Klingons are a bipedal hominid race of slightly larger size and mass to humans. Compared to their human counterparts, Klingons are approximately 30% larger and heavier. A typical Klingon male is 6'6" in height and weighs around 320 lbs. The median range, of course, varies from this norm, with some individuals seen as short as 5'11" or as tall as 7'3". A typical Klingon female is 6'2" in height and weighs at around 250 lbs. The difference in size is most likely due to the increased gravity on Qo'noS, which is 125.387% when compared to the gravity of Earth. This would also account for the increased strength of Klingons, which is nearly equal to that of Vulcans. Visual acuity is slightly less than that of humans, primarily due to the tinted atmosphere of Qo'noS. Body temperature is slightly lower than humans, averaging at 93.7 degrees - most likely due to the warmer environment found on their homeworld.
The Klingon skeletal structure is unusual in that it incorporates both an internal skeleton as well as epidermal bony plates. Klingon bone is much denser than that of humans, making it extremely difficult to break. Many joints and exposed bones, such as the knuckles and knees, are reinforced with a second layer of bone as well. The skull is protected by an extra thick layer of bone on the forehead and face, resulting in the cranial ridge and prominent facial bones that Klingons are so well known for.
Most Klingons also have a second layer of bone over the nasal cavities, giving them a series of "ridges" along their nose, much like that of the Bajorans. This extra layer of bone provides protection for the sinus passages, making Klingon noses harder to break. Of course, given the Klingon propensity for brawling and fisticuffs, many noses end up broken regardless.
Hair does not typically grow over the bony cranial ridges and cheeks, although it has been seen in some cases. An exception are the eyebrows, which grow over the lower portion of the cranial ridges. Klingon hair does not grow as quickly as human hair, and Klingons therefore pride themselves on possessing long hair - a sign of maturity and age.
On the back of a Klingon can be found several overlapping epidermal plates comprised of thick bone. These plates apparently protect the spine and prevent crippling injury to the individual. Back injuries are unusual among Klingons, apparently due to this natural adaptation. In one noted case, Lt. Worf of the USS Enterprise-D was injured while on duty and his spine was replaced with a replicated copy in 2368. He survived the controversial operation only due to the redundant organ structure of the Klingon species, which leads us into our next section.
Klingons are very unusual in that they possess a redundant organ structure, resulting in a "backup" for every major organ and many minor ones. For example, a third lung is present and the heart possesses five chambers when only four are needed. The liver is duplicated as well as the kidneys.
Even the brain possesses an extra synapse for every one used, and many nerve clusters are heavily reinforced as well - likely accounting for a Klingon's high tolerance for pain. This tolerance for pain has become enmeshed in their culture and is reinforced in many rituals, including the Age of Ascension rites and the mating process. The Klingon pictured at left is shown wielding a "painstick" - an electronic device used to apply electric shock that is used in many rituals. Of course, their ability to withstand pain is also one of the reasons they are considered formidable warriors. Klingons have been known to continue fighting in battle despite major wounds and blood loss or even the loss of major limbs.
The stomach and digestive tracts are heavily lined with muscle, thereby allowing a Klingon to digest foods which a human could not. In addition, the various acids found in the stomach are nearly twice as powerful as those found in a human stomach. Due to the reinforced muscle, however, the digestive tract is not as efficient as its human counterpart. Klingons cannot absorb nutrients as easily as humans can, which has resulted in the typical Klingon diet consisting of many live foods and few preserved.
Klingon reproductive organs are nearly identical to those of humans. Once impregnated, a Klingon female gestates for ten months, giving live birth to one or two offspring. Twins are not common among Klingons, but they are not unheard of either. If fact, according to legend the greatest figure in Klingon culture - Kahless the Unforgettable - was himself a twin. Because of this, if a woman gives birth to twins, it is considered a good omen for the House.
Klingon reproductive systems are physically and genetically compatible with that of many other species, including Vulcan, Romulan, and Human. This is highly unusual, but it does allow for inter-species births. Notable examples of Human/Klingon pairing are K'Eleyhr, B'lanna Torres, and Alexander Rozhenko. An example of a Klingon/Romulan pairing is Ba'el, pictured at right. Unlike other inter-species relations, such as Vulcan-Human, no genetic manipulation is necessary. Such couplings are rare, however, due to the rigorous physical demands of a mating Klingon. In fact, the mating rituals of Klingons are considered by many races to be abusive and violent. Klingon females are nearly as strong and durable as the males, and even among a purely Klingon couple, mating usually results in minor injuries to both parties.
Klingon physiology is much more reliant on a chemical biosystem than that of humans. A Klingon body produces hormones at a nearly constant rate, and pheromones are used to signal various emotions at all times. Klingons have three adrenal glands (compared to a human's one) and become nearly unstoppable during adrenaline production. Some aspects of Klingon culture have evolved around this reaction, commonly referred to as "the bloodrush". Because of this, Klingons are also highly susceptible to what biologists call the "fight or flight" syndrome. A cornered Klingon is not a pretty sight. Since the past thousand years of Klingon culture have trained countless generations that running is the mark of a coward, a typical Klingon's first response to any stress is agressive hostility. Many believe that the Klingon's agressive tendecies indicate a lack of control, but it is simply a physiological reaction to stress.
There is a rare but well-documented sub-species of Klingon which briefly rose to power during the middle of the 23rd century. This sub-species more closely resembled humans than Klingons, lacking the familiar cranial ridge and bone structure. Internal scans of the few recovered specimens also indicated a lack of redundant organ systems. The sub-species still possesses the chemical biosystems of the parent species, and appear to have the same dense skeleton. For the sake of classification, the parent strain is now referred to as "Imperial" Klingon.
Unfortunately, very few specimens of the sub-species were obtained, and the strain appears to have disappeared from the Klingon Empire in modern times. Diplomatic inquiries regarding this matter have met with obstruction, as the Klingons claim the question of the sub-species is an "internal matter" and not subject to discussion. At least one case of a surviving member of this sub-species has been documented - that of Kor, a former Klingon captain. Prior to his death in the 24th century, Kor appeared as any other Imperial Klingon. Dr. Julian Bashere of Federation space station Deep Space Nine had occasion to perform a medical scan on Kor, which appears to have only deepened the mystery. At the time of the scan, Kor possessed all physiological features inherent to Imperial Klingons. How this is possible remains one of the great unanswered mysteries of medical science.