Turtles and snakes must cope with unusual challenges to bring forth young; turtles have rigid shells that don't open very wide, while snakes' eggs can't limit their locomotion before they are deposited. To overcome these challenges, snakes deposit oblong eggs that better fit their abdominal cavity, and turtles deposit small, round eggs that easily pass through their shell openings.
Snake eggs are leathery or rubbery to the touch and feel like a little bag of jelly or liquid. The eggs usually are elongated and deposited in a sheltered spot, such as a tree hollow. In the early stages of incubation, snake eggs contain relatively little albumin; the albumin develops over time as the permeable egg draws water from the environment. Because of this, snake eggs increase greatly in size over the course of their incubation.
Turtle eggs are round to slightly oblong, and white to cream in color. Turtle eggs may be rubbery or leathery to the touch -- like snake eggs -- or hard like the egg of a bird. The rigidity of a turtle eggs varies based on how much water the eggs must absorb from their environment; in general, harder eggs are less permeable to water than softer eggs are. Because turtle eggs must pass through the narrow gap in their shells, turtles are limited to producing relatively small eggs and offspring. To help maximize the amount of available space inside their small eggs, the shell of developing turtles is pliable, and often bends to fit inside the egg. While small turtle species only deposit a few eggs at a time, large turtles produce very large clutches, and sometimes produce many clutches per season. The world’s largest species -- the leatherback sea turtle (Dermochelys coriacea) -- deposits an average of 400 to 500 eggs per season.
Variation in Reproductive Methods
Given that the world holds more than 10 times as many snake species as it does turtle species, it's not surprising that snakes exhibit much greater reproductive diversity than turtles. All turtles practice internal fertilization and deposit eggs for external incubation. Most snakes practice internal fertilization as turtles do, but scientists have documented at least one species -- the flowerpot snake (Rhamphotyphlops braminus) -- that is an all female species and reproduces asexually, through parthenogenesis. Most snakes are oviparous, or egg laying, but others give live birth. Two modes of live birth occur in snakes; ovoviviparous species nourish their developing young with egg yolk, while viviparous species nourish their young through a placental connection. Both modes feature young that are enclosed in transparent, unshelled eggs, which hatch at or near the time of birth.
Two primary methods exist for determining the gender of developing embryos; the combination of the parents' genes determine the gender of some embryos, while environmental conditions surrounding the egg determine the gender of other species. Snakes tend to exhibit genetically determined sex. Turtles, by contrast, largely employ environmentally determined gender. Turtle embryos become male or female based on the temperatures surrounding the egg; above a threshold temperature embryos become one gender, and below that threshold the embryos become the other gender. Some species produce both genders when incubated at moderate temperatures.
- United States Fish and Wildlife Services: Leatherback Sea Turtles (Dermochelys coriacea)
- Reptile Database: Species Numbers
- Arkive.org: Brahminy Blind Snake
- Copeia: Temperature Dependent Sex Determination in Sea Turtles
- Evolution: Sexual Competition Among Brothers May Influence Offspring Sex Ratio in Snakes
- Journal of Embryology: Growth and Metabolism in Snake Embryos
- David De Lossy/Photodisc/Getty Images