Changeling

For the Earth legend regarding a Changeling child see Changeling (folklore); for the TOS episode see "The Changeling".

The Changelings were liquid-based, shapeshifting lifeforms native to the Gamma Quadrant of the Milky Way Galaxy. Their homeworld was, apparently, a rogue planet within the Omarion Nebula (which has since been rendered uninhabitable by the Obsidian Order and the Tal Shiar in the Battle of the Omarion Nebula in 2371). (DS9: "The Search, Part I", "The Die is Cast")

Physiology

The Founders were at least partially composed of morphogenic enzymes, the molecules responsible for their shapeshifting ability. (DS9: "Things Past")

Common characteristics of Changelings included their biomolecular structure and morphogenic matrix.

In its natural state, a Changeling's body was a formless gelatinous mass, to which it had to revert every eighteen hours (Odo's example) in order to regenerate. (DS9: "The Forsaken", "The Storyteller", "The Alternate", "In Purgatory's Shadow"). If a Changeling remained in solid form for longer than sixteen hours, their body began to deteriorate and peel or "flake' away.

In "A Man Alone", it was first stated that Odo had to rejuvenate every eighteen hours; afterward, it was consistently stated as being every sixteen hours. Reversion to a gelatinous state was considered by DS9's producers to be "a limitation unique to Odo" due to not having yet mastered all of his abilities. (AOL chat, 1998)
This could be proven true as the Female Changeling claims that she didn't take another form in weeks, when Weyoun suggests that she might relax in her liquid form in "What You Leave Behind".

Since Changelings do not eat, "Robert Wolfe used to theorize that the Changelings pulled their energy directly from subspace (or some [other] quasi-scientific realm)." (AOL chat, 1997) His theory also stated that "Changelings use some sort of subspace "pocket" to store additional mass during a morph." (AOL chat, 1997)

Changelings could take virtually any corporeal form, ranging from humanoid aliens to fog to reflective surfaces. They did not seem to be able to exist as or change to forms of energy. (DS9: "Chimera", "The Way of the Warrior") They, or at least the Founders, also had the ability to closely mimic humanoid personalities to the point where even close friends and family often could not distinguish them from the originals. (DS9: "By Inferno's Light", "Apocalypse Rising", "Paradise Lost") They could transform parts of themselves into smaller electric devices, like a combadge and make it operational, and were also capable of at least mimicking the shape and operation of a computer. Changelings could also mimic the form of fire and clouds.

Since it was established many times that any part of a Changeling's body reverts to its gelatinous form if removed, it is unclear how they could take the form of clouds or other vaporous substances; as soon as the cloud moved even the smallest amount, the Changeling would automatically revert to its natural gelatinous form over whatever area the "cloud" had covered.

Starfleet sensors as of 2371 failed to recognize Changelings when they mimicked other forms. However, it was established that changelings could mimick humanoid physiology, as in the case of Odo. The Obsidian Order created a prototype quantum stasis field generator that could prevent a Changeling from shape shifting. (DS9: "The Adversary", "The Die is Cast")

The thoughts and emotions of Changelings could not be read by Betazoids. (DS9: "Fascination")

Changelings could survive in the vacuum of space, and were highly resistant to disruptor fire. (DS9: "Chimera", "Apocalypse Rising")

Changelings have often been portrayed as extraordinarily resistant to things that would be highly hazardous to species composed of cells and requiring metabolic processes to sustain themselves. However, in the mirror universe 'our' Julian Bashir shattered that universe's Odo into pieces with a single shot. (DS9: "Crossover") However, in 'DS9: "Vortex"' Odo was seen to have the ability to regroup himself after being shattered. But, since neither the Mirror Universe Odo, nor the Changeling who took Martok's form reformed, it is safe to say they were dead. In 'DS9: "Treachery, Faith and the Great River"' Odo also talked as if he could die from lack of air or cold, even though he has no metabolism that would need either.

Changelings – or at least Odo – had no sense of smell. (DS9: "If Wishes Were Horses", "Improbable Cause") But, Laas stated that if his hands had Klingon blood on them, they would emit a stench, showing that after living among solids for years, he was at least familiar with the concept. When Odo was forced to assume solid form by the Founders, he gained a sense of smell and taste, which suggests that changelings are capable of mimicking those senses. Even when in humanoid form, Changelings did not need to take in sustenance. Odo noted that he once attempted to eat after assuming humanoid form for the first time, but that he found the experience not only unsatisfying due to his lack of tastebuds, but that he also described it as 'messy' for reasons he chose not to elaborate. (DS9: "Meridian")

A telepathic energy matrix (which caused the crew of Deep Space 9 to fight for control of the station) was rejected by Odo's lack of a humanoid brain. It did have the ability to warp his face and knock him unconscious, though he retained his humanoid shape. (DS9: "Dramatis Personae")

There was an apparent telepathic aspect to the species, evidenced in Odo's being drawn toward the Omarion Nebula, which was once home to the Founders. (DS9: "The Search, Part II") However, it is possible that the instinctive desire to visit the nebula was "hardwired" into him. Changelings had the ability to sense, in most cases, the presence of other Changelings. (DS9: "The Search, Part I", "Homefront", "Chimera") The Jem'Hadar may also have the ability to sense Changelings (DS9: "The Abandoned"), but this ability not be telepathic but an imprinted response to a Changeling's gelatinous state, as the child Jem'Hadar did not respond to Odo until he temporarily reverted to that state.

