There is a slang phrase in Serbo-Croatian that means “doing nothing; being idle; wasting time”, and it is “hladiti jaja”, which means “cooling (one’s) balls”. So, if you see a guy just sitting there, clutching a beer bottle and gazing into the distance, you may ask him “Hey, man, whatcha doin’?” and he may reply ” ‘ladim jaja”, i.e., “I’m coolin’ me balls”.
Well, this slang phrase, indicating a thermoregulatory behavior, has its origin in the real theromoregulatory physiology. Yes, mammals have to cool their balls. That is why mammalian testes are located outside the body inside the scrotum.
Many cells, tissues and organs have an optimal working temperature. Some, like muscles, work best at a temperature a little higher than the core body temperature – hence the need to warm up before excercise. Brain cells are particularly sensitive to fluctuations in temperature, especially to heat. That is why large mammals have large sinuses inside their skulls – this is a heat-stroke prevention mechanism.
Mammalian testis works best at temperatures a couple of degrees below the body temperature. At the core body temperature the sperm get overheated and die. That is why, although having testes outside the body complicates development (vas deferens has to loop over the ureter) and exposes a tender organ to harm (ouch!), most mammals keep them outside nonetheless. That is also why, in some places and at some times in history, sitting in a hot tub for an hour every day, was used as a method of male contraception. That is also why tightie whities and tight jeans are looked at as a possible culprit explaining the reduction in male fertility in the Western world over the past several decades. Global warming is next.
Although the average core body temperature of birds is a couple of degrees higher than the average core body temperature of mammals (yup, birds are hot!), the testes in birds are kept inside the body cavity, roughly in the same spot where ovaries are located in females. This in no way reduces the fertility in birds – evolution has struck on a mechanism of sperm development that works best at a higher body temperature. Unfortunately for those of us doing martial arts, evolution is a blind process and never discovered the same mechanism during mammalian evolution. Thus, the scrotum.
Having a week of blogging about manatees and dolphins, made me think about aquatic mammals a lot. If you ever went swimming or got drenched in a rain, you know that it is so much easier to lose body heat in water or when wet, than in air or when dry (convection is faster than radiation).
But, unlike bulls and lions and elephants and men, walking around with their balls dangling, aquatic mammals cannot have their testes outside of their bodies in a scrotum because it would seriously impede their hydrodynamics. Each testis of this dolphin is 0.5 meters long:
So, cetaceans (whales and dolphins), pinnipedians (seals, sea lions and walruses) and sirenians (dugongs and manatees) have evolved a different mechanism for cooling their testes – the vascular heat-exchanger. This is nothing new in evolution – heat exchange mechanisms just like this are used by many animals whenever there is a need for regional heterothermy, i.e, for some body parts to be warmer or colder than the rest of the body. So, what is the principle on which heat exchangers operate? It is called a “counter-current” mechanism and it works roughly like this:
Blood, warmed by its passage through heat-producing organs (intestines, liver, muscles, lungs, heart) enters the aorta and from there goes to arteries that take blood to all parts of the body. If this warm blood comes to the surface and enters skin capillaries, the heat is dissipated into the environment and there is a net energy loss for the animal. This is something an animal may need if it is overheating, e.g., during exercise, but in general it forces the animal to eat more, thus forage more, thus spend more time exposed to predators and spend even more energy chasing prey.
So, in many animals, the veins that bring cool blood from the skin surface back into the body are located tightly next to the arteries that take blood from the body to the surface. When a warm object and a cold object are close to each other, heat exchange occurs – the warm object gets a little cooler and the cold object gets a little warmer. This is what happens to blood going through these vessels. As the warm blood in the artery encounters the cold blood in the adjacent vein, it cools a little. Further out, it encounters even colder venous blood and gets even cooler. Closer to the surface and it encounters even colder veins and gets even colder. Finally, once it reaches the skin, the arterial blood is as cold as the outside environment. No heat energy is lost. After branching out into the capillaries (in order to exhange materials and gasses with the skin cells), blood enters the veins and goes back into the body, getting warmer and warmer as it receives heat energy from the adjacent artery. Once it reaches the trunk, it is again at the core body temperature and no net energy loss was recorded.
Here it is in a little bit more detail of a dolphin flipper:
As you can see, on the top of the graph is the situation in which all the venous blood passes by the artery. At the surface the blood is cold so no energy is lost. On the bottom of the graph is a situation when an overheated animal needs to lose some heat energy to the environment – the venous blood is shunted to the surface veins instead of the deep veins. The arterial blood does not get cooled as much and reaches the surface while still warm. The heat is lost to the surrounding water (or air). The veins now carry warm blood and they are located at the surface, thus losing additional heat energy to the environment.
