Eastern fence lizard

This species has sexual dichromatism, which is a difference in coloration between the sexes. Males are blue and black on their ventral side. In some lizards, this coloration appears green or turquoise. In males, these patches assist intraspecific sex recognition and signaling. The size of patches correlates with the size of a lizard, and therefore may be a signal towards females or other antagonistic lizards.[11] Males are somewhat uniformly brown elsewhere with a reddish tinge.[12]

Females and juveniles are slightly colored blue and black on the ventral side or not at all. Their coloring is more complex, with "rows of dark brown or black chevrons set against a background of gray and brown."[12]

This sexual dichromatism is correlated to and somewhat regulated by plasma levels of testosterone in juveniles and adult females. Higher levels of testosterone support stronger blue and black ventral coloration.[12]

Male eastern fence lizards can also experience short-term color changes on their dorsal side and badge after exposure to temperature changes. The color of a lizard affects its social dominance or inferiority, camouflage, and self-regulation of temperature. Social dominance as a result of lizard color may exist because coloration would then possibly provide an honest indicator of lizard temperature and health.[13]

It is thought that sexual dimorphism and sexual differences in behavior may be due to differences in androgen concentration and receptor immunoreactivity.[14]

The eastern fence lizard has several adaptations to better survive in colder regions of its distribution, including larger eggs, more efficient embryonic development, and shorter incubation times. The larger egg sizes are not a result of greater maternal investment — even when forcibly made smaller, efficiency and incubation time were similar to the originally larger eggs.[21]

On the other hand, female lizards in southern regions produce second clutches more often, although their clutches are smaller than northern ones. The season at which eggs can incubate and lizards may be active is longer in the southern regions.[22]

The overall reproductive success of both northern and southern lizards are approximately equal, given that all else (i.e. body size) is equal.[22]

The running speed of these lizards is not limited by their mechanical power output. As the slope of the terrain increases, running speed and stamina of lizards decreases. Eastern fence lizards run faster uphill than downhill at a fixed degree of incline or decline. Lizards with higher running speeds also have higher stamina in regard to time and distance.[25]

Male eastern fence lizards will establish a territory in the early spring, doing push-ups to flash their blue scales and deter other males.[26] Males will headbob more at males with smaller ventral patches, but aggressiveness is not affected by male patches.[11]

The size of a male's territory (home range area) is directly correlated with his body size. The territory of smaller males in their first breeding season tends to be smaller. Territories may overlap; about half of any given male's territory is shared by another male.[26] Within their territory, males cluster around the ranges of females, who are a limited resource. As such, the range and distribution of males is predominantly determined by the distribution of nearby females.[26]

Courting begins after female fence lizards encounter a male within his territory. Males express sexually dimorphic ventral abdominal coloration during courting. This coloration is used to attract mates, as more and brighter colors have been recorded to increase the likelihood of courtship.[27]

In 60% of instances where a female and male encountered each other, males displayed courting behaviors. Older and larger males tend to win contests with other males, and therefore gain access to any nearby females. Any contest victories or defeats did not affect a lizard's future location.[26] Larger bodied males are more likely to mate with multiple females.[28] Contests with other males and courtship towards females elevates plasma levels of corticosterone, an indicator of stress, in adult males.[26]

Feeding behaviors

Fence lizards are ambush, sit-and-wait, predators.[29] They are insectivores that typically eat arthropods such as ants and grasshoppers.[30] Studies have shown that eastern fence lizards display an ontogenetic shift in consumption of native and invasive prey. Although eastern fence lizards are an ant specialist, invasive imported red fire ants can pose a risk to young, smaller-bodied juveniles if they consume them, especially because young fence lizards are more common consumers of ants than adults.[31] It was found there was an increased consumption of fire adults in the adult life stage compared to earlier life stages.[31] These lizards are known to forage generally twice a day. Females will eat more during the breeding season for greater energy storage to help produce offspring.[32]

The female finds a suitable location to lay her eggs, usually in a rotting log or similarly damp area, and deposits them without any further parental care.[33] Young females will only produce one clutch of three to sixteen eggs, while a large female can produce up to four.[34] The eggs take approximately ten weeks to hatch and emerge near the end of summer. The young lizards grow quickly and are able to reproduce the next year.

Unfavorable nesting conditions can cause females to retain their eggs longer than they would have otherwise. This phenomenon is referred to as egg retention. Egg retention in the eastern fence lizard produces heavier eggs with more advanced embryos and with higher posthatching survival rate, but does not influence phenotype.[35]

Greater reproductive energy allocated to first clutches is common at higher latitudes, whereas more energy is dedicated to later clutches in lower latitudes.[36]

After the eggs are laid by a female, the development of the embryo and yolk within differ from that of birds, which has long been assumed to be identical. Instead, the process is more similar to snakes.

Strands form a vascularized network within the yolk sac cavity. The embryo and yolk sac cavity then progress through three distinct developmental stages.

In the first stage, endodermal cells develop and invade the nourishing yolk in order to digest it. They may clump within and begin filling the cavity.

In the second stage, the vasculature of the yolk sac produces blood vessels that expand throughout the cavity.

In the third stage, the cells organize themselves around the blood vessels. They can now more quickly and easily digest yolk. The products of digestion reach the embryo via release into blood vessels.[37]

The pressures of predation can select for a variety of escape tactics. In their natural habitat, these lizards encounter invasive ants, red imported fire ants (Solenopsis invicta), the bites of which threaten the lizards with envenomation and, possibly, reduced growth in juveniles. Fire ants also compete with eastern fence lizards for their nesting habitats and predate on their eggs. [38] These encounters can be dangerous and even lethal to the lizards. When attacked by these stinging fire ants, fence lizards twitch their bodies to throw off the attacking ants before fleeing.[39] The learning of these lizards after repeated exposure to fire ants has also been studied. Lizards that were more familiar or exposed to attacking fire ants tend to twitch more and flee sooner compared to lizards that were not exposed. This may also be an adaptive behavioral response induced by higher stress levels (and thus, elevated corticosteroids). However, exposure to fire ants did not increase the percentage of lizards that demonstrates escape behavior. Instead, those that had priorly shown escape behavior were more likely to show escape behavior earlier and more strongly. In other words, this behavior is not learned, but can be shaped if already present. The number of twitches performed by a lizard is also not affected by previous exposure. Juveniles, which are younger and smaller, flee more rapidly than adults in any encounter.[39] Furthermore, within the past 70 years, according to a study published in 2009, eastern fence lizards in certain regions have adapted to have longer legs and new behaviors to escape the red imported fire ant, which can kill the lizard in under a minute.[40]

Given the incubation period of the eggs, one study suggests that 61% of nests may be at risk of predation by these invasive fire ants. Nest choice and distance to the nearest fire ant mound did not significantly impact predation; thus, because there is little that eastern fence lizards can do to resist such predation on their eggs, there is conservation efforts toward stopping invasive species such as this fire ant.[38]