Frogs are amazing amphibians that have unique ways of storing waste and reproductive materials. If you’ve ever wondered where frogs keep their pee and poop or how they preserve eggs and sperm, you’ll find all the details here.
In short, frogs expel liquid waste through their skin and cloaca, turn solid waste into mucus packets that are expelled through the cloaca, store eggs externally after laying them, and store sperm inside the male’s testes and cloaca.
Liquid and Solid Waste Storage in Frogs
Expelling Liquid Waste Through Osmosis
Frogs have a unique way of expelling liquid waste from their bodies through osmosis. Osmosis is the process where water moves across a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration to equalize the solute concentrations on both sides.
In frogs, the lining of the bladder acts as a semi-permeable membrane, allowing water but not solutes to pass through.
As urine containing salts, urea, and ammonia is produced and enters the frog’s bladder, the solute concentration inside the bladder increases. This creates an osmotic gradient where there is a higher solute concentration inside the bladder compared to outside in the frog’s body.
Water then gets pulled from the frog’s body into the bladder through osmosis to dilute the urine inside the bladder and equalize the solute concentrations.
The influx of water into the bladder causes the bladder to expand, pressurizing the contents inside. When the frog is ready, it can voluntarily relax the sphincter muscle opening the cloaca to release the liquid waste.
Some frogs are also capable of reabsorbing water from the bladder back into the body when necessary, allowing them to recycle water efficiently.
Forming Mucus Packets With Solid Waste
Unlike liquid waste which can be freely expelled, frogs need a way to store solid waste safely until they are ready to release it. To do this, they form mucus packets that encapsulate the solid fecal matter within.
Frogs have glands in their intestines that produce mucus, a thick and slimy substance. As solid wastes pass through the large intestine, mucus is secreted and coats the waste. This allows the mucus to bind the waste together into a cohesive packet or pellet.
The mucus coating provides several beneficial functions. First, it lubricates the fecal packet, allowing for easier passage through the cloaca. Second, it isolates the waste from direct contact with the frog’s tissues, protecting against irritation or bacterial infection.
Finally, it allows the frog to retain and store the fecal packet for an extended time within the rectum until the frog is in an optimal location to defecate.
When the frog is ready to release the waste, muscles surrounding the cloaca contract to expel the mucus-coated fecal packet out of the body in one intact piece. The sticky mucus helps keep the waste contained even after it has been voided.
This creative use of mucus demonstrates the ingenious solutions frogs have evolved to manage solid waste.
Egg Storage After Frog Mating
Laying and Fertilizing Eggs Externally
After mating, most frog species lay their eggs externally in water or moist environments. This allows the eggs to be fertilized externally by the male releasing sperm over the eggs as the female deposits them.
Leaving the eggs exposed to the elements provides both benefits and risks for frog reproductive success.
A key advantage of external fertilization is that frogs can produce large numbers of eggs – up to 20,000 in some species! By laying many eggs, frogs maximize their chances that some offspring will avoid predators, desiccation, and other environmental threats.
However, the trade-off is that each individual egg has lower odds of survival than eggs cared for internally or protected in nests.
Another potential benefit is that laying eggs externally frees adult frogs from providing parental care. The lack of care means adults can mate with multiple partners, producing more genetic variety in offspring.
However, some frog species have evolved strategies to protect or provision their externally laid eggs, including guarding egg masses, transporting eggs, or allowing eggs to develop in pouches on the body.
Leaving Eggs Exposed to Elements
Most frogs deposit egg masses in water or very damp locations. This protects eggs from drying out, which is one of the biggest risks of external exposure. However, eggs laid in shallow water may overheat in the sun or get attacked by aquatic predators like fish.
Some tropical tree frogs lay eggs in arboreal nests, avoiding both dry land and risky water bodies.
Another challenge of leaving eggs exposed is that they are vulnerable to fungal or bacterial infections. Especially in stagnant water, diseases can rapidly spread between eggs in a mass, killing embryos before they hatch.
In some cases, male frogs have been observed sprinkling sterile urine over eggs after mating, which contains antifungal compounds that may reduce infection risk.
While exposed eggs can’t do much to protect themselves, parents have some options to improve survival odds. In addition to guarding eggs, some poison dart frogs transport freshly laid eggs to pools collected in plant axils.
Other frogs like Surinam toads have pouches on their backs where eggs develop until hatching, protected from external threats.
Sperm Storage Within the Male Frog
Producing Sperm in the Testes
Male frogs produce sperm in structures called testes. The testes contain seminiferous tubules where sperm are generated through the process of spermatogenesis. Spermatogenesis begins with spermatogonial stem cells that undergo mitosis to produce primary spermatocytes.
These cells then go through the first meiotic division to become secondary spermatocytes, followed by the second meiotic division to become spermatids. Spermatids undergo further maturation steps like condensing of the nucleus and growing a flagellum to become mature sperm cells, or spermatozoa.
This entire process takes around 12 days in most frog species. The developing sperm cells are supported by Sertoli cells that reside in the walls of the seminiferous tubules. Once mature, the sperm are released into the lumen of the tubules and transported to a storage area called the cloaca.
Storing Sperm in the Cloaca
The cloaca is a common chamber that serves as the opening for the digestive, reproductive, and urinary tracts in frogs and many other animals. In male frogs, the cloaca stores mature sperm received from the testes until they are ready for use during mating.
Specifically, sperm are stored in sac-like pouches called Bidder’s organs that protrude from the upper wall of the cloaca. The Bidder’s organs provide a safe and nurturing environment to keep sperm alive between matings.
They are lined with secretory cells that release protective fluids and nutrients to support the sperm.
The number and size of Bidder’s organs can vary between frog species. For example, according to a study published in Journal of Mammalogy, the Bidder’s organ pair of the bullfrog Rana catesbeiana makes up over 1% of its body weight!
The cloaca has muscles that allow the frog to pump sperm from the Bidder’s organs into the cloacal chamber during mating. This provides an external release of sperm so they can fertilize eggs as the female extrudes them.
Remarkably, some frogs like the Australian green tree frog Litoria caerulea are able to store viable sperm in their Bidder’s organs for over two years, according to a study in JEB!
Conclusion
In summary, frogs have specialized waste removal systems involving osmotic diffusion through their skin for liquid waste and mucus secretion through the cloaca for solid feces. They also reproduce using external fertilization, with females depositing eggs externally in water and males storing sperm internally in the testes and cloaca until it is time to fertilize them.
