Parietal-Eye
Horned lizards, along with most other lizards, have a Parietal-Eye (also known as third eye) on the top of their head. Not much is known about why they have them, but research suggests that it is useful for thermoregulation and burrowing.
Photoreceptor cells in human eyes can only see either red, blue, or green pigments. In contrast, lizard parietal eyes contain photoreceptors with both blue and green pigments in each cell. As can be seen in the diagram above, parietal eyes also lack bipolar cells, so information from photoreceptors go straight to the ganglion cells. When blue is detected by the parietal eye, gustducin is released, which causes hyper-polarization and in turn sends an off signal to the connected nerve. When green light is detected by the parietal eye, Go is released, which causes de-polarization and in turn sends an on signal to the connected nerve. (Simpkins). The significance of this complex is thought to serve as an indicator for dawn/dusk. The low amount of light at dawn triggers green pigments, which causes the complex to become de-polarized. When the day becomes brighter, the light triggers the blue pigments, which causes the complex to become hyper-polarized (Solesslo).
Brumation
During mid-November to mid-January, the horned lizard enters a state called Brumation. Similar to hibernation (which is limited to endotherms, such as bears), brumation is a period of inactivity during the winter season in which reptiles undergo lowered metabolism, energy demands, and heart rate. By doing this, organisms are able to go for long periods of time without food or water, and therefore don't expend an excessive amount of energy trying to stay alive in the winter (Holden).
Corticosterone Levels in Brumating Ectotherms
A popular measure of stress in non mammals (birds, amphibians, and reptiles) is by studying their corticosterone levels. Corticosterone is a steroid that regulates glucose intake levels, which is a beneficial factor in preparation for months of brumating for these Horned Lizards. This is because having a stored amount of glucose in the body saves energy and allows the lizard to rest for the winter months (Holden).
The image above is from Holden's study on the physiological effects of brumation in reptiles. The X axis represents temperature (Celsius), and the Y represents the amount of Corticosterone in the bloodstream (ng/ml). As can be seen, as temperature decreases and Brumation begins, Corticosterone levels increase in order to increase the uptake of glucose in the body. As the temperature increases and Brumation comes to an end, Corticosterone regulates back down to the original level.
The HPI (Hypothalamic-Pituitary-Interrenal) axis is the system in which Corticosterone is released due to a stress response.
1) This starts with the Hypothalamus (located in the brain), which releases corticotrophin-releasing factor (CRF) to signal the Pituitary gland (Rollins-Smith).
2) Upon receiving this signal, the Pituitary gland releases corticotropin straight into the bloodstream, to head towards the Adrenal gland (Rollins-Smith).
3) After receiving this signal, the Interrenal gland releases Corticosterone (Rollins-Smith).
Here is a quick video to understand the HPI axis (Neuroscientifically Challenged). Please keep in mind that this video goes over the HPA axis found in mammals, so be sure to substitute the different glands and hormones listed in the above section that are specific to the HPI axis in non-mammals.
Ectothermy: Behavioral and Physiological Thermoregulation
Behavioral Thermoregulation
Horned lizards are strictly ectothermic, which means that they are unable to control their body temperature. To adapt to their varied surroundings, they utilize behavioral thermoregulation (Hill).
Here is a horned lizard sun-basking, which is the act of finding patches of sun to raise their internal body temperature. These lizards usually prefer to bask at dawn, to warm their body up from the previous night. Specifically, in a study where horned lizards were transplanted to different climates, they altered their basking time in order to adjust to new climates (Refsnider).
Physiological Thermoregulation
In addition to behavioral strategies to thermoregulate, horned lizards also have highly sophisticated physiological methods to cool down their body (not control, as they are ectotherms). Lizards have a large amount of cephalic blood vessels, as can be seen in the image below. They are able to transfer warmed or cooled blood to their head in an attempt to warm or cool themselves (Porter).
Digestion of Harvester Ants
A horned lizard's diet mainly consists of harvester ants, which are extremely poisonous. Horned lizards are adapted to get around these poisonous meals without getting harmed in any way.
