I think we can all agree that amniotic eggs are delicious. They also happen to be one of the best sources of energy out there, and this is at least partially why we, and many other animals, enjoy eating them so much. In addition, they rarely fight back, and they almost never have physical defenses, such as spines, or chemical ones, such as deadly toxins. In fact, on the inside they’re pretty much all lipids (a group of molecules including fats and cholesterol), surrounded by either a leathery (in monotremes and most reptiles) or a hard, calcified (in birds) shell. I’ve already written about a species of burying beetle that specializes on snake eggs, apparently with great benefit to its fecundity relative to other burying beetles that use carrion. Turns out, snakes aren’t above specialized oophagy themselves.

There are a few snakes that eat anamniotic eggs, such as the turtle-headed sea snakes (about which I’ve written before) and the South American goo-eaters. These have many amazing adaptations to eating shell-less eggs, but I’d like to focus on the amniotic egg-eating snakes for now. To review, an amniotic egg is one with a shell and several other embryonic membranes, called the amnion, chorion, and allantois. These structures physically protect the embryo and facilitate gas and waste exchange between the embryo and its surroundings, because the shell is too thick to allow the embryo to breathe and excrete by diffusion alone. These eggs are laid by birds, many reptiles, and monotremes (egg-laying mammals such as the platypus and echidna). In placental mammals (including humans), which are also amniotes, some of these structures are part of the umbilical cord, while others are vestigial. Amniotic eggs are adapted for being laid on land, and even the most aquatic of amniotes, such as sea turtles and pelagic birds, must come to land to lay their eggs.

Because of the resilience and self-contained nature of amniotic eggs, many organisms that lay them have done away with parental care. Choosing a nest site, usually under a rock, log, or pile of poop, or in a nest dug underground, is the extent of it. Beyond that, a female snake or turtle will most likely never see her kids hatch, let alone grow up, graduate, or become successful. This also means that their eggs are basically undefended from predators, except for being concealed and not smelling very much. Birds are slightly better parents, but they risk giving away the location of their nest to predators by flying back and forth to it many times a day. Experiments conducted by herpetologist Steve Mullin and ornithologist Bob Cooper have shown that gray ratsnakes locate bird nests over twice as quickly when parents are attending than when they aren’t, a phenomenon so prevalent that it has its own name (Skutch’s hypothesis) and is thought to influence the evolution of optimal clutch size in birds (because more offspring need to be fed more often, necessitating more trips to and from the nest and increasing the likelihood of detection by a predator).