Sexual selection in scaled reptiles studies how sexual selection manifests in snakes and lizards, which constitute the order Squamata of reptiles. Each of the over three thousand snakes use different tactics in acquiring mates. Ritual combat between males for the females they want to mate with includes topping, a behavior exhibited by most viperids in which one male will twist around the vertically elevated fore body of its opponent and forcing it downward. It is common for neck biting to occur while the snakes are entwined.
Male-male competition
Snakes
In the species Japanese striped snake (Elaphe quadrivirgata), competition involves males maintaining body contact with their opponent and exerting pressure by pushing, topping, or entwining in order to subdue him.
Male snakes employ a variety of strategies to help them entice the female into mating. The red-sided garter snake (Thamnophis sirtalis) population in Alberta, Canada hibernates for the majority of the year, emerging in early May to copulate and feed. The communal dens have been observed to reach populations of thousands, with females often dispersing from the den rapidly to try to avoid being attacked by a flurry of males. Males must then rely on pheromone-based tracking to locate the females. When multiple males come into contact with a female intra-sexual competition often occurs. Snake species that have largely male-biased sex ratios often have high levels of male-male competition. Males may be under strong selection for the development of a variety of characteristics that aid them in acquiring a mate. The most successful males are the ones that display the most vigor in their efforts. Males who are able to align their body and cloaca, the joint outlet of the reproductive and digestive system, with that of the female for the longest period of time are often the most successful. While attempting cloacal alignment, a male entices a female by pressing his chin along the length of her body and continually attempting to intertwine his own tail with hers. At the same time, the male tries to prevent other males from breeding with the female, using his body to block their access.
When copulation is achieved, a male implements a coercive mating strategy known as caudocephalic waving. The male garter snake strikes his own body against the female in quick repeated intervals. These strikes drive anoxic air from the non-respiratory lung of the female into her respiratory lungs, causing an increase in her stress level. The increase in stress level causes cloacal gaping, allowing for the courting male to more easily insert his reproductive organ into the female.
Snake mating systems are generally believed to be polygynous, where males copulate with multiple females. Many researchers have assumed that multiple male courtships are successful without providing paternity evidence, and mating systems may be more polyandrous, supported by studies of the green anaconda (Eunectes murinus). A population containing hundreds of marked anacondas was observed over a period of several years with forty-five mating aggregations studied. When a female anaconda is ready to find a mate, she lies in an area of shallow water or mud until a male approached her. Some mating aggregations recorded up to thirteen individual males attempting to court the female at once. These interactions sometimes lasted for periods of up to a month, showing that the males put a large amount of their energy into mating attempts. Observation after copulation recorded no evidence of a male anaconda mating with multiple females or even searching for another female after mating. High-energy cost of courting is also apparent in the red-sided garter snake, where the most vigorous males would succeed in copulation. The intensity and amount of energy that male snakes put forth into the courting of females provides evidence for a polyandrous mating system among snake species.
Lizards
Males spend most of their energy in fights over a female to establish dominance and impress females by demonstrating a high quality of fitness. In White's skink (Egernia whitii), in southeastern Australia, larger males with strength and size correlating positively, intimidate rivals with their size, ward off other males from a female and claim territory space. The losers of fights have an increased stress rate and fewer breeding opportunities while fighting among males, which only happens during the breeding season illustrates to a female that he would be able to provide protection for her and her eggs. Some weapons used are spikes on the body and tail, teeth, and claws. An iguana's bite force can result in injury to other males; this leads the weaker male to flee from the fight and abort his chance of trying to mate. Bite-force performance predicts dominance in males and who sires more offspring. Male dominance correlates with a large territory size and access to females. In the common collared lizard Crotaphytus collaris, males display their locomotive skills in order to attract a female by getting to territory and resources first. Faster males have energy to spend when it comes to obtaining food and territory and are protective of their female mate and have a higher reproductive success and mate with more females on a first-come, first-served basis. In Australian agamid lizards, coloration influences competitive success; the more intimidating a male is perceived based on his color, the more likely a weaker male would not want to compete with him for a chance to mate with a female.
Choice in lizards
Males in some lizard species can choose the female they want to mate with. Males prefer more-ornamented females displaying better fitness and fecundity. In striped plateau lizards (Sceloporus virgatus), females during the breeding season develop an orange color on their throat area signaling that they are ready to mate, and it represents a higher quality female (fewer ectoparasites, and larger egg mass).
