Bat breath reveals metabolic substrate use in free-ranging vampires

We analysed the stable carbon isotope ratio in exhaled CO2 (delta(13) C-breath) of free-ranging vampires to assess the type of metabolized substrate (endogenous or exogenous substrate) and its origin, i.e. whether the carbon atoms came from a C-4 food web (grass and cattle) or the C-3 food web in which they were captured (a rainforest remnant and its mammals). For an improved understanding of factors influencing the delta C-13(breath) of vampires, we conducted feeding experiments with captive animals. The mean delta C-13(breath) of starved bats was depleted in C-13 in relation to the diet by 4.6 parts per thousand (n = 10). Once fed with blood, delta C-13(breath) levelled off within a short time approximately 2.2 parts per thousand above the stable carbon isotope signature of the diet. The median time required to exchange 50% of the carbon atoms in exhaled CO2 with carbon atoms from the ingested blood was 18.6 min (mean 29.5 +/- 19.0 min, n = 5). The average delta C-13 of wing membrane and fur in free-ranging vampire bats suggested that bats almost exclusively foraged for cattle blood during the past weeks. The delta C-13(breath) of the same bats averaged -19.1 parts per thousand. Given that all free-ranging vampires were starving and that the delta C-13 of cattle was more in enriched in C-13 by 5-6 parts per thousand than the delta C-13(breath) of vampires, we conclude that the vampire bats of our study metabolised fat that was predominantly built from carbon atoms originating from cattle blood. Since delta C-13 of wing membrane and fur integrates over weeks and months respectively and delta C-13(breath) over hours and days, we also conclude that vampire bats of the studied population consistently ignored rainforest mammals and chose cattle as their prey during and prior to our study.