of four. Each snail was placed in a separate compartment connected to three others within a chamber, and the compartments were opened one at a time to allow the monitor to eat the four snails. On finishing the fourth snail, the lizard was allowed into another chamber containing four more snails.

After such conditioning, one snail was removed from some snail quartets. The lizard reacted by searching extensively tier the missing fourth snail, even when it was free to move on to the next group. Similar experiments with yawing numbers of snails in the groups showed that the monitors can count up to six. With groups larger than six, however, the monitors seemed to stop counting: they merely classified them as "lots," eating them all before moving on to the next chamber. Such an ability to count probably evolved as a consequence of raiding nests of reptiles, birds, and mammals, because the average clutch or litter size is about six.