There are over 5,500 species of lizards, the most diverse group of reptiles. Lizards live on all continents except Antarctica. Many species are restricted to a specific habitat niche. Lizards are largely absent on oceanic islands, though the Galápagos now hosts 7 species which descended from a single ancestral species.
Many lizards are meat eaters, with diets that range from insects (some even specialize in certain critters) to omnivorously eating anything they can catch or scavenge. Iguanas are herbivorous. The Galápagos marine iguana uniquely forages upon seaweed and marine algae in the sea off the rocky coasts of those islands.
Sight is a crucial sense for most lizards, both for predation and communication. Many lizards possess acute color vision, with eyes of high cone (color receptor) density. Some have pigmented oils in the cones that further enhance color sensitivity.
Black-and-white rod receptors, common in other tetrapods, are generally lacking in color-acute lizards. Like some insects, such as bees, lizard vision extends into the ultraviolet light spectrum.
Those lizards with exquisite color vision, and with a diurnal lifestyle, are typically brilliantly colored. Some lizards are capable of rapid changing their color.
In contrast, most nocturnal geckos are drab. Nocturnal geckos adapted to seeing in low light while retaining color sensitivity.
Human vision switches from cone-based color vision in bright daylight to rod-based black-and-white as darkness descends. Color reception lessens at lower light levels.
Color sensitivity of the helmet gecko is 350 times higher than humans at the low light threshold where color vision become available. These nocturnal geckos have larger cones, with distinct concentric zones of different refractive powers that constitute a multifocal optical system. Each zone is differentiated by 15 diopters, the same magnitude needed to focus light in the wavelength range which gecko photoreceptors are optimally attuned.
Diurnal gecko species are monofocal, and lack distinct, concentric, refractive zones. Geckos re-evolved diurnality numerous times, and their vision systems varied in doing so.
Though it is not poisonous, the vivid blue-headed agama lizard was long one of the most feared lizards in South Africa. The Muslims there loathed the lizard for its head-bobbing, which they believed was an intolerable mimicking of believers bowing to Allah.
The agama has monocular vision: independent eyes. Its head-bobbing betters depth perception.
To communicate, lizards employ pheromones and an extensive vocabulary of body language: postures, gestures, and movements. They communicate to entice mates, define territory, resolve disputes, and perhaps for other reasons unknown to us.
Some lizard species have patches of bright color to signal others: typically, under the throat or on the belly. These colorations can be hidden, so as not to be a painted target to predators.
Nearly all lizards use visual signals and chemical cues for social communication, but their relative use varies considerably. Iguanas, which arose before scleroglossans, illustrate by contrast.
Iguanas rely heavily on visual signals: some quite elaborately dramatic, considering the brilliant colorations found in many iguana species.
Scleroglossans – which includes worm lizards, geckos, anguids, skinks, and snakes – rely on chemical communication more than iguanas, though there is some spectacle there as well. Flagrant fragrant plumes from female cloacal glands convey to a male all the particulars of mating potential. Invisible chemical trails let males track females through maze-like terrain, most notably subterranean complexes, where visibility is poor.
Some geckos couple overt behaviors with sonic signals. Tactile communication is often mixed with other modes among numerous lizards. As with many animal species, social interactions among lizards are most pronounced and diverse during mating season. Mating rituals vary by species. As with humans, hormonal changes alter behavior patterns.
Lizards are far from loquacious, but they have well-developed ears and sense of hearing, which are often used to eavesdrop on potential predators.
Geckos are the most specious lizard family, with 1,500 distinct kinds. Geckos are typically small (ranging from 1.6–60 cm), with soft skin. They have adapted to a range of biomes, from deserts to jungles. Most live in warm climes.
Gecko vocalizations are unique among lizards. They talk to one another with clicking or chirping sounds which differ by species.
Almost all geckos lack eyelids, which are compensated for with tough corneas. Geckos lick their eyes to keep them clean.
Gecko adhesion is highly dependent on surface wettability and the presence of water or air between the toe pad and the contact surface. ~ American zoologist Alyssa Stark
Some 60% of geckos can walk up walls and on ceilings, even glass. Gecko soles have ridges (lamellae) that can become adhesive at will. The ridges of each foot have half a million hairs (setae). The end of each hair splits into 100 to 1,000 tiny spatulas which can only be seen using an electron microscope.
Surface contact with the setae spatulas creates billions of fragile molecular attractions, called van der Waals interactions, after Dutch theoretical physicist Johannes Diderik van der Waals. The van der Waals force may be attractive or repulsive. It emanates from molecular dipole moments.
van der Waals interaction is relatively weak compared to other chemical bonding, such as covalent bonding or electrostatic ionic interaction. van der Waals intermolecular attraction depends on the relative orientations of the molecules involved.
Electrostatic forces also effect gecko adhesion, but not as a dominant force. Geckos can adhere to surfaces where electric charges do not accumulate, such as bare steel.
Geckos have complete control over their van der Waals soles. At a certain angle, the subtle attraction is lost, the spatulas detach and roll up like a party favor.
Organic fats (phospholipids) secreted on the feet protect the delicate hairs and provide liquidity that assists adhesion and its release. Geckos can clean their feet of debris by hyperextending their toes.
Namibian Desert Gecko
Bradfield’s geckos live on the coast of Namibia, where cool winds and fog are the norm. Unlike most desert geckos, which are nocturnal, these Namibian geckos are diurnal. Their specially adapted scales efficiently soak up the Sun’s rays and keep the geckos warm.
