Even on land, crocodiles are no fish out of water. While these reptiles might look lazy and slow sunning on the bank, they can easily pick up speed when necessary, and a scary number can gallop or bound like a horse or a dog.
Bounding is when an animal's forelimbs hit the ground at the same time, with the back legs pushing off soon after; meanwhile, a gallop is a four-beat sequence whereby the fore and hindlimbs take turns landing.
Freshwater crocodiles from Australia (Crocodylus johnstoni) were historically thought to be the only species capable of doing both. But that's not actually true. Not even close.
It turns out even scientists have underestimated these creatures. Past research suggested only a handful of croc species were able to gallop, but a new study now adds five more to the mix, suggesting it's a whole lot more common than we ever thought.
Setting up video cameras around a zoological park in Florida, veterinary scientists analysed the gaits and speeds of 42 individuals from 15 species of crocodylia, which includes true crocodiles (family Crocodylidae), alligators and caimans.
While alligators and caimans were only able to trot on land, the team noticed eight species of crocodile capable of galloping or bounding.
They claim their study is the first to properly document galloping in the Philippine crocodile (Crocodylus mindorensis), the Cuban crocodile (C. rhombifer), the American crocodile (C. acutus), the West-African slender-snouted crocodile (Mecistops cataphractus) and the dwarf crocodile (Osteolaemus tetraspis).
Judging by how common this skill appears to be, there might even be more species that can do the same. There have already been anecdotal reports of galloping in species such as the marsh crocodile (C. palustris) and the New Guinea crocodile (C. novaeguineae).
"We were really surprised at one major thing – despite the different gaits crocodiles and alligators use, they all can run about as fast," John Hutchinson, a specialist in evolutionary biomechanics at the Royal Veterinary College (RVC), told PA.
No matter what their size, almost every species studied was able to reach nearly 18 kilometres per hour (11 mph), whether it be through trotting, galloping or bounding.
Only crocodiles, however, could use their legs asymmetrically, providing longer stride frequencies, especially among those with smaller body sizes. Why alligators cannot do this remains uncertain, but the researchers think this skill is probably ancestral and has less to do with speed than we thought.
"We suspect that bounding and galloping give small crocodiles better acceleration and manoeuvrability, especially useful for escaping from danger," explains Hutchinson
"It seems like alligators and caiman stand their ground rather than run away with an extreme gait."
Similar to other studies, the researchers think the crocodile's unusual asymmetrical gait came from a long-lost ancestor that lived on the land and had longer legs.
If this is right, it could mean that the ancestors of the alligators somehow lost this ability or no longer express it.
But there's also another possibility that is rarely acknowledged: the common ancestor of today's 20 crocodile species may have actually evolved this asymmetrical gait as opposed to inheriting it.
Looking at related species could clear up some of the confusion – the gharial is an Asian fish-eating crocodile that lies outside the Crocodyloidea and Alligatoroidea ancestry, so if they can be shown to have asymmetrical gaits, it could shed light on how this skill appeared.
But similar to crocodiles and alligators, the gaits of the gharial's are not well documented, so there's clearly a lot more research that needs to be done.
"Together, our new observations of asymmetrical gaits and our broader dataset on locomotor kinematics spanning the clade Crocodylia considerably expand our knowledge of their behaviours and natural history," the authors conclude.
"Importantly, this combined evidence strongly refutes the popular notion that only a few crocodiles use asymmetrical gaits."
The study was published in Scientific Reports.