The concept of a placenta is usually associated strictly with mammals, serving as the biological link that sustains a developing fetus inside its mother. The idea that a reptile, such as a lizard, might possess this structure seems contradictory to the classic image of reptiles as egg-layers. However, the world of lizard reproduction holds a surprising level of complexity that challenges this simple classification. Some species have evolved a temporary organ remarkably similar in function to the mammalian placenta. This reproductive adaptation represents one of the most fascinating examples of evolutionary convergence found in vertebrates.
The Spectrum of Lizard Reproduction
Reproduction in the order Squamata, which includes all lizards and snakes, is a spectrum ranging from laying eggs (oviparity) to giving live birth (viviparity). The majority of reptile species reproduce through oviparity, where the female deposits eggs that contain a large yolk sac. The eggshells in these species are typically leathery and permeable, providing protection while allowing for gas exchange.
In contrast, viviparity, or live birth, has evolved over 100 independent times across squamates. In viviparous species, the developing embryo is retained within the mother’s reproductive tract until it is fully developed and ready to hatch. This live-bearing strategy shifts the burden of embryonic maintenance from an external egg to the maternal body.
A common intermediate stage is prolonged egg retention, where the female holds the eggs inside her body for a longer period before laying them. Even most oviparous squamates retain embryos for about one-third of the total gestation period, providing a clear evolutionary pathway toward full viviparity.
Why Viviparity Evolved in Lizards
The transition from egg-laying to live-bearing is a major evolutionary shift, primarily explained by the “cold-climate hypothesis.” This theory suggests that retaining developing embryos internally allows the mother to regulate the incubation temperature through behavioral thermoregulation. By basking in the sun and moving between warm and cool spots, a gravid female can maintain a more constant and often higher temperature for her developing young than the surrounding soil.
This maternal control is advantageous in high-altitude or high-latitude environments, where soil temperatures are often too low or variable for successful external incubation. Warmer temperatures accelerate embryonic development, which helps ensure the young hatch before the onset of extreme cold. For instance, a seven-degree Celsius difference can reduce the required incubation period by over 40 days in some species.
Viviparity also offers protection from external threats, such as predators and pathogens, which target vulnerable nests. An embryo retained inside the mother is constantly mobile and shielded, unlike a stationary egg clutch deposited in the soil. Furthermore, retaining the young allows the mother to seek out better foraging areas or safer refuges without abandoning her offspring.
Structure and Function of the Lizard Placenta
In viviparous lizards, internal embryo retention requires the development of a temporary structure for exchange between the mother and offspring—effectively a placenta. This organ forms through the intimate apposition of extraembryonic membranes from the embryo and the mother’s uterine lining. Unlike the single type of placenta found in most mammals, lizards develop two primary placental regions derived from distinct embryonic membranes.
The first is the yolk-sac placenta, or omphaloplacenta, which arises from the embryonic yolk sac that initially provides nutrients. The second is the chorioallantoic placenta, formed by the fusion of the chorion and the allantois, a membrane that handles waste storage and gas exchange. In the vast majority of viviparous lizards, the placenta’s primary roles are to facilitate gas exchange and to regulate the transfer of water, calcium, and sodium.
For most species, the bulk of embryonic nourishment still comes from the large yolk sac, a strategy known as lecithotrophy. However, in specialized lineages, such as certain Australian skinks and Neotropical mabuyine lizards, the placenta supports a significant transfer of organic nutrients from mother to embryo, a process called matrotrophy. These matrotrophic species often have eggs with very little yolk, making the placenta a true source of sustenance.
Structurally, the lizard placenta is much simpler than the complex, highly invasive placenta of placental mammals. Nutrient transfer often occurs via histotrophy, where the embryo absorbs nutrient-rich secretions from the uterine wall, rather than the deep, direct intermingling of maternal and fetal blood vessels seen in mammals. Although the lizard placenta performs the same fundamental functions of gas exchange and nutrient transfer as its mammalian counterpart, it represents an entirely independent evolutionary solution to the challenge of internal embryonic development.