The vulture definitely had the size advantage. With an eight foot wingspan and a head larger than a softball it was a formidable opponent for the jackal—a mammal smaller than your average Scottish terrier. The lappet-faced vulture grabbed the jackal around the waist and lifted off, raising the tiny carnivore into the air. About a foot off the ground, though, the jackal wiggled loose, managed a full flip head-over-paws and landed back on the grass. This was the moment of truth. In a flash it grabbed a large piece of meat—the prize it had been trying to claim—and took off.
Scavenging is not an easy job but someone has to do it. When an animal dies, the carcass generates an enormous amount of food, triggering considerable competition between a diverse array of species. In a paper we recently published in the Journal of Animal Ecology, Adam Kane and I took a look at how these dynamics play out—particularly how ground-based scavengers use soaring birds of prey to find their next dining spot. This study also has implications for our own evolution. After all, early hominids were scavengers as well as hunters and probably have a lot more in common with today’s hyenas than we might like to admit.
Carrion is a widely dispersed and ephemeral resource, which means you have to search a large area quickly to find it. Flight gives birds an incredible advantage; a vulture can soar 200 km in a day using very little energy and can see for several kilometers in all directions as it searches for its next bite. A jackal or hyena has no such advantage, but it might, I suspected, employ vultures as their eyes in the sky.
By putting out hunks of goat meat in Masai Mara, Kenya, I could assess how scavengers use each other for information. My most exciting discovery occurred one day when I watched an unusual set of circumstances. I noticed a spotted hyena wandering around in the distance as we set up our carcass. I sat back in the car and watched, assuming we would soon be witnessing a hyena crunching down on goat bones.
But that isn’t what happened. Instead the hyena meandered along unable to find the bait. Shortly afterward, a small eagle, a Bateleur, landed on our carrion. These eagles turned out to be the best at discovering carcasses, likely due to their huge eyes and low-flying search patterns, but tend to go for mice or lizards—not exactly a meal big enough to be shared (or stolen). The hyena looked over, clearly saw the raptor land, but continued walking in the opposite direction.
It wasn’t until a white-backed vulture also took the bait that the hyena came lumbering over to finally steal the prize. My hypothesis was confirmed – the mammals were spying on vultures, gaining valuable information about carrion resources. But even more impressive, they could evidently differentiate vultures, which are reliable indicators of relatively large dead animals, and eagles, which look similar but tend to prefer smaller creatures.
Even though I rarely saw a hyena or jackal rush to a carcass until a vulture had landed, I wanted further evidence that this was really what was occurring. This is where Adam Kane, a researcher from the University College Cork, came in, creating a model to further test these ideas and see if they lined up with my field data. He used estimates of mammalian ground speed and density, as well as their likely detection distances, to assess how quickly they could find a dead animal with and without the use of a vulture. He found that jackals and hyenas would discover carcasses a lot faster if they could use the birds and indeed the arrival times that I had recorded for jackals and hyenas suggested that they followed vultures but not eagles.
If these present-day carnivores could tell the difference, then surely early hominids could as well. In fact, some anthropologists have even speculated that one advantage of an upright posture for our early ancestors might have been that they could see vultures landing in the distance and increase their chances of getting to a carcass before another larger, and possibly dangerous, scavenger beat them to it.
With vultures disappearing from the African continent at an alarming rate, largely due to efforts to poison carnivores, which inevitably kills lots of scavengers too, this means the whole scavenging guild might be in trouble. Without vultures, we are missing a key player in carrion consumption and it might mean that other scavengers discover fewer carcasses and take longer to find them, leaving more meat available to fuel bacterial infections and disease-spreading insects like flies.
Vultures may have shaped our evolution—and we still need them today to keep the environment safe and healthy.