Science1 publisher3 min readPublished
Dated livestock bones tie Madagascar's megafauna losses to herds that spread 1,200 years ago
Oregon State and Columbia researchers found livestock spread across Madagascar about 1,200 years ago, as its large native herbivores declined. People may have arrived millennia earlier, so the timing argues against hunting alone as the cause of the extinctions.
The Scientist · Science desk
What happened
- Sean Hixon of Oregon State and Leanne Phelps of Columbia co-led a study in Proceedings of the Royal Society B that dated ancient cattle, sheep and goat bones from across Madagascar.
- The dates show livestock spread rapidly across the island about 1,200 years ago, roughly when elephant birds and pygmy hippos disappeared.
- Pollen preserved in sediments indicates grasses likely became more abundant across much of Madagascar within the last millennium.
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Why it matters
- contradiction The release says the timing suggests a causal link, while co-leader Phelps allows that livestock may not have directly caused forest loss, so the evidence supports the timing more firmly than any causal role.
- exposure The case against hunting alone rests on an early human-arrival estimate; later dating that puts arrival closer to the herds would shrink the gap that argument depends on.
- constraint Climate-only explanations now have to account for extinctions clustering near livestock arrival when drying reached different regions at different times.
- decision Phelps says the history argues for adding forage in existing open habitat and protecting the remaining forest, since herds are still part of how people on Madagascar make a living.
The argument against hunting as the only cause turns on sequence. Evidence cited alongside the study suggests humans may have reached Madagascar thousands of years before livestock did [11]. If hunters alone had wiped out the giant lemurs and elephant birds, the losses should line up with the first people [11]. On the new dates, they line up with the herds [4]. "The comparatively late arrival of livestock and the megafauna extinctions, coupled with the early human arrival estimate, suggests that it wasn't just hunting by humans that drove the extinctions," Hixon said [12].
Those dates come from bone. "Over the past five years, we brought together the largest collection to date of ancient, introduced livestock bone samples from across Madagascar for radiocarbon and stable isotope analysis," Hixon said [3]. Radiocarbon gives each animal an age. The release also reports that over the past 1,000 years the island's cattle likely came to rely more on C4 grasses, whose photosynthesis suits warm, dry weather [7]. The release does not say how many bones were dated or how many sites they came from.
Climate was the older suspect. Drying had been thought a key driver of past large-animal extinctions, but paleoclimate reconstructions place it at different times in different parts of the island [6]. A die-off driven by drying should be staggered region by region. According to the study, endemic large herbivores instead disappeared around the time livestock and dogs were introduced, whatever the regional climate was doing [10]. Cattle also moved into dry country early. "Although cattle might have preferred wet habitat, they successfully exploited relatively dry habitat during their earlier spread across the island," Hixon said [8].
The thing this doesn't tell you is which of the new arrivals did the harm. Livestock, dogs, spreading grassland and shrinking forest all fall in roughly the same window [1][5][10]. Hixon's own word for the fit with the extinctions is "approximately" [4]. Overlapping dates cannot divide the losses among grazing competition, predation by dogs and forest clearing. The pollen evidence is coarse, too: those reconstructions infer past plant communities at regional scales and beyond [5].
The release is more confident than one of its co-leaders. Its summary says the pattern "suggests a causal link" between livestock proliferation and past extinctions [10]. Leanne Phelps of Columbia said that even if livestock did not directly cause forest loss and the growth of grasslands, their spread has historically been associated with important changes in vegetation and biodiversity [13]. "The next step is to understand more about the causal relationships between these associated human and environmental changes, how their relationships varied across Madagascar's diverse landscapes, and the practical relevance of that for supporting healthy livelihoods and biodiversity today," Phelps said [14].
What to watch
- New dating of the first human arrival on Madagascar: an arrival closer to 1,200 years ago would weaken the argument against hunting alone.
- The region-by-region causal work Phelps described, testing whether livestock arrival came before forest loss in each landscape.
- Publication of sample counts and site coverage for the livestock bone database, showing how much of the island the dates represent.