Sexual selection in lizards

In lizards, sexual selection is the consistent relationship between traits and fertilization success or fertility.^[1] Some traits that are favored by sexual selection are bright colors, intimidating weapons and fast locomotive skills. Mechanisms of sexual selection are female choice, male choice, male-male competition and cryptic female choice.

Female choice

Females in many lizard species have the choice to mate with or reject males.^[2] Females spend energy in investigating a male’s traits in order to determine if he is healthy and has good genes.^[2] In the species Side-blotched lizard (Uta stansburiana), males chosen by females had 76 percent less ectoparasites.^[3] 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.^[4] In the species Anolis pulchellus, females chose males who defended territory 89% of the time.^[5]

Females can determine a male's fitness based on the pheromones he leaves in an environment.^[6] These chemical cues play a major part in female choice because they inform females of a male’s quality.^[7] Males try to impress future mates by displaying elaborate colors which portray a male’s qualities. The elaborate colors of a male can signal to a female that he is healthy, disease free, or has good genes^[8] as seen in Podarcis melisellensis, also called Dalmatian wall lizards, are found in Croatia. These lizards are identified based on their morph color: yellow, orange and blue. A male that is an orange color morph is seen as a more dominant male than any other morph in intrasexual competition, since the orange color displays the lizard as more aggressive. Orange morph 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.^[9] In this species of lizards, the females prefer the orange males since the orange males are bigger and healthier and can give a female’s offspring high quality indirect benefits.^[9] Even though females prefer to mate with orange morphs, females will still mate with yellow morphs. Yellow morph lizards give females more direct benefits like protection and small territory than indirect benefits. Meanwhile, blue males are only able to mate by intruding on another male’s territory and mating with other male’s females. What these lizards portray is the rock-paper-scissors game.^[9] 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.^[10] Their throat colors are blue, orange, and yellow. These colors can indicate much about the lizards’ behaviors. Orange-throated males are more dominant and defend their territories; yellow-throated males defend their females; blue-throated males do not defend territories or females but breed by raping the females of the orange and yellow-throated males.^[11] In this species, females prefer yellow-throated males since they are not as aggressive as the orange throated males, but they protect the female better than blue throated males.^[11] This is another example of the rock-paper-scissors game.^[11] Lacerta viridis, also called European green lizards, male lizards develop a blue throat patch, which shows high reflectance in the ultraviolet (UV) range. Female lizards prefer males with UV throat reflectance since throat brightness correlates with a larger body size, a larger head size and less ectoparasites.^[12] Having high throat UV reflectance is costly in energy for male European green lizards. Therefore, male lizards are only able to gain this benefit if they are healthy (only high quality males will exhibit this trait). Females can then rely on throat color in male European green lizards to be an honest signal.^[13]

Male choice

Males in some lizard species can choose the female they want to mate with.^[14] Female ornamentation plays a role in male choice. Males prefer more-ornamented females since those females display a better fitness as well as better fecundity.^[14] In striped plateau lizards (Sceloporus virgatus), females develop an orange color on their throat area signaling that they are ready to mate (only during the breeding season). A brighter orange color represents a higher quality female (fewer ectoparasites, and larger egg mass).^[14]

Male-male competition

Intrasexual competition between males for female mates is very common among different lizard species. Males spend most of their energy in fights over a female to establish dominance and/or to impress a female by demonstrating a high quality of fitness. In Egernia whitii, a type of skink found in southeastern Australia, larger males (strength and size correlating positively) have advantages over smaller males. Larger males are able to intimidate rivals with their size, ward off other males from a female and claim territory space.^[15] The losers of fights have an increased Stress rate and less breeding opportunities.^[4] Fighting among males illustrates to a female that he would be able to provide protection for her and her eggs.^[4] Males mostly use their weapons and become more aggressive only during the breeding season when they are competing over a female. Once the breeding season is over they rarely use their weapons. Some weapons used are spikes on the body and tail, teeth, and claws.^[16] In combat, an iguana lizard’s teeth are used as a weapon since their 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.^[16] Male dominance correlates with a large territory size and access to females. Males with a stronger bite force sire more offspring in comparison to weaker biting rivals as well.^[16] In the collared lizard Crotaphytus collaris, males display their locomotive skills in order to attract a female by getting to territory and resources first.^[17] Faster males have energy to spend when it comes to obtaining food and territory.^[17] Faster males are protective of their female mate as opposed to slower males they also have a higher reproductive success then slower males because faster males mate with more females on a first come, first serve basis.^[17] 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.^[18]

Post-copulatory mechanism

Cryptic female

Females store sperm after sexual intercourse from multiple males before ovulation, but how sperm is stored is still unknown.^[19] There is a time gap between sperm copulation and fertilization. This time gap allows the female to mate with multiple males. The most competitive sperm will fertilize the female’s eggs while the remaining sperm will be discarded.^[20] Females also gain nutrients from sperm storage so the more a female mates with different males the more nutritional access she will obtain.^[21] After copulation, some female lizard species chose the sex of their offspring based on the male’s sperm. Females produce sons with sperm from larger sires, and daughters from sperm with smaller sires.^[22] A reason would be to ensure that her sons will have good genes that can display stronger more elaborate traits in order to maximize his reproductive success.^[22] An increase in the production of sons demonstrates a preference for larger males.^[22] Daughters are produced from smaller sires since females do not need to use a large amount of energy to attract a male; more males are ready to mate than females.^[22] However, if females store sperm from a variety of sires, females can produce both sons and daughters with high fitness.^[22]

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