Mesotherms are an intermediary between ectothermy and endothermy, and I won't lie trying to research them is a pain in the ass because a lot of mesothermic species do not exist anymore (hi dinos) or are discussed primarily in terms of endothermy.
The characteristics of mesotherms are as such: they can raise their internal body temperature via metabolism, and they cannot exert particular metabolic control of a particular temperature. This means they have a greater range of internal temperatures they're subject to than most mammals (aka they're poikilothermic or heterothermic), and can keep their body temperature as much as 20 degrees higher than their environment in some species, but it and their metabolism will drop as ambient temperatures drop. They do not burn fat or shiver to warm up. They are more active than ectotherms, and require less food than endotherms.
Real life mesotherms include: hyraxes, naked mole rats, echidnas, leatherback sea turtles, tuna, certain sharks, and possibly the Argentine giant tegu (it's more accurately heterothermic).
Supposing ectothermy and endothermy are two poles on a spectrum, one could lay out these three species, from left to right, as such: Cardassians, Trill, then Bajorans.
Bajorans evolutionarily descend from heterothermic mesotherm underground rodents. Their temperature tolerance range varies slightly depending on where on Bajor their ancestry is from, but the primary factor is where they and their parents grew up. Even with this variance, knitting and crochet are popular in Bajoran textile production for warmth.
Ideal temperature: 65f-80f accommodates most Bajorans even with the varying tolerance.
Treatments/concerns: their temperature range has overlap with the climate controls on many Federation vessels, so most Bajoran crew don't have to seek out accommodations when serving in Starfleet. If a Bajoran has temperature concerns, adjustments to quarters climate controls are usually sufficient. In emergencies, the first thing to do is move the patient to an area with a 10 degree temperature greater or lesser than the current one (depending on which issue it is). If not possible, employing temperature packs, dressing and undressing, and so on can usually alleviate the matter greatly.
Cardassians are the most visually obvious mesotherms. This owes not only to their reptilian appearance, but also to the fact that their skin lightens or darkens depending on temperature as a method of regulation: darker for cold temperatures, lighter for hotter. Different “base” colors cluster roughly to geographic heritage, however all Cardassians have more or less the same tolerance medically (some Cardassians train to function better in cold temperatures, but the health affects remain similar). Cardassians are poikilotherms.
Ideal temperature: 86f-95f, with a basking temperature of 110f minimum.
Treatments/concerns: as mesotherms are subject to environmental temperature, all three exhibit some degree of basking behavior, however Cardassians particularly have developed a culture around it (and often will partake in behavioural fever). Federation vessels are always about 10 degrees too cool and unless crewed by ectothermic species, have little or no basking opportunities. Cardassians are fine on Federation vessels for limited time frames without accommodation for this and other reasons, but long term residence requires accommodation to prevent serious health problems, such as malnutrition and gout. It is inadvisable for non-medical personnel to undress a Cardassian experiencing temperature issues as their clothing insulation helps keep temperature constant, not just warm.
There are two different major populations of Trill: those on the northern contenients who have protrusions on the face and those on the southern contenients who have spots. The North-South distinction can be seen in how Southern Trill like Jadzia wear thicker uniforms when avaliable, and that the dress of civilians is made of heavy fabrics like brocade. The Northern Trill’s face protrusions are sinus related. Trill are poikilotherms.
Ideal temperature: 60f-75f for Northern Trill, 70-85f for Southern Trill.
Treatments/concerns: joined Trill are even more sensitive to temperature shifts than unjoined Trill, as the internal temperature the symbiont can tolerate is a much narrower range (about 5~ degrees, much like humans). They also often run hotter than unjoined Trill, so Jadzia and Lenara need to try and keep their temperatures around 80-85f. Part of the concern of the joining process is making sure a symbiont and its host having matching internal temperature ranges; the populations of different pools have symbionts with different tolerances. This is a limiting factor to enabling symbionts a variety of experiences and is an area of research the symbiosis commission has flagged as of interest. Trill from one population with an abnormal temperature range are often more likely to be selected for joining, but aren't especially common.
Cardassians and Trill, in addition to temperature, generally need more humidity in their environment than is usual on Federation vessels not operated with such needs in mind, though for different reasons. Also like Cardassians, it is inadvisable to undress a Trill experiencing temperature regulation issues for the same reason previously stated.
