REPRODUCTIVE STRATEGIES

Unlike turtles, tortoises, crocodiles, alligators, and the tuatara, which all lay eggs, squamate reptiles employ both oviparity (egg-laying) and viviparity (live-bearing). There are advantages and disadvantages to each of these strategies.

Every spring, thousands of Red-sided Gartersnakes (Thamnophis sirtalis parietalis) emerge from communal dens in Manitoba, Canada. The males emerge first to await the females, whereupon they will compete with other males for the largest females. Some males mimic females to draw other males away and allow them to mate the females.

COURTSHIP, MATING BALLS, AND SKEINS

Sexually receptive female snakes emit pheromones, leaving a trail for males to follow with their extremely sensitive forked tongues, sometimes multiple males pursuing a single female. At the famous Manitoba snake pits, thousands of male Red-sided Gartersnakes (Thamnophis sirtalis parietalis) emerge from hibernation, only to linger around the dens to await the later emergence of the females.

Multiple male Green Anacondas (Eunectes murinus) will often court a single female, forming a mating ball around her as they jockey for position with their tails in an attempt to mate. These mating balls are usually found in shallow water, and the female may mate with several males. A similar arrangement has been observed in more active species such as the Keel-bellied Whipsnake (Dryophiops rufescens) and the Paradise Flying Snake (Chrysopelea paradisi). Here, a single female is courted as she moves through vegetation by two or three males, which form a continually writhing entanglement, known as a “skein,” about her body.

Green Anacondas (Eunectes murinus) may form great mating balls in the shallows or on land, one large female attracting up to a dozen smaller males, all jockeying for the best position to mate with her.

Male pythons and boas possess cloacal spurs (above right), the vestiges of their ancestral hind limbs, with which they will stroke the females during courtship.

The spurs of females are small or absent. Similarly, male Southern Turtle-headed Seasnakes (Emydocephalus annulatus) have a spike on their snouts, which they use during courtship to stroke the female’s back.

MALE COMBAT

It is quite common for two male snakes, in pursuit of the same female, to engage in combat—this behavior has been observed in adders, rattlesnakes, the King Cobra (Ophiophagus hannah), and the Black Mamba (Dendroaspis polylepis). These combats are wrestling matches that involve the competitors entwining around one another, raising up and trying to force their opponent to the ground. Combats between venomous species do not usually lead to injuries, but those between large pythons may involve spurring and result in deep wounds.

THE MECHANICS OF MATING

Snakes reproduce sexually via copulation, the male snake inserting one of his paired hemipenes into the cloaca of the female, which will lift her tail to make penetration easier. The female’s ova are fertilized internally with the male’s sperm, although this may not happen immediately—females of species that mate in the autumn are able to store sperm until more favorable conditions return in spring. Sperm is usually stored for only a few months, but much longer periods have been documented—six years in the case of the Banded Cat-eyed Snake (Leptodeira annulata).

Most tropical and subtropical snake species reproduce once a year, but in colder climates, where prey is only seasonally available and weather conditions are not ideal, the period during which a post-parturition female can recover her optimal body weight may be short. In these situations, species such as the Northern Adder (Vipera berus) in the Alps or the Carpet Python (Morelia spilota) in southern Australia may reproduce only biannually. Species in warm climates with abundant prey year-round, such as the Common Saw-scale Viper (Echis carinatus), may produce two litters each year.

A pair of male Dharman Ratsnakes (Ptyas mucosa) writhe and wrestle with one another in an attempt to win the right to mate with a nearby female. Males of many species engage in this behavior, including King Cobras (Ophiophagus hannah) and Black Mambas (Dendroaspis polylepis).

OVIPARITY OR VIVIPARITY

Birds are oviparous (egg-laying), as are turtles, crocodilians, and monotreme mammals, while all remaining mammals are viviparous (live-bearing). But this clear-cut division of reproductive strategies becomes blurred in the Squamata, especially in the snakes. Oviparity is the primitive condition for snakes, and it is the sole strategy exhibited by 15 of the 33 snake families (see here), including many of the small, fossorial, and relatively primitive snakes. Live-bearing is the only strategy found in ten snake families, primarily in the Henophidia. The remaining eight families contain both oviparous and viviparous genera, and in some cases, genera that contain both oviparous and viviparous species—for example, the viviparous Smooth Snake (Coronella austriaca) and oviparous Southern Smooth Snake (C. girondica).

A female Osage Copperhead (Agkistrodon contortrix phaeogaster) with her litter of neonates. The neonates are only days old but they will soon disperse into the surrounding undergrowth.

A female Reticulated Python (Malayopython reticulatus) with her clutch of leathery-shelled eggs. She will defend her eggs against egg thieves such as monitor lizards and incubate them by “shivering thermogenesis”—the continual twitching of her muscles which will elevate her body temperature by 13–23 °F (7–13 °C).

