Vampire bats, as their name implies, feed almost exclusively on blood. A bat must feed every day or it will starve to death, but bats are not always successful at finding a blood meal. Perhaps for this reason, they roost during the day in communal groups and sometimes share blood by regurgitative feeding. Researchers measured whether hungry bats were more likely to receive regurgitated blood than were partially fed bats (Wilkinson 1984). Eight bats were captured in the evening before they had fed and were held without feeding until the next morning. As a control, six bats were captured after naturally feeding at night, and they were also held until the following morning.

At that time, the bats were returned to their groups. Five of the eight hungry bats were given regurgitated blood meals by group-mates, but none of the six well-fed bats were given a blood meal by other bats. What statistical test would you use to address the question “Does the probability of being fed by roost-mates vary according to hunger status?