Creature Feature: Horned lizard

Eat or be eaten. Although Iggy Pop won no accolades for this lesser-known, undeniably bizarre 80’s punk rock track, animals in the natural world have been following this tune for untold millennia. The simplicity of this law of nature belies the complexity of ways that animals have evolved strategies to avoid being eaten. Impressed by the defensive tactics and subterfuge employed by characters in Game of Thrones? Well, there’s a host of characters on our very own planet that fend off their enemies using far more thrilling methods. [Disclaimer: GoT references to follow. It’s that time of the year. Sorry, not sorry.] Think poison is a woman’s weapon? Old news for some newts that have mastered the art of murder by deadly skin toxins. Faceless men can morph into anyone in the realm, you say? I see you have yet to meet the mimic octopus. Oh, but surely no animal can defend itself using a loaded, dragon-killing harpoon! Enter, the humble yet beautifully deadly cone snail. True, the wildlings often craft grungy attire that doubles as camouflage and the Sand Snakes wear flashy but lethal weaponry. But some invertebrates are deft trash-camouflage artists and others know a thing or two about deadly adornment. Alright, so you get the idea…Still unimpressed? Well, Mother Nature has another ace up her sleeve: a few lizard species can actually shoot out a high propulsion stream of blood through their eyes! Phrynosoma lizards have perhaps one of the most bizarre yet under-appreciated defensive strategies in the animal realm.

Talk about bloodshot eyes. Horned lizards, such as this coast horned lizard (Phyrnosoma coronatum), can shoot out streams of blood through their eyes to ward of predators. [Source]

The horned lizard (Phyronosoma spp.), sometimes erroneously referred to as a horned toad (in Greek, phyrhos = toad, soma = body), includes several species that have evolved this extreme adaptation to defend themselves against predators [1]. Many reptiles have what is called a nictitating membrane, or third eyelid, that protects their eyes against debris (as an aside, that little pink membrane in the corner of your eye is a vestigial third eyelid!). However, some Phrynosoma species have a blood sinus at the base of this membrane that fills up with blood when the lizard is threatened [1]. Muscles around the eye compress, causing blood to squeeze through capillaries into this cavity [1]. When the eyelids are completely swollen, the built-up pressure causes a fine jet stream of blood to shoot out at the lizard’s would-be attacker, sometimes up to 6 feet away [1]. If you thought that scene in GoT with Oberyn Martell was gruesome, imagine if your eyeballs went through that pressure every time you felt threatened by an enemy. Yup, that’s right, this fascinating line of defense is repeatable, and lizards can sometimes shoot blood out of both eyes hundreds of times [1].

The horned lizard’s extreme defensive strategy is most commonly used against predators such as coyotes and other canids. [Source]

Although this defensive response might seem extreme, the horned lizard only uses it against particularly menacing predators like canids (i.e. dogs, coyotes, etc.). When other predators threaten this slow-moving lizard, it will hunker down and flatten out against the ground, while positioning its spiny head in a defensive posture [2]. A hungry snake is likely to avoid a mouthful of ossified spines and go after more palatable, less painful prey. That crown of fierce-looking horns is a unique trait in the Phyronosomatid lizard family: those spines are actually bony extensions of the cranium [3]. Although these behavioral and morphological traits likely protect Phrynosoma lizards against predators, it is thought that they evolved such distinctive skeletal morphology due to their prey-base: ants! Despite significant inter-specific variation, horned lizards’ diet consists primarily of ants and the more myrmecophagous (i.e. ant-eating) species tend to have more blunt, non-cusped teeth, reduced jaw musculature and distinct skull morphology [4]. While the percentage of ants in different Phrynosoma species’ diets varies considerably, it is likely that the ancestor of this genus was a specialist ant predator [3].

Horned lizards are thought to have evolved specialized skull and dental morphology adapted to ant-eating: they have non-cusped, blunt teeth, flattened heads, and reduced jaw muscles. Inter-specific variation within the genus Phyrnosoma is thought to be related to the percentage of ants in their diet. [Fig. 1 from Meyers et al. 2006, Source]

These bizarre lizards have a host of unique traits that have inspired a variety of research topics which sometimes require some innovative jerry-rigging on the part of the researchers. Herpetologists (biologists who study reptiles and amphibians) have developed different versions of a “backpack” to remotely monitor the behavior of horned lizards [5], [6]. This backpack includes makeshift materials (such as the innertube of a bicycle) and the finished product holds a radiotransmitter powered by a battery that can last several months [6]. Such innovative technologies can allow us to study the movement patterns of lizards without disturbing the animals in their natural habitat [7]. We can answer specific questions about the social behavior of these blood-squirting lizards and and also elucidate how changes in their habitat might affect their survival in increasingly fragmented habitats. One thing’s for sure: if you come across one of these critters on a hike, beware their spiny crown and try not to disturb them, lest you elicit a bloody counter-attack.

If you live in California, you might come across Phrynosoma coronatum, the coast horned lizard, a much less menacing yet equally-bizarre lizard compared to some of its closely-related cousins. Watch out though: those unassuming eyes are primed for bloody battle! [Source]

References:

  1. Sherbrooke, W. C. (2003). Introduction to horned lizards of North America (No. 64). Univeristy of California Press.
  2. Sherbrooke, W. C. (1987). Defensive head posture in horned lizards (Phrynosoma: Sauria: Iguanidae). The Southwestern Naturalist32(4), 512-515.
  3. Pianka, E. R., & Parker, W. S. (1975). Ecology of homed lizards: a review with special reference to Phrynosoma platyrhinos. Copeia1975(1), 141-162.
  4. Meyers, J. J., Herrel, A., & Nishikawa, K. C. (2006). Morphological correlates of ant eating in horned lizards (Phrynosoma). Biological Journal of the Linnean Society89(1), 13-24. https://doi.org/10.1111/j.1095-8312.2006.00654.x
  5. Fisher, M., & Muth, A. (1995). A backpack method for mounting radio transmitters to small lizards. Herpetological Review26(3), 139-139.
  6. Richmond, J. Q. (1998). Backpacks for lizards: a method for attaching radio transmitters. Herpetological Review29(4), 220.
  7. Fair, W. S., & Henke, S. E. (1999). Movements, home ranges, and survival of Texas horned lizards (Phrynosoma cornutum). Journal of Herpetology, 517-525.

Additional resources: http://www.californiaherps.com/identification/lizardsid/phrynosoma.id.html

Main featured image of Phrynosoma cornutum taken by Tom Barnwell [Source]

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s