Bradfield’s Namib day geckos need only 1/4th as much energy as other desert geckos. Slackers to the max, they move leisurely and eat extraordinarily little. Further, these desert geckos need little to drink, as 70% of their water intake is absorbed through their skin from the thick morning fog.
Adults chirp at a high pitch when approaching each other, as statements of claimed territory and other information.
As with communication proclivities, iguanas and scleroglossans have contrasting lifestyles. Iguana are highly territorial, with a defined home range. These lizards both advertise and defend their territory via ritualized displays. Scleroglossans are relatively non-territorial: defending only their homes.
Territorial boundary establishment and maintenance is a common cause of aggressive interactions between lizards, which usually involves sexually mature males. These behaviors vary by species: visual displays, posturing, chasing, grappling, and, at the extreme, fighting with potential for serious injury.
Non-territorial scleroglossans males act aggressively over scarce resources. One resource is especially valued: choice females.
Relative body size often tells the prospects for success for aggressive encounters between males, though hormone levels and established residency are also factors. Home court advantage counts.
Striped Lava Lizards
Boulders in northeastern Brazil host a dense community of striped lava lizards. A dozen or more may be on a single rock. Each social group boasts a dominant male, as evidenced by enlarged testes, along with subordinate males, who have smaller cajones, and several females, as well as numerous juveniles.
The dominance hierarchy maintains social order and allocates rock space, including spacing between individuals. When an intruder approaches, the group hustle across the rock in a wave, into crevices. These flat lizards pack themselves into crevices like volunteer sardines.
The dominance hierarchy is suspended while in a crevice. Emerging from the crevice onto the rock reestablishes social order.
All lizards in the group keep a lookout for predators. When one responds to a perceived threat, others follow that lizard’s lead and enter crevices. Survival trumps social niceties.
Lizards are typically polygynous: males mate with multiple females. Polygynous mating is ubiquitous in short-lived species, and those that breed multiple times within a season. Monogamously oriented lizard mating systems are known but poorly documented.
Either mating system is a matter of social status as well other fitness characteristics. Males compete to attract the most desirable female(s).
The Australian skink known as the sleepy lizard is monogamous, though extra-pair matings do occur. Monogamous relationships may last for years, not just for the breeding season.
Long-term pair bonding has advantages, including lower probability of predation owing to coupled vigilance by male and female. Sleepy lizard offspring tend to remain within the female’s home range, adding complexity to this skink’s social organization.
Lizard tails are often a useful if expendable appendage. Some lizards have tails that they may use to mimic their heads, such as with the sleepy lizard; others may mimic leaves. The tails of many arboreal lizards are prehensile. Iguanas and monitors beat their attackers with their tails.
Many lizards can practice autotomy: lose their tail to break the grasp of a predator. The detached tail continues to wiggle, providing a distracting deception that facilitates escape.
A lizard can regenerate a new tail in ~2 months. The new tail is not a perfect replica. The replacement tail lacks the fine motor control of the original.
Lizard tail loss is not without social consequence, particularly for juveniles of territorial species. High-status juveniles lose face with a lost tail. That lost status may be regained but losing a tail as a juvenile can have a cascading effect on a lizard’s success prospects.
Several invertebrates, including spiders, lobsters, sea stars, crabs, and octopi, are also autotomy artists in being able to regenerate a lost limb.
Lizards exhibit behavioral flexibility, using multiple strategies and reversal learning, plus rapid associative learning. This degree of flexibility is not predicted for a species that lacks complex social structure and has a relatively simple foraging strategy (i.e., sit-and-wait). ~ American zoologists Manuel Leal & Brian Powell
Many studies of reptile cognition in the 1950s and 1960s were inherently flawed, as they had been designed for mammals, which have different reactions to stimuli. With an accumulation of negative results from obtuse researchers, reptiles were generally considered dim-witted. The irony of scientific stupidity is unending.
In the 1960s, American neurobiologist Paul MacLean fantasized that the human brain had 3 levels: a triune brain. The lowest was the reptilian complex, followed by the limbic system, with the neocortex on top. This sophistic notion carried currency in some corners, particularly psychiatrists (not the brightest flock of psychologists), but never gained acceptance among thoughtful zoologists.
American science writer Carl Sagan imprudently popularized the triune human brain in his 1977 Pulitzer Prize-winning book: The Dragons of Eden. The Pulitzer Prize awarded was not given for fiction, which Sagan’s book was. Thanks to Sagan’s prosaic tripe, the notion of a reptilian part of the human brain is still popularly believed.
The sine qua non of intelligence is survival. Most lizards are well camouflaged in their natural surroundings and know to keep still until a predator passes by. With the ability to change their look, chameleons are especially careful to blend in to their location when danger is sensed.
Other lizards distract or startle a predator to give themselves a chance to escape. The Australian frilled lizard, for instance, opens its big mouth, loudly hisses, and flashes its expansive neck frill before scampering away.
Though lizard savvy has not been studied extensively, lizards are at least as clever problem solvers as social birds, such as those in the Paridae family, which includes tits and chickadees.
Monitor lizards are among the smartest of lizards. They can count at least up to 6, which is likely as high as they’ll ever need to count.
Monitors are adapted for hunting live prey. They have a venomous bite. 2 or more may hunt together cooperatively.
One monitor was observed luring away a female crocodile from her nest while its partner opened the nest to feed on the eggs. The decoy was then given its chance to an egg feast.
Sociality demands mental acumen in keeping track of relationships. Lizards are gregarious.