Firstly, you need to understand why snakes poo. Faeces is made up of indigestible and undigested material, such as fur, mature skin and very solid bits of bone and teeth. The amounts of these materials varies with the food item. Pinky mice and rats are potentially entirely digestible leaving no wastes to be eliminated. This is not an issue so long as they are not maintained indefinitely on such a diet.
The digestive tube of snakes does not have the layers of muscle we have. Snakes rely on moving their bodies over objects to help push the contents of the gastrointestinal tract along. This is why an exercise session on the back lawn will frequently produce a good old fashioned dump.
The rate of digestion is dependent on quite wide range of factors but the standout factor is temperature. Snakes need to be able to get to their preferred body temperature to optimise the rate of digestion.
The most frequent cause of constipation is insufficient hydration. Snakes will not want to water that has stood for more than a day. So daily change of water or every second day is the way to go and constipation should never be a problem.
Production of urates is entirely separate from digestion. Urates are the nitrogenous wastes that are produced when excess proteins are broken down and used for energy. They are excreted via the kidneys and carried to the cloaca. They are normally voided when faeces passes through the cloaca. However, if there is sufficient build up they can be pushed out of the cloaca by movement of the body over objects.
Unlike a human meal, which is usually through n a day and certainly no longer than three, the rate of digestion in snakes, as has been indicated, is dependant on a wide range of factors, such as the nature of the prey item, its size, the temperature of snake, its rate of metabolism, its age, its length, its girth, its activity type and duration, its hydration level, its state of health and stress levels, whether it is coming up to shed... I cannot think of anymore off the top of my head at the moment, but I am sure you get the idea. So it is perfectly normal for a snake to defecate anywhere between 4 days and 4 weeks following a meal. Food items that contain no roughage, such as pinkie mice and rats, may not produce any waste at all to be defecated.
With respect the size of feed items, snakes have been known to eat animals two or three times their own body girth. However, it is important to realise that is not a regular thing and is not without its hazards. Captive snakes are much better two smaller items than a single large item. Food items with a girth of one and a quarter to one and a half times the maximum girth of the snake are as large as you would want to go. They should leave a clearly perceivable bump not much bigger than about one and a quarter times the normal diameter once swallowed. Weight is the most accurate way to go. Prey items in the range of 5% to 25% of the weight of the snake are acceptable.
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