Mating. One of the key differences between reptiles and amphibians is in the mating process - or, to be precise, the fertilisation of the eggs. In amphibians, the fertilisation is external (in the widest sense of the word - e.g. newts where fertilisation is internal but the male does not directly impregnate the female) whereas with reptiles it is internal. This is clearly evidenced in the mating behaviour. I keep very few species of amphibian and these are Anurans - Frogs and Toads. In these, simplistically, the male mounts the female gripping her sometimes with his forearms just behind hers (e.g. Common Frog, Pool Frog, Common Toad, Green Toad) or just in front of the hind limbs (e.g. Bombina spp). As the female lays her eggs or spawn the male ejects his sperm over them to fertilise them. The close proximity of male and female maximises the chance of fertilisation. I say "simplistically" because even amongst the few species I keep interesting variations occur. In the Common Toad the well known "Toad knot" might be seen where several males are trying simultaneously to mate with a single female - in these cases just about any part of the anatomy of any other toad in the knot might be grasped.
With reptiles mating is a more intimate process. In Lacertids, the male will usually grasp the female's tail in his jaws and then slowly move his grip up until he is grasping her in the middle of the body. She may well walk around during this period and seldom appears to simply submit. On occasion, indeed, she will turn round and bite him which he will normally accept as a signal to desist. Otherwise, once he is gripping her body, she lifts her tail slightly and in an extremely fast move he places his cloaca adjacent to hers. He inserts one of his hemipenes (a lizard's penis is split into two hemipenes which can be used alternately) and ejaculates his sperm. In the case of many Agamids the male seizes the female at the back of the neck but the remainder of the process is the same. The coupling can be over within a couple of seconds with some species but can last for several minutes in others. Often in the mating season, females can be seen to bear bruising or scars on the body (lacertids) or missing/damaged scales and spines behind the head (Agamids) although this is very seldom a matter for concern.
Egg laying or giving birth. Again, dealing first with the amphibians, the spawn is produced during the mating process. Generally, this is simply laid in large numbers in the water as, for example strings of spawn in toads, rafts of spawn in the typical frogs, single eggs in the bombinas and small clumps in the hylids. In the case of the Midwife Toad, a short string of only 40 to 50 eggs - or less - is laid and the male wraps this around his hindquarters and carries it around until the tadpoles are ready to emerge. Each egg in this case is surrounded by a semi-permeable membrane so the male only has to visit water occasionally to moisten the eggs. Once the tadpoles are ready to emerge the enters the water to allow them to. One of the results of this is that the newly hatched tadpoles are considerably larger than most species which rely on large numbers. Another exception is the Foam nesting frogs, where as the eggs are laid the female whips up a froth with her hind legs which looks rather like a meringue in which the eggs rest. This gives the eggs a degree of protection and something of a controlled environment for their development. This foam nest is normally built over water and the newly hatched tadpoles drop from it into the water below. Clearly, in most cases, development of the eggs is dependent on the ambient temperature of the water. Only species such as the Midwife can actually take action to control the incubation temperature.
The vast majority of reptiles are egg layers but there are a few exceptions. The best known in the UK are the Viviparous Lizard and the Slowworm. These retain the eggs which are laid when they are ready to hatch. This is not true viviparity as the young are still encased in a membrane and have a yolk-sac. Nonetheless, they emerge from this within only a few minutes and are then ready to fend for themselves. A small number of other species which are occasionally available are also live bearing. These are mainly some of the skink species and some of the small Iguanids. Generally, in live bearing species, the female seeks out somewhere secluded and under cover to give birth as both she and her young are vulnerable at this point. In captive animals it is always essential to ensure they have somewhere to hide. If such a hiding place is available with a layer of damp moss the female is most likely to give birth there.
Egg laying lizards, like most amphibians, are dependent on their environment to produce the temperatures necessary to incubate their eggs. The simplest ploy is to lay their eggs in a location where they have sufficient moisture (the egg shells are semi-permeable) but the medium in which they are laid reaches sufficiently high temperatures. One of the most commonly used mediums amongst most species is sand. It is safe to say that most species will lay their eggs in a suitable sand box. It must be stressed that unless somewhere suitable is provided they will certainly suffer stress and may even suffer egg-binding, blocked ovaries and, eventually, a painful death. So, should you suspect your female lizard is close to laying eggs ensure she has somewhere suitable to do so.
In a large enough vivarium and certainly outdoor or greenhouse vivaria then simply a sandy area can be provided. Ensure it has sufficient depth - 6 inches is enough for most small to medium sized lizards. It should receive full sunshine and retain some moisture at least below the surface. A piece of roofing slate laid on the surface or embedded at about a 45 degree angle will help. For indoor and smaller vivaria a box will literally do the job. Largish plastic lunchbox containers will do and in these circumstances it is best to partially fill them with sand, cut a hole large enough for the lizard in the lid and put the lid on. The size of the container is determined by the lizard - she should be able to fit in comfortably but with still a good depth of sand beneath her. For example, I have used boxes measuring about 23 x 16 cms with 75mms depth of sand for most Lacertids, whereas the Water Dragons have an open sand area measuring about 30 x 30 cms with 30 cms depth of sand.
Egg incubation: Once again, dealing briefly with the amphibians, with the species I keep it is simply necessary to keep water levels up and background and water at the "normal" temperature for maintenance of the animals.
Matters become far more complex with reptiles. Incubation periods tend to be much longer and both temperature and humidity have to be carefully controlled. Firstly, the eggs should be very carefully dug up and every effort made to ensure that you do not disturb their orientation. Eggs that are stuck together should not be separated. They should be dug up as soon as possible after laying - this also minimises the chances of damaging them. They should be placed in a suitable container laying to half their depth in either vermiculite (with an equal weight of water added) or silver sand moistened enough to ensure that it keeps its shape. In my case, I always ensure that the container is large enough to have an equal volume of air to sand and I then place the lid on it and put the whole container in the incubator. This ensures that the humidity level is retained and the air within the container is more than enough.
If eggs are harvested immediately after laying they will be quite small and often wrinkled - even feeling a little flabby. If, however, they are fertile, they will absorb moisture from the incubation medium over the first couple of days of incubation and increase considerably in size. They will also become much firmer - although poking them is not recommended!
Incubation temperatures are hugely variable - not only from species to species but, in natural circumstances from day to day - even hour to hour. Nowadays I try to emulate this which also means I can incubate eggs from a very wide range of species from temperate to tropical in a single incubator.
Follow this link for details on how I choose and regulate temperatures for my egg incubation.
During incubation an egg might fail. Typically if this is the case it will turn yellow and collapse. Such eggs should be removed as otherwise mould might develop. Occasionally, due to the humidity, mould might appear on an apparently healthy egg. This can be removed by wiping it off VERY gently with cotton wool or soft tissue dipped into an extremely diluted bleach solution.
As hatching approaches most lizard eggs start sweating (the exception being hard shelled eggs such as those of some Geckos). This can be for as little as a couple of hours, or exceptionally 48 hours. Typically, however, after sweating for about 24 hours, the young lizard will cut one or more slashes in the egg shell. The egg collapses somewhat at this stage while the hatchling sits within it with possibly only its nose sticking out, or occasionally its whole head. Again it will remain like this, if not disturbed, for the same sort of time span as the sweating before it finally emerges. Some animals will still have a small amount of yolk sac attached, but this quickly dries out and falls off. NEVER try to remove it yourself.