The Basilisk Lizard

Basiliscus plumifrons (male); photographed in situ [1]

The genus name Basiliscus is derived from the Greek word for “little king,” and also shares its name with a serpent from European mythology, so venomous and dangerous that simply fixing its gaze on someone would kill them. Basilisk lizards (Basiliscus spp.) are famous for their ability to run across the water’s surface. This behavior is often employed as an escape tactic, jumping off a nearby perch to slip across the surface of a stream to safety. While running on water, basilisks exhibit bipedal locomotion, a type of sprinting gait not uncommon in lizards. On land many species will transition into this theropod-like locomotion, including iguanids, agamids (especially the frillies!), and teiids. Analysis of remarkable fossil tracks from an ancient lizard in South Korea takes bipedalism back 110 million years (Lee et al. 2018). There is even evidence for bipedality in a fossil anapsid from the Early Permian, with a body form that suggests a parasagittal digitigrade posture (fancy term for standing upright and putting weight on the digits). There are many hypotheses for lizards evolving bipedality for efficiency of movement and an increase in speed. However, a few studies on agamid locomotion and biomechanical modeling suggest that in many cases bipedality may simply result from the lift generated above a certain acceleration threshold.

Basiliscus plumifrons (male) overlooking a stream near a leafcutter ant mound; photographed after pursuit [3]

What is uncommon though is the ability to “walk” on water, earning basilisks yet another common name, the Jesus Christ lizard. In contrast to other bipedal lizards, basilisks have more elaborate hindlimb movement in order to keep the lizard above the water’s surface (Hsieh, 2003). The lizards slap their feet down into the water, creating a pocket of air above the feet. This force prevents the lizard’s body from sinking while the lizard performs exaggerated leg movement to the back and side of the lizard. The challenge for running on water is staying upright, since the aqueous substrate doesn’t provide good support for sustaining an upright posture. When placing its foot down, it would be easy for the lizard to sink down to the side. The lateral movements in particular are thought to enable the lizard to maintain balance. By pushing to the side with one leg, the lateral force generated shifts the body’s weight to the other side, where it will be met with a complementary leg stroke — basically slipping across the water’s surface.

Basiliscus plumifrons (female); photographed before release [4]
Basiliscus plumifrons (female); photographed in situ [1]
Basiliscus basiliscus (male); photographed in situ [1]
Basiliscus basiliscus (male); photographed in situ [1]
Footage of head-bobbing in a plumed basilisk (Basiliscus plumifrons) and black spiny-tailed iguana from my first trip to Costa Rica in 2016

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