Space invaders: Extrovert lizards’ role in evolution

Biological scientist Dr Jack Brand has been closely studying certain lizards with intriguing behaviours – and has now shown that an Australian lizard may be better at invading foreign lands because it’s extroverted.

In a recent research paper published in Nature Communications, Monash University’s Dr Brand and lead co-authors Dr Annalise Naimo and Professor David Chapple investigated the behaviour of the delicate skink (Lampropholis delicata), which has managed to move from eastern Australia to islands throughout the Pacific, becoming the only Australian lizard that’s successfully invaded overseas.

The research team travelled across the Pacific, measuring the behaviour of 520 lizards from 14 different populations to see if the delicate skink was in any way “primed” to succeed as invaders of other countries.

The thinking is that the invasion process itself might be a kind of “filter” promoting biological traits that make for a successful invasion. Introverts, the idea goes, are filtered out.

The skinks have moved from their native eastern Australia, via air and sea cargo, to Lord Howe Island, Hawaii, and New Zealand.

These skinks are particularly active, bold, and prone to explore. Is that why these lizards are so good at invading?

These qualities may make them more likely to end up in luggage, a ship, aircraft or in cargo in the first place, but what about when they get out at the other end?

Read more: Hundreds of Australian lizard species are barely known to science. Many may face extinction

The idea was originally canvassed more than a decade ago in a paper led by Monash’s School of Biological Sciences asking whether the invasion process itself “selected” the braver animals in a population, and whether this led to evolutionary changes.

“If you can imagine that invasions are composed of lots of different stages,” says Dr Brand. “An animal has to first be taken up by a ship, a plane, or in someone’s personal effects.

“It must then survive being transported, sometimes for months in precarious conditions. After this, the really tough part begins.

“The species has to survive the introduction into a foreign land and find a new place to live far from home. In doing so, it must cope with new predators, strange food sources, and different diseases.

“We think that each of these stages of an invasion may select for those extroverted individuals, where only the most exploratory skinks end up making their way onto cars, ships, and planes,” he says.

“You’re then taking a really biased sample of the population overseas, which alters the traits of those invasive populations, a process called the ‘selective filter hypothesis’.”

This may result in a so-called “Olympic Village Effect”, where the most active and exploratory skinks end up mating with other similar individuals.

“These may be like the Olympic villages of the lizard world, where only the most successful skinks survive the invasion process. When these skinks then interact and breed, they also have offspring who carry those same traits.”

A similar process was observed in the invasion of cane toads in Australia, where toads with the longest limbs that could jump the farthest ended up dispersing the furthest.

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“When you get to the very front of these populations where they’re actively expanding, you often find individuals with really large limbs that travel great distances in very straight lines,” he says.

Eventually, the large, fast ones all reach the front of the pack, and mate. Once these toads interbreed, they create offspring with similar traits, resulting in generations of “super-invaders”, rapidly speeding up the invasion process.

The Nature Communications paper stops short of finding a definitive evolutionary process at play, but the researchers predicted and found a consistent pattern of results.

“On average, the invasive populations were far more exploratory than the native populations,” says Dr Brand.

“We found repeated patterns in all three independent invasions into Lord Howe Island, Hawaii, and New Zealand. I didn’t think the results would be this striking.”