Why Pigeons Do Not Urinate like Humans?

 

Early this morning I was walking to the shop in my housing estate where there are always a lot of pigeons in the morning perching on the wire lines of the lamp posts waiting to be fed by residents. 

I saw one of the residents throwing food behind her house to feed the pigeons. However, another  neighbour a few doors away who saw it was not very pleased. He complained to me as I passed by, that these pigeons are a nuisance as they dirty the place by peeing everywhere. 

But he was wrong. Pigeons like birds do not urinate. This may surprise almost everybody including even biologists and scientists. But I do not wish to tell this to my neighbour, as I do not wish to offend him and neither would he understand scientific language if I were to explain why pigeons do not pee.  

However, the danger with pigeons lies not from urination, but their droppings that can cause lung diseases such as cryptococcosis, histoplasmosis and psittacosis. A person can become infected with these diseases by breathing in the dust that is created when cleaning droppings. Cryptococcosis is an illness caused by infection with Cryptococcus fungi in the brain or lungs. It can also spread to many areas of the body (disseminated cryptococcosis). Histoplasmosis is another infection caused by breathing in spores of a fungus often found in bird and bat droppings. People usually get it from breathing in these spores from pigeons and birds.

Psittacosis is another zoonotic bacterial infectious disease caused by the obligate intracellular organism called Chlamydia psittaci. Psittacosis, which is also called parrot fever and ornithosis is transmitted from contact with infected birds and causes a wide-ranging spectrum of disease and severity.

Nevertheless, it is not my intention to write about these diseases here even though diseases caused by pigeons are far more serious than them peeing. But what I want to write now is on why pigeons, birds and some other animals do not pee. 

So immediately now after I got home, I am writing out the reasons why it is biologically and technically incorrect to say pigeons pee all over the place as my neighbour complained.

First of all, before I proceed, the word “pee” is not a particularly good word to use to describe urination. A more appropriate and better scientific and medical term for urination is micturition.

Having clarified that word in simple English language, let me now use only very slightly scientific language to explain why pigeons and birds do not urinate like us.

In humans and in all mammals, we have a urinary bladder to contain the urine and allow urination in an intermittent period. The urinary bladder is absent in feathered animals that have beaks under Class Aves such as pigeons and birds. Birds and pigeons do not have a urinary bladder. So, without a urinary bladder how do they contain their urine?

In humans and in mammals, the immediate waste products of food protein metabolism is ammonia.  Ammonia is toxic to the body which is then converted into urea that is soluble in the urine. Urea requires a large amount of water for excretion through the urine. Uric acid is the least toxic and requires less amount of water.

Because the waste products of their metabolisms are insoluble uric acid, they are excreted mixed up with their faeces which is their droppings.

Another reason for their absence of a urinary bladder to carry urine is also to reduce their body weight as they need to be as light as possible to fly about. This will also help them maintain stability while flying.

 Reptiles, birds, land snails, and insects excrete nitrogenous waste in the form of uric acid which is excreted in the form of a pellet or paste mixed with water to minimise the loss of water. These are called uricotelic animals. Excretion of uric acid is known as uricotelism. Uricotelism is vital in land vertebrates that lay eggs because if the embryo within the shelled egg had produced ammonia or urea as a waste product it would have accumulated to the toxic level. This is resolved by being uricotelic, as uric acid is almost insoluble, gets precipitated, and remains with the shell only. This problem is not faced by eggs laying fishes or amphibians by having shell-less eggs (ammonia or urea can diffuse out) or mammals (as urea is carried away by the maternal blood at the placenta). Besides pigeons and birds, the urinary bladder is also not present in some lizards, snakes, crocodiles as they excrete semisolid waste which contains uric acid that is excreted along with the faeces that contains water, considered as their ‘urine’.  Ostrich is the only bird that has a urinary bladder. Hummingbird is ammoniotelic, meaning they excrete ammonia as the waste product at low temperature and high-water intake whereas they are uricotelic in elevated temperature and low water intake. Thus, they are facultative ammoniotelic. In these animals that have no urinary bladder their ureters directly join the cloaca or ‘anus’ through which they excrete nitrogenous waste along with their faeces. Please remember faeces contains water which can also be considered as urine though mixed up with faecal matter. All living things require water, and their water intake needs to be balanced with their water output either through urine, faeces, skin or lungs in whichever their mechanisms.

 What about fish that are constantly inside water. Do fish urinate? What about mammals like whales, dolphins, otters and other aquatic mammalian animals? Do they urinate?