There are both advantages and disadvantages to live-bearing for snakes. Among the disadvantages is the fact that the gravid female must carry her offspring for the entire gestation period, which could be three months, during which time she is unlikely to feed, and if she is killed her entire reproductive investment is lost—she literally has all her “eggs” in one basket. In the tropics, it is probably advantageous for the female to lay her eggs so that she can begin hunting again, but in arctic–alpine climates live-bearing is a much better strategy. While eggs are vulnerable to cold temperatures, a live-bearing female can shelter below ground during the night or cold periods and then emerge to bask in the sun. When locomotion is combined with melanistic (black) coloration, the female snake becomes an efficient mobile incubator, seeking out the best basking spots.

Live-bearing is also a useful adaptation for aquatic snakes—the Colubrine Sea Krait (Laticauda colubrina), for example, is an oviparous marine species that must come onto land to lay its eggs, whereas viviparous true seasnakes simply give birth in the ocean and are not tied to land. Similarly, oviparous pythons must lay their eggs on land, while aquatic anacondas give birth directly into the shallow water.

EGG-LAYING

Snake eggs are oval, with opaque white leathery shells. Some snakes lay their eggs soon after mating, when embryonic development has only just begun, while other species may retain the eggs until halfway through or near the end of the incubation period. The Persian False Horned Viper (Pseudocerastes persicus) has been recorded laying eggs that hatched 30–32 days later, half the usual 60–70-day incubation period, while the Sahara Sand Viper (Cerastes vipera) lays eggs only days before they are due to hatch. Most eggs are deposited when the embryo is at the 30 percent development stage.

A hatchling Green Tree Python (Morelia viridis) uses an egg-tooth on the tip of its upper lip to “pip” its egg. It will breath air for the first time but may wait some time before fully emerging. At approximately 15 months of age it will take on the green adult livery.

Snakes may lay their eggs in a rocky crevice or an animal burrow, and leave them to incubate on their own. Western Grass Snakes (Natrix helvetica) often lay their eggs in garden compost heaps, sometimes communally, where the eggs benefit from heat generated by the decomposing vegetation. A few snake species remain with their eggs post-oviposition. Female pythons coil around their eggs, both to incubate and to protect them. Incubation is accomplished via a process of endogenous heat production known as shivering thermogenesis, which involves a prolonged period of rhythmic muscular contractions. Brooding Diamond Pythons (Morelia spilota) have been recorded shivering up to 50 times per minute throughout the two-month incubation period. Shivering thermogenesis will maintain an incubation temperature of 88–91 °F (31–33 °C), as much as 13–23 °F (7–13 °C) above the ambient air temperature. The pythons do not feed throughout this entire period.

A female King Cobra uses her body coils to build a nest of leaves, laying her eggs in the center. She will then remain at the nest site to protect the eggs from predators, such as the Water Monitor Lizard (Varanus salvator).

At the end of the incubation period, the young snakes hatch by making slits in the leathery shell of the egg. They use an egg-tooth on the front of the upper lip to achieve this, a process known as “pipping,” although they may not actually emerge for some hours. Once out of the egg, the hatchling will shed its skin and become independent. Depending on the species and the size of the female, the clutch may vary from a single egg to more than 80 in the case of large pythons.

LIVE-BEARING

Just over 20 percent of snakes are live-bearers, a process that has evolved at least 35 times within the Serpentes. Two different forms of live-bearing have been defined—viviparity and ovoviviparity—but most recent authors do not differentiate between them. Boas, rattlesnakes, most vipers, American natricid watersnakes, true seasnakes, and the Red-bellied Blacksnake (Pseudechis porphyriacus) are all viviparous. Neonates are born coiled in a transparent egg membrane that ruptures soon after birth. Litter sizes for viviparous snakes vary depending on the species and the size of the female, from one to two in small species, to more than 100 in the Puff Adder (Bitis arietans).

A female Copperhead (Agkistrodon contortrix) giving birth to a litter of neonates which are born in membraneous packages from which they almost immediately escape to take their first breaths.

VIRGIN BIRTH

Parthenogenesis is a form of reproduction whereby a female produces exact clones of herself without the need of a male. There are numerous parthenogenetic lizards, but only one truly obligate parthenogenetic snake, the Brahminy Blindsnake (Indotyphlops braminus), for which no males are known. There are also cases of females from normally bisexual species—including pythons, boas, filesnakes, gartersnakes, and pitvipers—producing offspring without first mating with a male. This is termed facultative parthenogenesis, and is the last-ditch attempt of a female to propagate when no males are available. The result is usually a small clutch or litter with a high mortality rate, and the offspring are always unisexual—all female in boas and pythons, and all male in rattlesnakes.

A two-headed Red-backed Ratsnake (Oocatochus rufodorsatus). Two-headed or dicephalic snakes are effectively conjoined twins with two heads at the same end. Although most specimens are stillborn, or die soon after birth due to other defects, there have been many cases of dicephalic snakes being raised to adulthood.