Should we be afraid of aliens?

There you are. In a half-finished hotel room on the Croatian island of Vis, the windows are covered with sheets, and they stare at a karst lizard (Latin name: Podarsis myliciliensis) who didn’t know about me because of the mirror glass. I noticed every movement. Every time I saw the creak of his forked tongue, I would knock. I was in the middle of a behavioral observation of a lizard I had caught the day before on the rocks of Mali Barjak, the neighboring island less than 100 meters in diameter. This karst lizard was one of more than two hundred lizards that my colleagues and I captured.. It was collected at several Croatian sites. In total, I made about five hundred and fifty observations on those lizards. “Why in the world would a person care about that?” I hear your opinion. Well, because that little island lizard can tell me why his grandson on the Croatian mainland could and could not run away from the little bloodthirsty Indian mongoose.

The little Indian mongoose is considered an “alien invasive species” or – in Dutch – an exotic invasive. Alien organisms are organisms that have moved to a place where they did not originally belong. In some cases, this alien causes damage to its new environment. Then we call it exotic gaseous. Our small Indian ferret has been intentionally exported from Asia to Hawaii, Fiji, Japan, Mauritius, Croatia, etc. This is to combat poisonous snakes and the spread of mice. What no one has taken into consideration is that ferrets are carnivores. So he sometimes loves a lizard, a bird, as well as a mouse or a frog. Once in a new area, the animal often commits a massacre. And because the mongoose now travels alone, it makes sense that the little Indian mongoose has been called one of the most dangerous invasive alien species on our planet.

The small Indian mongoose was deliberately introduced to some islands and mainland Croatia. Credit: JN Stewart.

Specifically on the islands, invasive alien species such as the mongoose cause death and destruction. We don’t see this often on the mainland. If we know how the island’s animals differ from their continental relatives, we’ll better understand why they are so endangered. This knowledge will allow us to better protect the island’s animals. So I went looking for things that go wrong with the island’s animals. I focused on the first and most important step in avoiding risks: being aware of the risks. And that is through one of the oldest senses in the animal kingdom: the sense of smell. Sniffing a predator gives three advantages over seeing, hearing, and definitely feeling one:

  • Many predators smell the predators before they get too close, making escape impossible.
  • The predator is often a master of camouflage. So the smell is the only thing that betrays its existence.
  • The fragrance usually lasts for a while as well. In this way, the prey can identify and avoid high-risk places where there is a lot of traffic by predators.

Thus, an animal that lacks a sense of smell has a significant disability. This is why highly diverse animals use their sense of smell to evade predators. This ranges from birds to mammals to fish, insects and… lizards. But lizards in particular have some useful traits that make it easy to study the sense of smell in island animals. They can smell through their tongue. This curiosity makes it possible for me to discover in a uniform way the interest that an orchid has for a particular scent. I simply count the number of times the lizard sticks out its tongue. If you also notice signs of anxiety on top of that, I know exactly what my lizards think about the scent of a ferret. It’s the perfect biological model for my research.

So I sat there on a Croatian island, playing with smell with karst lizards for several hours. The goal of the game: to find the killer by his smell only. In addition to the scent of the mongoose, it also provided lizards with ginger, an unscented cloth, and a scent from the local predators (snakes) they were familiar with. As it turned out: the island lizards did not even take out their tongues from clothes, stones with the marks of a mongoose and even with the smell of snakes, not to mention frighten them. However, the mainland’s lizards had wiped out all the killers! So it seems that the island’s lizards have lost their ability to smell predators. Hence also one of their most important defense mechanisms.

A karst lizard in an observation yard discovers the scent of a stone.

But why does this happen specifically on the islands? Through brain scans, I discovered what was wrong with the sense of smell of the island lizards. And imagine what? The parts of the brain responsible for processing odor molecules are much smaller in island lizards than in mainland lizards. It is currently difficult to determine why this is, but there is a hypothesis that could explain it. The brain uses a lot of energy and carrots are notorious for their harsh conditions. Island lizards may have to conserve brain tissue at the expense of their sense of smell. In this regard, they are behind the mainland lizards, which, thanks to their superior sense of smell, are better armed against invading alien organisms.

Although many karst lizards fall prey to mongooses, we do not see any serious consequences for the survival of this species on the Croatian islands today. So I hope to get my crusty friends back by going back to the hunting site. Perhaps their sheer number is what plays in their favour. However, less common animals also suffer from the harsh island conditions. It is therefore highly likely that these island conditions will also affect them and limit their ability to evade invasive alien species. For those rare animals, the introduction of an invasive species could be an additional impetus toward eradication.

We can therefore conclude that in a globalized world where millions of animals are transported daily by road traffic, shipping or aviation, we should focus especially on preventing the introduction of invasive alien organisms into the islands. In this way, the Mali Bariak lizard can continue to sunbathe on its rock in the middle of the Adriatic with peace of mind.

Charlotte van Morlegheim is competing for the 2022 Flemish Doctoral Cup. Find out more about this research at

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