The same mechanism is used by wading birds to cool their legs so as not to lose heat energy that way. The same mechanism is used by large tropical mammals (e.g.,. giraffes and antelopes) to cool their brains. The same mechanism is used in the whale tongue in baleen whales – they keep their mouths open all the time and would thus lose a lot of energy as heat via tongues, necessitating even more foraging and eating to replenish that energy. This is how tuna warms up its swimming muscles, brain and eyes and how seals cool off their flippers and tails. This is how Dimetrodon and Stegosaurus are thought to have cooled their large bodies – through the sails and plates on their backs. Even in our arms and legs, the same mechanism operates to a small extent as blood passes through surface veins when we are hot and through deep veins adjacent to the arteries when we are cold.
And it is not just heat that is controlled in the same manner. The same counter-current principle is used in avian lungs to extract as much oxygen out of the air as possible, in avian and mammalian kidneys to concentrate urine as much as possible, and in fish swim bladders to fill or empty the swim bladder.
But back to dolphin balls now! A group of marine biologists at UNC – Wilmington has studied this problem in dolphin, seal and manatee testes and published a series of papers demonstrating that the counter-current heat exchanger – a set of arteries surrounded by many little veins – lowers the testis temperature in relation to the rest of the body. If you have a subscription to “American Scientist” you can read a cool popular-science article on their work in Reproductive Thermoregulation in Marine Mammals:
First, they measured the temperature in the area of the intestine adjacent to the testis while heating and cooling the fins:
A rectal probe housing a linear array of five copper-constantan thermocouples was designed to measure colonic temperatures simultaneously at positions anterior to, within, and posterior to the region of the colon flanked by the countercurrent heat exchanger. Colonic temperatures adjacent to the countercurrent heat exchanger were maximally 1.3°C cooler than temperatures measured outside this region. Temporary heating and cooling of the dorsal fin and flukes affected temperatures at the countercurrent heat exchanger, but had little or no effect on temperatures posterior to its position.
A rectal probe housing a linear array of seven copper-constantan thermocouples was designed to measure colonic temperatures simultaneously at positions anterior to, within and posterior to the region of the colon flanked by the CCHE. Immediately after vigorous swimming, temperatures at the CCHE decreased relative to resting and pre-swim values: post-swim temperatures at the CCHE were maximally 0.5 degrees C cooler than pre-swim temperatures. These data suggest that the CCHE has an increased ability to cool the arterial blood supply to the testes when the dolphin is swimming. This ability could offset the increased thermal load on the testes is an exercising dolphin.
Cetaceans possess cryptic testes that lie within the abdominal cavity, that are surrounded by primary locomotor muscles, and that are presumably exposed to core or above core body temperatures. It has remained a question as to how cetaceans produce and store viable sperm at these high temperatures. We offer anatomical evidence for a two layer arterio-venous countercurrent heat exchanger at the cetacean testis. Subcutaneous veins from the peripheral surfaces of the dorsal fin and flukes carry cool blood from the fins to the lumbo-caudal venous plexus. The lumbo-caudal venous plexus is juxtaposed to the spermatic arterial plexus, which supplies the testis. Venous plexus flow is from the ventro-lateral margins of the visceral cavity towards the vena cava. Arterial plexus flow is from the aorta towards the ventro-lateral margins of the visceral cavity and into the testis. The existence of a countercurrent heat exchanger suggests that cetaceans potentially compensate for detrimental effects of core temperatures on sperm viability and storage by regulating the temperature of blood flow to the testis.
So, blood cooled at the surface of the flippers and the dorsal fin takes the cold blood into the body and, on its way there, hugs the spermatic artery. The heat exchange occurs in which venous blood warms up, while the arterial blood gets cooled before entering the testis.
This arrangement of blood vessels is consistent with the fact that all aquatic mammals evolved from large terrestrial mammals. Thus, their testes used to descend into a scrotum and dangle outside, then gradually evolved to remain within the body cavity, taking all the associated blood vessels with them.
The group is now looking at the females – do their uterus and the fetus within it also get cooled by a counter-current heat exchanger:
Bottlenose dolphins possess a specialized vascular structure called a counter-current heat exchanger
(CCHE), that functions to cool their reproductive tissues. Heat is transferred from the warm arterial blood to the
relatively cool venous blood at a reproductive CCHE site in the reproductive tissue. This allows cooled arterial blood
to supply the intra-abdominal testes and the pregnant uterus. To test whether CCHE functions to also deliver
relatively cooled blood to the fetus, the following methods will be employed: 1) determine the position of the CCHE,
2) take body temperature at two positions (one at the CCHE and the other at an area unaffected by the CCHE), 3)
maintain a log of deep body temperature over time, and 4) collect other husbandry and health data.
So, next time at the beach when you ask a dolphin “Whatcha doin’?” you may get a response “Coolin’ me balls, man!” Or at least you will imagine having such a conversation.