The image above shows the fast process in which the lizard catches the ant. As can be seen, the whole process occurs in 512 ms. This feeding pathway occurs in one swift motion, therefore giving the ants hardly any time to retaliate. Also, the lizard doesn't use teeth as part of their feeding pathway, so this minimized ingestion time as well (Fertschai).
The red arrows in the image above highlight another very important factor in their successful prey catching techniques: their mucus. The horned lizard's pharynx and esophagus produces a large amount of mucus. This mucus immobilizes the ants as they are swallowed alive, which protects the lizard's buccal and digestive tract tissues from the ant's poison (Fertschai).
Mechanoreceptors
Horned Lizards, along with many other reptiles, have mechanoreceptors on their scales. Before we get into details about its functions, let's take a deeper look into how mechanoreceptors work.
Tactile mechanoreceptors are activated by mechanical stimuli, such as a light touch on the horned lizard's scales. When this happens, the stretching of the mechanoreceptor membrane activates a stretch-activated channel. This causes the inflow of Na+ and K+ cations (Hill).
The inflow of cations produces a receptor potential, which is the main response of a receptor cell to stimulation. The cations depolarize the sensory cell (make the inside less-negative than the outside). This then triggers action potentials inside the cell, which are (Hill).
In a tactile sensory receptor, light touch causes a low frequency of action potentials, while more pressure causes high frequency of action potentials. To sum it up: the pressure of tactile simulation is positively correlated with the frequency of action potentials (Hill)
What is an action potential?
An action potential occurs when there is a change in voltage between two membranes. The completion of a full action potential results in an impulse (Hill).
As can be seen in the picture above, stages of an action potential are resting, rising, falling, and recovery.
A) At rest, the only channel that is open (and is open all the time) is a K+ leakage channel, which results in a similar charge on both sides (Hill).
B) During the rising phase, a stimulus causes an increase in membrane potential. If the stimulus is strong enough (to pass the threshold line), then Depolarization occurs. This is when the voltage gated Na+ channel is opened, and Na+ rushes through the membrane into the axon (Hill).
C) To initiate the falling phase, or Repolarization, the Na+ channel closes and inactivates soon after opening, which then causes the membrane potential to fall (Hill).
D) During recovery, the K+ voltage channel remains open until the membrane potential returns back to its resting value (Hill).
These are mechanoreceptors present on the scales of Phrynosoma modestum (the Roundtail horned lizard), and are found on all other types of horned lizards.
(Photo credit: Sherbrooke)
Here is another view of a mechanoreceptors on a multi-scale complex (as shown by the black arrows)
(Photo credit: Sherbrooke,)
Directionality of the Ear
The auditory system of lizards differs from that of mammals in many ways, but one aspect that has a large affect on their hearing is their form of directionality.
Binaural Sound Localization
Binaural Sound Localization is the process of locating the source of a sound based on its distance and angle from the ear.
Above is a graphic that helps to explain sound localization. On the left side of the picture, there are two speakers playing sound at the same volume and distance, and so the person assumes that the noise is coming from straight ahead (represented by the triangle). On the right side of the picture, the left speaker is playing sound closer to the ear, and the right speaker is playing sound farther from the ear. As can be seen by the triangle, the perception of sound locality is aligned with left side speaker.
Here is a great example of sound localization (Binaulab Audio 3D). Be sure to wear headphones/earbuds and deselect mono stereo on your device.
How is Sound Localization Different Between Humans and Lizards?
Since lizards have a significantly smaller head than humans do, they have evolved a method to improve their sound localization. Their head size matters because many wavelengths have amplitudes that are simply bigger than their head, and therefore cannot be detected very effectively by their auditory system (louder sounds have a larger amplitude) (Christensen-Dalsgaard). In the video of Queen in the previous paragraph, the listener can tell where the sound is coming from based on the timing, amplification, and direction of the music. In lizards, since they are unable to do this very effectively, they have an enlarged Eustachian tube to supplement their localized hearing (Christensen-Dalsgaard).