In the course of evolution, the fish came first, then the reptiles, after which the birds that are considered “glorified birds” All these classes of animals like birds do not urinate in the sense that we know it.

Let me explain further to provide a more solid explanation why birds and certain other animals do not urinate in the traditional sense but are connected to uric acid excretion with the evolutionary adaptations that support efficient water conservation, particularly in species that lay eggs.

Birds like pigeons lack a urinary bladder is biologically correct as I have already explained. The reason for this is tied to both weight reduction and their mode of excreting nitrogenous waste as uric acid. This form of excretion is efficient in water conservation, which is crucial for flight, as carrying excess water weight would impede their ability to stay airborne. The excretion of uric acid in a solid or semi-solid form, often along with their faeces, eliminates the need for a separate urinary system like that of mammals.

Ostriches and hummingbirds as I have also mentioned are exceptions. The ostrich is indeed unique among birds for having a urinary bladder. Hummingbirds are fascinating in that they exhibit facultative ammonotelism, depending on water intake and temperature, as I have also mentioned earlier. This flexibility is rare and further illustrates the diversity of excretion strategies among animals.

As far as fish is concerned, fish do excrete waste, albeit in a way quite different from terrestrial animals. Fish primarily excrete ammonia directly through their gills, as it diffuses easily into the surrounding water. This process is known as ammonotelism. Some fish may also produce urea or uric acid, but ammonia is the main nitrogenous waste product because they live in water, which allows the ammonia to be quickly diluted and carried away.

Fish do have kidneys that play a crucial role in osmoregulation and the excretion of certain waste products. Their kidneys help maintain the balance of salts and water within the body, especially in freshwater and saltwater species that face different osmotic conditions.

Aquatic mammals, including whales, dolphins, and otters also do have kidneys and urinary bladders, just like terrestrial mammals. They undergo micturition (urination) like land mammals, with urine expelled periodically. The challenges they face are related to osmoregulation, as they must retain water and excrete salts efficiently. For example, animals like marine mammals such as whales and dolphins live in a saltwater environment, and their kidneys are highly specialized to excrete concentrated urine to minimize water loss while getting rid of excess salts. In contrast, freshwater mammals like otters excrete more dilute urine to deal with the lower osmotic pressure in freshwater environments, as they constantly absorb water.

This is interesting as human cannot drink sea water as their kidneys operating on osmoregulation are unable to excrete such high concentrations of salt in sea water. Through evolutionary adaptations marine animals must have an efficient kidney to excrete out the high salinity in sea water. They evolved first from the oceans in its salty environment before humans came into land where only fresh water was available. Humans may have lost that ability to adapt anymore, unlike aquatic mammals to efficiently handle osmoregulation in salty environments. Human kidneys, like those of most terrestrial mammals, are not capable of processing large amounts of salt from seawater, as this would require more water for excretion than is available in the body. Aquatic mammals, however, evolved specialized kidneys with the ability to excrete highly concentrated urine, allowing them to survive in saline environments without suffering from dehydration. Our ancestors likely adapted to the availability of freshwater as they moved onto land, while aquatic mammals retained and enhanced their salt-excreting abilities. Evolution certainly plays a major role in shaping the unique mechanisms of various species based on their habitats.

In evolutionary point of view fish came first in evolutionary history, followed by reptiles, birds, and then mammals. This evolutionary progression explains the variety of excretion mechanisms seen across these groups. Birds and reptiles, being descendants of common ancestors, share uricotelism, while mammals evolved to handle waste through urea (ureotelism), as water was more available for them compared to their bird and reptile counterparts.

Back to my explanation, although pigeons and birds do not urinate in the traditional sense, they must balance their water intake and output. Water is lost through their faeces, respiration, and, in some cases, minimal water excretion via the cloaca. In renal physiology there is osmoregulation in birds and reptiles that have mechanisms, such as salt glands in some species (e.g., seabirds, marine reptiles), which excrete excess salts directly, helping them maintain water balance, especially in salty environments.

To summarize my explanation especially in describing the adaptations that allow birds and reptiles to conserve water, fish however do urinate (excrete ammonia and sometimes urea), and aquatic mammals urinate just like their terrestrial relatives.

 "Uricotelism" and "ammonotelism" are not just limited to birds and fish, respectively, but also occur in various other species depending on their ecological needs.

I hope I have managed to explain in the simplest way for those who have no, or much knowledge in biology, physiology, or in evolutionary adaptation. That was why I did not want to tell my neighbour why pigeons do not pee.