“Absolutely disgusting, so grim, the worst fieldwork of my life, but also extremely rewarding in a very odd way,” said Julia Grootaers, describing her three months collecting data among rotting pig carcasses in Costa Rica’s Osa Peninsula.

For her master’s thesis at the University of Exeter, U.K., Grootaers was part of a study examining how vultures affect carrion (decaying flesh) decomposition in neotropical ecosystems. Their findings, published recently in the journal Ecology and Evolution, reveal that in the absence of vultures, carcasses take twice as long to decompose, and fly populations double, with significant implications for ecosystem health and potential disease transmission.

“There’s a reason why carcasses smell bad. We’re not meant to be near them,” Grootaers told Mongabay. The fieldwork involved spending day after day amid decomposing pig carcasses, some weighing up to 130 kilograms (nearly 290 pounds). “We’d have to obviously wear gloves and a mask, because you have so many flies flying around and they are trying to get into any orifice at all.”

The experiment consisted of 32 pig carcasses deployed in the southern Pacific region of Costa Rica, half in grassland and half in forest habitats. Eight carcasses were covered with exclusion cages for each habitat to prevent vulture access, while eight control carcasses remained uncovered. Half the experiment took place during the wet season and half during the dry season.

The pigs were sourced from local farms that supplied meat for human consumption, with the animals usually killed just an hour before the research team picked them up, Grootaers said.

The research team monitored insect activity using pitfall and bottle traps around each carcass, and used camera traps to monitor for other animal visitors and scavengers. Grootaers and colleagues weighed the carcasses daily until they fully decomposed, or for at least 20 days. However, the study used only the first 10 days of measurements in the analysis because, often, not much of a carcass was left beyond that point.

The results were striking. Carcasses available to vultures decomposed approximately twice as fast as those where vultures were excluded. Specifically, control carcasses lost an estimated 9.5 kg (21 lbs) per day. In contrast, exclusion carcasses (with no vultures) lost only about 4.8 kg (10.6 lbs) per day. And fly abundances doubled when vultures weren’t around.

One unexpected finding was how few vertebrate scavengers visited the carcasses, such as large cats or possums. While studies in temperate regions often find animal carcasses supporting diverse scavenger communities, this was not the case here.

“We kind of expected, OK, well, we’ve got ocelots, we’ve got pumas, we’ve got lots of birds of prey, you know, apart from vultures,” Grootaers said. “Surely we’ll get like an interesting sort of community. Turns out, not really.”

Camera-trap photos showed that vultures were the primary vertebrate drivers of decomposition, as they consumed all but one control carcass. Other vertebrates were rarely detected at the carcass sites, with opossums being the most common non-vulture visitor.

“It was quite surprising to have a low number of scavengers in the carcasses,” Sergio Lambertucci, a vulture expert from the National University of Comahue in Argentina, who wasn’t involved in the study, told Mongabay. “I was expecting in those tropical areas to have a lot of different species of scavengers.”

Fly populations doubled at carcass sites without vultures, a finding with potential public health implications. Slower-decomposing carcasses could have important consequences for infectious and zoonotic (animal-transmitted) diseases in the tropics. A previous study found that 27% of 85 wildlife carcasses brought in for epidemiological surveillance in Costa Rica were from animals that had died from an infectious pathogen.

“Flies maximize carcass utilization by laying hundreds to thousands of eggs which develop rapidly into larvae, forming intensively feeding maggot masses which eventually mature into adult flies,” Grootaers said. The longer a carcass lies around uneaten, the greater the number of generations of flies that can develop in its flesh.

“Blowflies, usually the first species to colonize a carcass, are known vectors of both botulism and anthrax,” Grootaers said. This study did not look fly species, but other microorganisms found in flies include antibiotic-resistant strains of E. coli, Salmonella and Shigella, the main cause of dysentery.

This study is also significant because vulture research has almost exclusively concentrated on Old World species, those found in Africa, Asia and Europe. Neotropical species, found in the tropical Americas, are represented in just 7% of existing vulture literature, exposing a prominent knowledge gap on New World vultures, despite the similar threats they face.

“There’s such a lack of information about them, so we don’t actually know what’s happening with them,” Grootaers said, “and what we don’t know, we can’t protect.”

“The neotropical vultures are not very well studied,” Lambertucci said. “We are lucky that the biggest ones, the Californian and the Andean condors, during the last 20 years were studied more, but is not the case for most vultures … Central America, in particular, is a place where we don’t know almost anything about them. Some of the knowledge gaps are related to their abundances, their population dynamics, demography, conservation threats.”

Nearly all (14 out of 16) species of Old World vultures and two out of seven New World vulture species are listed as threatened or near threatened with extinction.

Vultures face numerous threats, including power line collisions, poisoning (both intentional and unintentional), habitat loss, and increasing negative perceptions driven by misinformation.

“The biggest threat is deforestation, because we are literally cutting down their homes,” Grootaers said. She and her colleagues say they hope their findings will spur increased conservation attention for these understudied yet crucial ecosystem engineers.

The economic value of vulture conservation has been demonstrated in other studies. A recent report by the NGO BirdLife found that conserving vulture populations in Southern Africa could have potentially huge economic benefits, with ecosystem services worth around $251 million per year in just three countries (Botswana, Zambia and Zimbabwe).

“Vultures are valuable to the health of humans, to the health of livestock and the health of other wild animals as well,” Lovelater Sebele, senior vulture conservation officer for Southern Africa at BirdLife Africa, told Mongabay. “Vultures are saving us a lot of money that would be used to maintain the health of these three groups of living organisms. We should be actively conserving vultures.”

“Governments could save a lot of money if they act to protect vultures now,” Grootaers said, “rather than leave it too late and then realize that this amazing natural cleanup crew is gone.”

Banner image of a vulture in Costa Rica’s Osa Peninsula. Photo courtesy of Andy Whitworth.

Liz Kimbrough is a staff writer for Mongabay and holds a Ph.D. in Ecology and Evolutionary Biology from Tulane University, where she studied the microbiomes of trees. View more of her reporting here.

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Citations:

Grootaers, J., Hernández Campos, G., Marie Montenegro, V., Vega Quispe, R., Wicks, S., Campos Landázuri, S., … Beirne, C. (2025). Vulture exclusion halves large carcass decomposition rates and doubles fly abundance. Ecology and Evolution, 15(5), e71408. doi:10.1002/ece3.71408

Aguilar-Vargas, F., Solorzano-Scott, T., Baldi, M., Barquero-Calvo, E., Jiménez-Rocha, A., Jiménez, C., … Alfaro-Alarcón, A. (2022). Passive epidemiological surveillance in wildlife in Costa Rica identifies pathogens of zoonotic and conservation importance. PLOS One, 17(9), e0262063. doi:10.1371/journal.pone.0262063

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