The above graphic shows the reptilian ear (left) and the mammalian ear (right). The function of the Eustachian tube is to bring the environmental pressure and the ear pressure to an equilibrium. The tube is connected to the throat sinuses, and is the only cavity that can connect the two eardrums (from the throat). Since mammals are able to rely on the direction and angle of sound location to determine where it's coming from, they have a small Eustachian tube. On the other hand, since lizards are unable to affectively fully rely on spatial direction of sound, the enlarged Eustachian is able to couple the vibrations from both eardrums to internally localize sound. (Christensen-Dalsgaard).
Muscular Involvement in Autohaemorrhaging
Horned Lizards have an interesting form of self defense that seems straight out of a sci-fi movie -- they squirt blood from their eyes! Well, more specifically, from their ocular sinuses.
Video credit: (Nat Geo WILD)
Blood pressure builds up in the sinuses below the Horned Lizard's eye when blood flow from the head to the heart is cut off. This occurs with the constriction of the internal jugular vein by the jugular constrictor muscle (Porter).
The internal jugular vein is a major vein located at the base of the head that delivers blood from the head back to the heart. When this vein is constricted by the jugular constrictor muscle, blood pressure in the head increases, which causes sinuses below the eye to swell (as seen on the left). With enough localized pressure, the sinus membrane will burst and blood can squirt out up to 5 feet! (Porter).
In Figure A, the green circle represents a non constricted jugular constricter muscle. The thickness of the arrows relates to amount of blood flow. As can be seen, there is heavy blood flow from the head to the jugular vein, and little blood flow in the head and eye area. In Figure B, the red x represents a constricted jugular constricted muscle. With this muscle in action, there is decreased flow from the head to the heart, and an increased flow of blood in the eye and head area (Porter).
A Deeper Look of the Constriction of the Jugular Vein By the Jugular Constrictor Muscle
The contraction of skeletal muscles in the body start when they are signaled by the neurotransmitter Acetylcholine (ACh) that is innervating a fiber. This triggers an action potential to begin it's depolarization phase, which lets Na+ into the membrane (Lumen Learning).
When the action potential completes its cycle, this signals Ca++ from the Sarcoplasmic Reticulum. The presence of Ca++ then initiates muscle contraction by binding to Troponin.
Muscle Contractions are the result of thick and thin filaments in muscle fiber moving against each other in a process called the cross bridge cycle. Below is an image that gives a summary of the muscle contraction process, and then a video that shows the cross bridge cycle (Muscle Contraction - Cross Bridge Cycle, Animation).
Iliofibularis Muscle
Located in the hindlimb of lizards (shown by the dotted line on the left), the iliofibularis muscle's purpose is to provide strength during locomotion. In the horned lizard, research has shown that this muscle contains a very high percentage of fast oxidative glycolytic fibers (FOG) (Bonine).
Fast twitch fibers
A twitch fiber is produces action potentials which cause the muscles to twitch. Slow twitch fibers create long strong twitches which are more durable (like for running a marathon), while fast twitch fibers create short fast twitches which are not as strong or durable (like for sprinting). There are two types of fast twitch fibers: fast oxidative glycolic fibers (FOG), and fast glycolytic fibers (FG) (Hill).
The main difference between the two fast type fibers (FOG and FG) is that FOG fibers use oxygen to produce ATP while FG fibers do not. FOG fibers contain a larger amount of mitochondria, and since they produce ATP aerobically, they are more resistant to fatigue. On the other hand, FG fibers are twitch faster, but since they produce ATP through glycolysis, they are less resistant to fatigue (Hill).
Why do Horned Lizards Have Mostly FOG fibers in their iliofibularis muscle?
It is hypothesized that horned lizards have mostly FOG fibers in their iliofibularis muscle due to their lifestyle. Compared to other types of lizards, horned lizards are significantly slower, and they rely on crypsis (camouflage) as their main way of avoiding predators. Therefore, they have no need to evolve more FG fibers in their iliofibularis muscle, since they don't need to sprint very often. Also, horned lizards are "sit and wait predators", meaning that instead of chasing around their prey, they sit and wait near anthills. This is another hypothesized reason as why they have evolved more FOG fibers, because of their low amount of mobility (Bonine).
Primary Literature Sources
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