Further suggestion of a telepathic trait arose when Odo's remaining morphogenic enzymes were stimulated by a plasma field, creating a miniaturized "Great Link" with Benjamin Sisko, Jadzia Dax and Elim Garak. (DS9: "Things Past")

At the very least, there exists a "hive mind" among the species when joined with the Great Link. (DS9: "The Search, Part II", "Broken Link", "Sacrifice of Angels", "Chimera")

"During the Link, I sensed that the other Changelings were trying to hide things from me... faces, names." - Odo (DS9: "Broken Link")

The Founders of the Dominion were Changelings, and made up the majority of the Dominion's leadership. Changelings preferred to think of themselves as a drop (the individual) in the larger ocean (what they call the Great Link). A Changeling, therefore, typically had little sense of individual identity. Being a liquid-based lifeform, in order to link with other Changelings they simply reverted to this state and the liquid blends together, creating a link between them. (DS9: "The Search, Part II")

Changelings have been suggested to be biologically immortal and therefore never die of old age. (DS9: "Children of Time", "Behind the Lines")

History

Eons ago, Changelings were monoforms like solids, limited to one shape and according to the Founders, at one time they were peaceful and explored the galaxy. But they were persecuted by some of the solids, and they came to believe that the only way to protect themselves was to conquer those around them. It isn't stated how they feel about other shapeshifting species. Solids (mono-form solid species) called the Founders "Changelings" as an insult, but the Founders took the name for themselves as an act of defiance. (DS9: "The Search, Part II") The story came from one of the Founders herself.

Long ago, the Founders sent out one hundred infant Changelings in order to gather information and explore, to return later and share what they have learned. The Founders did not expect any to return until the late-27th century. Odo and Laas were among the hundred. (DS9: "The Search, Part II", "Chimera")

The Great Link was soon infected by a morphogenic virus that threatened the Changelings with extinction. The Federation's secret police, Section 31, intended to wipe out the Founders, leaving the Dominion powerless and wavering. This was cured by Odo. (DS9: "Extreme Measures", "What You Leave Behind")

No Changeling ever harmed another until Odo killed one to save his crewmates in late 2371. (DS9: "The Adversary")

The Changelings seemed proud of their heritage and considered themselves subtly superior; in contrast, Odo (who had been separated from his people for most of his existence) was almost their polar opposite, experiencing self-doubt and internal conflict regarding his people's warlike nature.

People

Mirror universe

In the mirror universe, the Bajoran wormhole remained undiscovered as late as 2375, therefore Changelings and other species native to the Gamma Quadrant remained unknown to the peoples of the Alpha Quadrant. The only exception was Odo, who was killed on Terok Nor during a workers' revolt in 2370. (DS9: "Crossover")

Most likely, Odo was, like his counterpart, one of the hundred and was found in the Denorios belt.

Background Information

Changelings other than Odo were first mentioned in DS9: "Vortex". They were referred to again in DS9: "Shadowplay", before being introduced in DS9: "The Search, Part I".
DS9: "Chimera" suggests Changelings have a lifespan of at least two hundred years, although if Odo was indeed two hundred years old when he traveled back with the USS Defiant in an alternate timeline in DS9: "Children of Time", that would indicate Changelings could live for at least four hundred years. It has also been suggested that Changelings might be immortal with the Female Changeling remarking "you are a Changeling, you are timeless" – a logical assumption, given that Changelings don't appear to have any cells that might decay over time.
Though Odo and Laas have been described as male and the Female Changeling as female, it is unclear what anatomical differences, if any, exist between male and female Changelings. Also, although an infant Changeling appeared in DS9: "The Begotten", it was never made clear exactly how Changelings reproduce. The most popular theory is that they reproduce asexually, and simply maintain a male or female form to make it easier for other species to relate to them. At one point, Odo assumed the form of the Female Changeling, which would support this hypothesis. (DS9: "Tacking Into the Wind"). Odo did display romantic affection for some female humanoids, most notably Kira Nerys, and was capable of sexual intercourse with humanoid species, but wasn't able to reproduce. He even had a sexual encounter with the Female Changeling, suggesting that two Changelings can mate in humanoid form, though the Female Changeling considered this version of intimacy unnecessary for their race, and pale compared to the Great Link (DS9: "A Simple Investigation", "Favor the Bold").
The Changelings frequently assumed a form similar to that of Odo; however, Odo's unique appearance was the product of his difficulty with assuming convincing humanoid forms, not mandated by his racial makeup. Since other Changelings have no difficulty in convincingly assuming humanoid forms, they must therefore assume an "Odoid" form out of choice. This may be because before meeting Odo and the races of the Alpha Quadrant, they had no need to reveal themselves individually to outsiders, and thus had no need for a unique humanoid form to differentiate themselves from other species. Odo would have provided them with a useful template. Ronald D. Moore commented: "Odo modeled his look after Dr. Mora and the Founders then modeled their look after Odo. They did this initially as a compliment and way of reaching out to their long-lost Changeling, and later they kept doing it as a dig and reminder to him of his own limitations." (AOL chat, 1997)
Although much older than Odo, Laas also seemed to have similar facial features, probably because of the same reason. Having been found by the Varalans it can be assumed that the unique bumps on his forehead are typical Valaran features. (However Odo doesn't have Mora's (or others') Bajoran features; he commented in DS9: "Past Prologue" that he always had trouble with their noses.)
The Female Changeling appears to be left-handed in "What You Leave Behind", however Odo seems to be right-handed during the course of the series. It is unclear that when assuming humanoid form, a Changeling has a dominant hand, or simply chooses which hand to use for different tasks.
Odo assumes the shape of a computer in "Paradise Lost", but it is unknown whether he could simply mimic the computer's form or whether he could actually perform the complex calculations a computer makes.
Changelings do grow physically, to a point; as Changelings don't eat or drink, it is unclear exactly where the 'new' mass and energy come from. Julian Bashir asked Odo for a sample of his liquid form, and Odo only agreed on the condition that he would get it back after Bashir's tests were complete.