The question of whether all small lizards lay eggs highlights the diverse reproductive strategies within this large group of reptiles. While the majority of small lizard species reproduce by depositing eggs (oviparity), this method has many exceptions across the globe. Understanding lizard reproduction requires examining this primary strategy, the characteristics of their eggs, and the evolutionary pressures that led some species to evolve live birth.
The Primary Method: Oviparity in Small Lizards
The vast majority of the world’s lizard species, including most small ones, reproduce through oviparity. This process involves laying eggs that complete their development outside the mother’s body. Oviparity is considered the ancestral reproductive mode for most reptiles and is widespread across many common families. Many small lizards, such as Anoles, lay a single egg at a time, often repeatedly over a breeding season.
Other small species, including many geckos, typically lay one or two eggs in a clutch, while some skink species commonly produce two eggs per reproductive cycle. Clutch size is often correlated with the size, age, and overall condition of the female lizard, with a clutch of four to eight eggs being typical for many small to mid-sized species. Females can produce multiple clutches in a single mating season, allowing their populations to thrive despite environmental challenges and high predation rates.
Characteristics of Lizard Eggs and Nesting Locations
Lizard eggs vary in their physical properties depending on the species, which influences where they must be deposited for successful incubation. Most lizard eggs have a leathery and porous shell, allowing them to absorb moisture from the surrounding environment as the embryo develops and expand in size. An exception is found in the majority of egg-laying geckos, whose eggs possess a shell that hardens shortly after being laid, preventing any further change in size or shape.
The female lizard carefully selects a nesting site that offers the necessary temperature and moisture levels for incubation, often providing protection from predators. Typical nesting sites are dark and moist, such as underneath logs, buried in soil, beneath rocks, or within dense vegetation. Some common house geckos will even attach their eggs in hidden crevices or behind objects on walls. The incubation period typically takes several weeks to a few months for the hatchlings to emerge, after which most lizard species do not provide parental care.
The Exception: Small Lizards That Give Live Birth
Some lizard species have evolved a reproductive strategy that involves giving birth to live young. This includes viviparity, where the young are nourished internally and born fully developed, and ovoviviparity, where eggs hatch inside the mother’s body just before birth. Viviparity has evolved independently over 100 times in the group of reptiles that includes lizards and snakes.
Approximately 20 percent of all lizard species are viviparous, including a significant number of small species. About one-third of all skink species are live-bearers, with many living in colder climates. The Common Lizard is a small Eurasian species that often gives birth to live young and lives further north than any other non-marine reptile. In rare cases, such as the Yellow-bellied Three-toed Skink, populations can exhibit both egg-laying and live birth, demonstrating an evolutionary transition.
Environmental Influences on Reproductive Strategy
The presence of live-bearing species, which are the exception to the rule of egg-laying, is largely explained by the “cold-climate hypothesis.” This suggests that low environmental temperatures provide the primary selective pressure for the evolution of viviparity. In cold habitats, eggs laid in a nest are exposed to low temperatures that can slow embryonic development, delay hatching, or even be lethal.
By retaining the developing embryos internally, the female lizard can actively thermoregulate by basking in the sun. This maternal manipulation of temperature leads to a shorter developmental time for the young and an increased chance of survival in colder environments or at high altitudes. This mechanism provides a selective advantage over external nesting where low temperatures would otherwise compromise the viability of the offspring.