Females in many lizard species have the choice to mate with or reject males. Females spend energy in investigating a male's traits in order to determine if he is healthy and has good genes. In the species Side-blotched lizard (Uta stansburiana), males chosen by females had 76 percent less ectoparasites. Females who are not sickly can spare the energy in investigating their potential mate's qualities. Females prefer males that can afford to spend the most energy in displaying their traits because it is difficult to fake good genes. In the species Anolis pulchellus, females chose males who defended territory 89% of the time.
Male painted dragon lizards, Ctenophorus pictus, are brightly conspicuous in their breeding coloration. However their color declines with aging. Experiments in which antioxidants were administered to these males led to the conclusion that breeding coloration reflects innate anti-oxidation capability that protects against oxidative damage to DNA. This finding indicates that color acts as a “health certificate” allowing females, during mating, to visualize the underlying oxidative stress induced damage in potential male mating partners.
Pheromonal cues
In snakes
Snakes have two distinctive chemical sensory systems, which have unique roles in the reception and deciphering of various chemical signals: the main olfactory system and the Vomeronasal system, which is used to decipher distinct chemosensory information. In snakes this has a much greater complexity than its olfactory system and is essential for prey tracking as well as mate distinction and courtship.
Snake tongues are bifurcated, or forked, to enhance their ability to pick up pheromones by means of tropotaxis, and so are able to detect different stimuli intensities simultaneously using the two tips, flick them to pick up chemical cues from surrounding objects, which are delivered to the vomeronasal ducts upon reentry of the tongue into the oral cavity, activating the regions of the brain responsible for translation, allowing them to be processed and used for both mate and prey location. In nearly all snake species, the males are the mate-searching sex, and sometimes must travel a great distance to find a female.
Examples. A study done on prairie rattlesnakes, Crotalus viridis, indicates that males who maintain a constant search for females over fixed areas tend to be more successful in copulating. This population is very male-biased, so males had to focus more on locating a mate and were less concerned with male-male competition. In red-sided garter snakes, it has found that the amount of rival males, the length of time the chemical substrates last, and the distance over which the trail is evident are significant factors influencing a male's success rate in finding a female. Thamnophis sirtalis parietalis often prefer the pheromone trails of females who occupied the same communal den as males.
In less male-biased scenarios, a male's tongue length plays an essential role in sexual selection. Dimensions of the copperhead, Agkistrodon contortrix, show a significant tongue size difference, with males having a greater tongue bifurcation length, supporting the hypothesis that sexual selection is acting on tongue size in male snakes in order to enhance their ability to locate a potential female mate.
In lizards
Females can determine a male's fitness based on the pheromones he leaves in an environment.Martín, J. & López, P. "Supplementation of male pheromone on rock substrates attracts female rock lizards to the territories of males: a field experiment" PLoS ONE 7, e30108 (2012). These chemical cues inform females of a male's quality, with males trying to impress future mates by displaying elaborate colors, which can signal to a female that he is healthy, disease free, or has good genes,Martín, J. & López, P. Multiple color signals may reveal multiple messages in male Schreiber’s green lizards, Lacerta schreiberi" Behavioral Ecology and Sociobiology 2009; 63, 1743–55 as seen in the Dalmatian wall lizard Podarcis melisellensis, whose identity is based on their morph color: yellow, orange, the dominant colour as such lizards have a larger size and bite force so they can ward off competing males in order to mate with a female of choice and claim territory, and blue.Huyghe, K., Vanhooydonck, B., Herrel, A., Tadić, Z. & Van Damme, R. Female lizards ignore the sweet scent of success: Male characteristics implicated in female mate preference. Zoology (Jena, Germany) 115, 217–22 (2012). The females prefer the orange males since they are bigger and healthier and can give a female's offspring higher quality benefits. Even though females prefer to mate with orange morphs, females will still mate with yellow morphs, who give females direct benefits like protection and a small territory. Blue males are only able to mate by intruding on another male's territory and mating with other their females. What these lizards portray is the rock-paper-scissors game. In the species Uta stansburiana, throat coloration was also used as a way for males to elaborate their good health and body conditions to females. This is another example of the rock-paper-scissors game. The male European green lizards develop a blue throat patch, which shows high reflectance in the ultraviolet (UV) range, which females prefer since throat brightness correlates with a larger body size, a larger head size and less ectoparasites, though it is costly in energy. Females can then rely on throat color in male European green lizards to be an honest signal.
Sexual conflict in snakes