Sexual selection in scaled reptiles
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.[1] 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.[2]
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.[2]
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.[3] 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.[4] Males may be under strong selection for the development of a variety of characteristics that aid them in acquiring a mate.[5] The most successful males are the ones that display the most vigor in their efforts.[1] 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.[6] 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.[1]
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.[1]
Snake mating systems are generally believed to be
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.[8] The losers of fights have an increased Stress rate and fewer breeding opportunities[9] 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.[9] Some weapons used are spikes on the body and tail, teeth, and claws.[10] 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.[10] Male dominance correlates with a large territory size and access to females.[10] 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.[11] Faster males have energy to spend when it comes to obtaining food and territory[11] 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.[11] 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.[12]
Choice in lizards
Males in some lizard species can choose the female they want to mate with.[13] Males prefer more-ornamented females displaying better fitness and fecundity.[13] 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).[13]
Females in many lizard species have the choice to mate with or reject males.
Male painted dragon lizards, Ctenophorus pictus, are brightly conspicuous in their breeding coloration. However their color declines with aging.[17] 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.[17] 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.[18] 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.[19]
Snake tongues are bifurcated, or forked, to enhance their ability to pick up pheromones by means of
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.[20] 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.[21] Thamnophis sirtalis parietalis often prefer the pheromone trails of females who occupied the same communal den as males.[22][23]
In less male-biased scenarios, a male's tongue length plays an essential role in sexual selection. Dimensions of the copperhead,
In lizards
Females can determine a male's fitness based on the
Sexual conflict in snakes
Sexual dimorphisms are phenotypic differences between males and females of the same species. The objective of many sexual dimorphic studies done on snakes focuses more on broad comparisons between species from different regions and less on individual species themselves.[32] Size dimorphisms are common in snakes; females tend to be larger in populations where the production of large liters is feasible. Males tend to be larger in mating systems in which male-male competition is a large factor.[33]
Post-copulatory
In snakes
Snake sperm morphology and function is highly influenced by their ability to find, interact with, and fertilize eggs. Snake species display extended copulations and higher gonad mass/body mass proportions in males than other reptilian taxonomic groups.[36] Furthermore, their mating systems have a wide range of variability, depending on the temporal availability and predictability of the females. These factors influence sperm competition levels in both intense male-male combative species and those species that participate in prolonged mate searching.[36]
Often in snake species, females will copulate with multiple males in one mating aggregation.[6][37] To increase his chances in paternity, a male will sometimes try to inhibit the female from re-mating. A common tactic in many species is to obstruct the reproductive pathway of the female in order to physically prevent additional copulations. The mating plug of T.s. parietalis is a jellylike blob produced by males that is implanted within the females reproductive tract. Retention of the plug can last from two days to two weeks, with maximal effectiveness declining after the two-day mark.[37] The plug functions to prevent the leakage of the male's sperm from the female's cloaca, reduce the attractiveness and receptivity of the female to further copulations, and to physically block the reproductive tract to prevent immediate re-mating. The plug is not always 100% effective, but re-mating is a rare occurrence while the plug is in place.[37]
Cryptic female lizards
Females store
Inbreeding avoidance
When the female sand lizard Lacerta agilis mates with two or more males, sperm competition within the female's reproductive tract may occur. Active selection of sperm by females appears to occur in a manner that enhances female fitness.[42] On the basis of this selective process, the sperm of males that are more distantly related to the female are preferentially used for fertilization, rather than the sperm of close relatives.[42] This preference may enhance the fitness of progeny by reducing inbreeding depression.
Environmental stimuli in snakes
The semi-aquatic black swamp snake,
After the drought period larger snakes, both male and female, were less likely to survive, with a larger decline in the female population, mainly because female snakes had a greater body size compared to males. This correlates with the majority of Seminatrix pygaea giving birth at the start of the dry season, giving them some time to recover from the energy depletion they suffered from birthing clutches of offspring,[43] showing that selection favors smaller snakes in times of drought and larger snakes during years of high food abundance, and providing evidence that environmental factors affect sexual selection.
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