Bat Breath Reveals The Identity Of A Vampire's Last Victim

Farmers are observing vampire bats satisfying their need for blood by attacking cattle instead of wild mammals. To document this change in behaviour, Dr Christian Voigt and colleagues analysed the stable carbon isotope ratio of exhaled CO₂ in vampire bats.

They fed captive vampires blood that was labelled with the stable (non-radioactive) isotope carbon-13 and then monitored the time period between the blood meal and the appearance of labelled carbon atoms in the exhaled breath. “Vampire bats used the freshly ingested blood very fast to fuel their metabolism; after less than an hour the stable carbon isotope signature of the vampires’ exhaled breath was similar to that of their latest diet” Dr Voigt states.

Then the researchers collected breath of free-ranging vampire bats in Costa Rica and analysed its stable carbon isotope signature. “The potential victims of vampires in Costa Rica are either cattle or rainforest mammals such as tapirs and peccaries. These two groups of animals feed on isotopically distinct plants which are grasses in the case of cattle and herbs or shrubs in the case of rainforest mammals. Therefore, we expected that the stable carbon isotope signature in bat breath would change according to their diet” Dr Voigt continues. The vampires’ breath clearly indicated that their last blood meal almost always originated from cattle, although rainforest mammals were also present.

The authors argue that the vampire bats do not necessarily prefer cattle blood, but that cattle are much easier to find for vampires than rainforest mammals. Cattle are held fenced-in on open pastures, whereas rainforest mammals roam in dense vegetation. Converting rainforests into pasture has a large impact on many native mammals of Latin America, usually not to the benefit of the original mammal fauna.

Vampire bats live only in Central and South America and weigh 30 - 40 g. Vampires share food among unrelated group members, a behaviour known as reciprocal altruism, which vampire bats have in common with humans.

Reference: C C Voigt, P Grasse, K Rex, S K Hetz and J R Speakman (2007). Bat breath reveals metabolic substrate use in free-ranging vampires. Journal of Comparative Physiology B, DOI : 10.1007/s00360-007-0194-z is published online on 16 August 2007.