Did you know that different animals have different-sized bladders? For example, a cat’s bladder is about the size of a golf ball, a human’s is like a coffee mug, and an elephant’s is as big as a kitchen trash can! But here’s the cool part: even though an elephant’s bladder is huge, it doesn’t take much longer for an elephant to pee than it does for a cat. How is that possible? Let’s dive into the science behind it, known as the “Other Golden Rule.”
This rule is all about how mammals, which are animals that excrete liquid waste, manage to pee quickly no matter their size. Two main things affect how fast they can do this: bladder pressure and gravity.
Bladder pressure happens when animals use their muscles to push urine out. This pressure is pretty much the same for all mammals, no matter how big or small they are. But gravity can change things up a bit depending on the animal’s size.
Mammals have a special tube called the urethra that carries urine from the bladder out of the body. The urethra uses something called fluid dynamics, which is a fancy way of saying that the flow of liquid speeds up when the liquid is higher up. Imagine you have two containers with the same amount of water, but one is taller. If you poke a hole at the bottom of each, the taller one will empty faster because of the height difference.
The length and width of the urethra also matter. For example, a female elephant has a urethra that’s about a meter long and 35 millimeters wide. In comparison, a human’s urethra is much shorter and narrower. The elephant’s longer and wider urethra helps it pee in about the same time as a human. If an elephant had a urethra as small as a cat’s, it would take much longer to empty its bladder.
This “Other Golden Rule” is important because peeing quickly can help animals avoid predators and get rid of harmful germs. However, the rule doesn’t apply to all animals. It only works for mammals that live on land and weigh more than three kilograms. Smaller animals tend to pee in tiny droplets because of something called surface tension.
Even among larger land mammals, there are some exceptions. For instance, male dogs might not empty their bladders all at once because they like to mark their territory with short bursts of urine. Male pandas sometimes do handstands to pee higher up on trees, which takes longer because of gravity. And when animals pee underwater, it can take longer due to the water pressure outside their bodies.
No matter how they do it, animals have developed efficient ways to get rid of urine, thanks to this fascinating system!
Gather some objects like a golf ball, a coffee mug, and a small trash can. These represent the bladders of a cat, a human, and an elephant, respectively. Discuss with your classmates how these sizes relate to the animals’ ability to pee quickly. Consider why size doesn’t always mean longer peeing time.
Using two plastic bottles of different heights, poke a hole at the bottom of each and fill them with water. Observe how the water flows out and discuss how this relates to the urethra’s role in different animals. Think about how the height and width of the urethra affect the speed of urination.
In groups, choose different animals and act out how they might pee in the wild. Consider factors like avoiding predators, marking territory, or peeing underwater. Discuss why these strategies are important for survival and how they relate to the “Other Golden Rule.”
Use a dropper to release small droplets of water onto a flat surface. Observe how the droplets behave and discuss why smaller animals might pee in tiny droplets. Relate this to the concept of surface tension and how it affects animals under three kilograms.
Research an animal that doesn’t follow the “Other Golden Rule” and prepare a short presentation for the class. Explain why this animal is an exception and how its urination process differs from other mammals. Consider factors like territory marking or underwater peeing.
Here’s a sanitized version of the provided YouTube transcript:
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A cat’s bladder can only store about the same amount as a golf ball. For humans, it’s roughly the size of a coffee mug, and for elephants, it’s comparable to a kitchen trash can. An elephant’s bladder is significantly larger than a cat’s, but it doesn’t take an elephant proportionally longer to urinate. So, how does this work? The answer lies in what scientists refer to as the “Other Golden Rule.” This rule applies to mammals, which excrete some of their waste in a liquid form. Other classes of animals have different systems for waste excretion.
The Rule explains how urinary organs and forces change with body size, allowing various mammals to achieve similarly quick relief. There are two main factors that influence urination speed: bladder pressure and gravity. Bladder pressure is generated when mammals contract their muscles to expel urine. This pressure is consistent across different body sizes, as it scales proportionally. However, the effects of gravity can vary among species.
Mammals have a tube-like organ called the urethra that transports urine from the bladder to the outside of the body. The urethra utilizes principles of fluid dynamics, which indicate that flow rate increases with the height of the liquid column. For example, if two containers holding the same volume of liquid are punctured at the bottom, the one with greater height will drain faster due to the increased elevation difference.
The urethra’s length and diameter also play a role in flow rate. For instance, a female elephant has a urethra that is about a meter long and has a diameter of thirty-five millimeters. In contrast, a person with a vagina has a urethra that is significantly shorter and narrower. The elephant’s longer and wider urethra allows it to urinate for a duration similar to that of a human. If an elephant had a urethra the size of a housecat’s, it might take much longer to empty its bladder.
It is suggested that the Other Golden Rule is maintained because quicker urination may reduce vulnerability to predators and help eliminate harmful pathogens. However, the Rule has its limitations. It has only been observed in non-aquatic mammals and does not apply to mammals weighing less than three kilograms, as they tend to urinate in droplets due to surface tension.
Even among non-aquatic mammals above three kilograms, various behaviors can disrupt the Other Golden Rule. For example, male dogs may not fully empty their bladders and instead urinate in short bursts to mark their territory. Male pandas sometimes urinate while in a handstand to mark higher up on trees, which takes longer due to the effects of gravity. Additionally, urination underwater can also take longer because of the external water pressure.
Regardless of the method, this system efficiently delivers urine to the outside world.
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This version maintains the core information while removing any potentially sensitive or inappropriate language.
Bladder – A hollow organ in the body that stores urine before it is excreted. – The bladder can expand to hold about 400-600 milliliters of urine in an adult human.
Mammals – A class of warm-blooded animals with hair or fur, and females produce milk to feed their young. – Humans, whales, and elephants are all examples of mammals.
Urine – A liquid waste product produced by the kidneys and excreted from the body. – The kidneys filter blood to produce urine, which is then stored in the bladder.
Urethra – A tube that carries urine from the bladder to the outside of the body. – In males, the urethra also carries semen during ejaculation.
Gravity – A force that attracts objects toward each other, especially the force that makes things fall to the ground. – Gravity is the reason why rain falls from clouds to the Earth’s surface.
Pressure – The force applied to a surface per unit area. – Atmospheric pressure decreases as you climb higher up a mountain.
Dynamics – The study of forces and motion in physical systems. – Understanding the dynamics of ecosystems helps scientists predict changes in animal populations.
Predators – Animals that hunt and eat other animals for food. – Lions are predators that often hunt in groups to catch their prey.
Surface – The outermost layer or boundary of an object or area. – The surface of a leaf is covered with tiny openings called stomata, which help in gas exchange.
Tension – The force that is transmitted through a string, rope, or wire when it is pulled tight by forces acting from opposite ends. – Surface tension allows small insects to walk on water without sinking.
