When your monitor "cycles"? What's going on inside, and why does it look like this or like that on the outside? There are many posts that refer to external indicators of cycling in female monitors, I think mostly correctly, but I haven't seen an overview of the nuts and bolts of cycling events in posts or on FAQs.
These comments aren't based solely on monitors, but there seems to be rather little variation among lizards (or even among reptiles) in the basics of reproductive cycling. I will try to point out those aspects where monitors do things a bit differently. There is a good site covering cycling in birds (which have generally similar features but lay one egg at a time, vs. their entire clutch) at http://www.jobird.com/eggdevel.htm
Bit of anatomy, first. Monitors have paired ovaries, which are located on the roof of the body cavity a bit behind mid-trunk, in front of the kidneys and on top of the rest of the viscera. Extending along the dorsal body wall from near the ovaries to the cloaca are paired oviducts, which are thin and straight in immature animals, but become thicker and longer (sinuously folded) at maturity. The front end of each oviduct (called the infundibulum, or ostium) is funnel-like, and lies near an ovary but does not enclose it. The lining of the middle third of each oviduct contains specialized cells that produce the 'white' of the egg (albumen), then the fibrous layers of the shell, which are mostly made of the protein collagen. In some reptiles the downstream part of the shell gland also secretes calcium compounds that make the shells more or less brittle or stiff.
The ovaries contain many oocytes, stem-cells that will give rise to ova, after having shed half of their genetic material. As oocytes develop into ova they enlarge, and lie in cavities (follicles) whose inner surface is made up of the cells that produce the yolk. All of the yolk is laid down while the ova are enclosed in the follicles; yolk (which consists of various proteins and lipids) is synthesized in the liver, drawing in part on lipids that are stored in the fat bodies. The fat bodies are pads of varying size and shape that are located alongside the intestines in the lower half of the body cavity. They become very heavily vascularized while ova are being provisioned. Some fat may also be stored in the base of the tail in stockier species of monitors.
As the follicles mature the ovaries become quite distended, while the abdominal fat bodies become reduced in size. This changes the external appearance of a female to a 'fuller' look ahead of mid-trunk, which is further enhanced by the lungs becoming pushed a bit forward. Females often show shallow breathing at this time, and usually cut back on feeding, or will eat and regurgitate. Some of this is just because there's less room for anything but ova, but the animal's circulatory system is also working overtime now to provision the ova, and less is available to be expended in transporting nutrients from the gut to the liver.
Maturing follicles release various hormones that cause the oviducts to enlarge and elongate. These hormones and others triggered by them also affect behavior. Males can detect certain hormones by odor, and copulation attempts usually begin well before ovulation occurs. Ovulation refers to the process in which the follicles burst, releasing mature ova into the body cavity near the openings of the oviducts – this is a discrete event, usually of short duration (a few hours at most). When it happens there may be a fair amount of internal bleeding and release of follicular fluids into the body cavity. Females will often stand high with the belly hanging, and move rigidly if at all, while actually ovulating. Monitors ovulate all of the eggs they will lay in one clutch at about the same time.
Most or all of the free ova are swept into the infundibulum of the oviduct within a short interval, and this is where fertilization occurs. Ova that fail to be captured by the oviducts remain in the body cavity, where they will be invaded by blood vessels and resorbed; however, if the thin enclosing membrane of an ovum that is not taken into the oviduct is ruptured and the yolk is released into the body cavity the result is often fatal.
It is important to realize that fertilization can only occur during the short period that the ova are passing through the 'mouth' region of the oviduct. Before that, the ova are enclosed in the follicles or free in the body cavity, and soon after entering the infundibulum the fertilized ovum (now properly called an egg) is covered by proteins secreted by the walls of the upper oviduct. Once this covering (part of the 'white' of the egg, containing albumen and other proteins) is in place, sperm cannot physically reach the cell nucleus of the ovum. Eggs are fertile or not from this point on.
Sperm can remain viable in the upper region of the oviduct, but for a relatively short time in monitors, perhaps a few days at most. Many other reptiles can "store" viable sperm, in exceptional cases for years (as in some snakes), but monitors seem not to do so.
Once the albumen and other constituents of the 'white' are in place, the egg (which already has a developing embryo) gets a thin membrane secreted around it, and enters the middle third of the oviduct. Once that membrane is formed, the egg is for the first time no longer a "part" of the mother, but an independent body. This is an important distinction, because the ensuing phases of egg development involve nothing but losses to the female's body. The albumen now absorbs water from the walls of the oviduct, and the egg swells to is final size. This water is being taken from the female's body, and because it goes into the now-isolated eggs, a female can rapidly experience (and show) dehydration. This is most evident around the tailbase and thighs, where a caved-in look is a good sign that egg deposition is fast approaching. Mineral salts are also added at this time, and calcium depletion can also be an acute problem for animals that are in poor condition.
Hydrated eggs now pass into the shell gland region of the oviduct, where several layers of felted collagen fibers are laid down around the membrane surrounding the egg. Monitor eggs may also have small specks of calcium compounds (such as aragonite) deposited on the outer layer of the shell, but these do not form a complete covering (save for tiny pores) as happens in many other reptiles, and in birds.
Because these processes in the oviduct occur in an assembly-line fashion, the first egg is fully shelled perhaps days before the last ovum even enters the infundibulum. Since monitors lay all of their eggs at once, the embryos are of slightly different ages at laying. How long the 'first' egg is retained depends on the number of eggs that are in the clutch. However, monitor embryos are not known to be able to enter diapause, and eggs that are retained much beyond the minimum lag needed to finish a clutch mostly seem to die. This is the basis of the oft-repeated statement that bad nesting situations result in dead eggs. Other lizards are able to retain viable eggs internally for weeks or months, but there is no indication that any monitor can do so.
There is a lot of variation in the number of eggs laid per clutch, between cycles in the same animal, among individuals, and among species. This comes down to what controls the number of follicles that develop at the start of a given cycling event. On an individual basis this is probably a function of size and nutritional status, while genetic factors assume a greater role in defining clutch size differences among species, as influenced by natural selection. That's a topic for a different post, but the basic message is that species that lay huge clutches (such as most African monitors) must do so to counteract miserable juvenile survival rates, whereas species of similar size that lay few eggs (such as V. salvadorii) simply do not experience the same level of hatchling and juvenile mortality.
There are a number of things that are "bad for" female monitors in this cycling period. Some have long lead-in times, such as nutritional state and calcium balance, while others are rather short-term items, such as hydration. A few are "instantaneous" things, mostly having to do with increased potential for internal injuries. When fat bodies are being resorbed and large follicles exist it is fairly easy to rupture something important, and ovulation itself is a very risky time.
This whole post is related to other threads, including multiclutching, UV light, 'social' status and stress in captive monitors, but those topics can't be properly discussed unless people are on the same page about the reproductive biology of monitors.
