Historic pack — Fever and survival: Kluger's lizards, 1974–1978 — POSSIBLE

In an infected reptile that cannot generate its own heat, does the freedom to seek warmth — behavioral fever — change survival?

Shelf #374 · gate-verified and approved by the developer

POSSIBLE — a historic pack, carded at the developer's request (Nov 10, 2026): One research programme at the University of Michigan asked, in the 1970s, whether fever is worth anything to an animal that cannot make its own heat. Desert iguanas (Dipsosaurus dorsalis) infected with the bacterium Aeromonas hydrophila develop a fever of about 2°C — behaviorally, by seeking warmer surroundings. Kluger 1975 placed infected lizards at low (34–36°C), neutral (38°C) or high (40–42°C) ambient temperatures: the warmer the lizards could be, the significantly better they survived. Bernheim & Kluger 1976 ran the mirror experiment with an antipyretic: of 12 infected lizards given sodium salicylate, the five that still managed to raise their temperature at least 0.6°C survived the week, and the seven whose fever was prevented died within three days. Grieger & Kluger 1978 supplied a mechanism: infection drops serum iron, the bacterium grows unchecked at fever temperature only when iron is plentiful, and giving infected lizards extra iron increased their mortality. Three small laboratory experiments, one programme, one direction — the founding demonstration that fever is an evolved defence, not a malfunction. The stamp is POSSIBLE: tiny groups, 1970s reporting, a single laboratory species — and a claim this library states narrowly, about ectotherms choosing their temperature, not as a sepsis treatment for any patient.

Kluger 1975: infected lizards kept warm survived significantly better than lizards kept cool

Experimental infection study (ambient-temperature arms)

Who: Desert iguanas (Dipsosaurus dorsalis) injected with live Aeromonas hydrophila, a gram-negative aquatic bacterium; the lizards develop a fever of about 2°C after injection. To test whether that elevation in body temperature increases resistance to the infection, infected lizards were placed in a neutral (38°C), low (34°C or 36°C) or high (40°C or 42°C) ambient temperature — the laboratory version of a sick lizard choosing, or being denied, a sun-warmed rock. Setting: experimental physiology laboratory, University of Michigan.

Result: An elevation in temperature following experimental bacterial infection resulted in a significant increase in host survival. The abstract reports the direction and significance without printing group survival percentages; this card does not supply numbers the record does not state.

Watch out: The founding experiment, with a 1975 abstract's economy: no group sizes or survival percentages in the record, no randomisation or blinding language — none was customary. Its strength is the design's simplicity (same infection, only the temperature differs) and its convergence with the programme's two follow-up experiments carded below.

Kluger MJ, Ringler DH, Anver MR. Fever and survival. Science. 1975;188(4184):166-8. PMID: 1114347. Registry ID: none — pre-registration-era animal study. MeSH: Aeromonas/growth & development · Animals · Bacterial Infections/mortality · Body Temperature · Disease Models, Animal · Fever · Lizards · Temperature

Bernheim & Kluger 1976: blocking the fever with salicylate killed the lizards that stayed afebrile — 7 of 7 dead in 3 days; the 5 that still spiked a fever lived

Experimental infection study with antipyretic intervention

Who: Twelve lizards (Dipsosaurus dorsalis) infected with live Aeromonas hydrophila and given varying, nontoxic doses of sodium salicylate, an antipyretic drug, to test whether preventing fever changes survival. The drug did not block the fever in every animal — which turned the experiment into a natural comparison between infected lizards that mounted a fever despite the drug and those that did not.

Result: Five animals increased their mean body temperature at least 0.6°C and survived the week; seven did not develop a fever and died within 3 days. The authors conclude that in these lizards, prevention of fever by an antipyretic drug increases the mortality rate from bacterial infection.

Watch out: Twelve lizards, and the fever/no-fever split was the drug's variable effect, not a randomised assignment — a lizard that failed to mount a fever might also have been sicker to begin with. Read it as convergent evidence beside the temperature-placement experiment, not as a stand-alone proof. Salicylate dosing in reptiles is not a practice recommendation; this is a physiology demonstration.

Bernheim HA, Kluger MJ. Fever: effect of drug-induced antipyresis on survival. Science. 1976;193(4249):237-9. PMID: 935867. Registry ID: none — pre-registration-era animal study. MeSH: Aeromonas · Animals · Enterobacteriaceae Infections/drug therapy/mortality/physiopathology · Fever/physiopathology/prevention & control · Lizards · Sodium Salicylate/adverse effects/therapeutic use

Grieger & Kluger 1978: the mechanism — infection withdraws iron, and at fever temperature the bacterium only thrives if iron is there

Experimental mechanistic study (in vivo and in vitro)

Who: The same model (Dipsosaurus dorsalis, Aeromonas hydrophila) used to ask how a raised temperature could help: serum iron measured in infected and uninfected lizards at normal (38°C) and febrile (41°C) body temperatures; the bacterium grown in vitro at both temperatures with normal and reduced iron; and infected lizards supplemented with iron.

Result: Infection significantly reduced serum iron levels, comparable to the fall seen in mammals, and the fall was independent of the lizard's body temperature. In vitro, A. hydrophila grew equally well at 38°C and 41°C when iron was plentiful — but when iron in the growth medium was reduced, bacterial growth was diminished at the febrile temperature and not significantly affected at the afebrile temperature. Adding iron supplements to infected lizards increased their mortality. The authors' conclusion: one mechanism behind the adaptive value of fever is the fever-coupled decrease in iron available to the pathogen.

Watch out: One mechanism, demonstrated in one bacterium–lizard pair; fever's adaptive value does not reduce to iron (heat also accelerates host immune processes), and the in-vitro arms are bench work, not patients. The iron-supplementation result is a warning against casual iron repletion during active infection in this model — not a dosing rule for any species.

Grieger TA, Kluger MJ. Fever and survival: the role of serum iron. J Physiol. 1978;279:187-96. PMID: 678329. Registry ID: none — pre-registration-era animal study. MeSH: Aeromonas/growth & development · Animals · Bacterial Infections/blood/mortality · Body Temperature · Fever/blood/mortality · Iron/blood · Lizards/physiology

Synthesis: a sick lizard that can reach the warm rock lives; one that cannot, dies — the oldest hard evidence that fever is a defence

Synthesis

Who: Synthesis of the Michigan programme's three carded experiments: Kluger 1975 (temperature placement), Bernheim & Kluger 1976 (antipyretic blockade), Grieger & Kluger 1978 (iron mechanism), all in Dipsosaurus dorsalis infected with Aeromonas hydrophila. Fenced, named but not carded: Vaughn, Bernheim & Kluger 1974 (Nature 252:473-4, PMID 4431469), the programme's first report that infected lizards behaviorally raise their body temperature — its PubMed record carries no abstract, and this library does not card what it cannot read.

Result: Put together: an ectotherm cannot shiver its way to a fever, so it walks to one — and that walk is worth its life. Denied warmth, infected lizards died; allowed fever-range temperatures, they survived significantly better; pharmacologically prevented from fever, they died within days; and a mechanism (iron withdrawal biting only at fever temperature) explains part of why. For veterinary practice the durable point is husbandry, not pharmacology: a hospitalised reptile needs a thermal gradient — real access to a warm zone it can choose — especially when infected, because thermoregulation is one of its immune effectors. POSSIBLE: three small 1970s laboratory experiments from one programme, never replicated as a survival trial in a clinical reptile population, and stated here as the physiology landmark it is.

Watch out: Do not let the romance outrun the record: this is a desert iguana and one bacterium in a 1970s laboratory. It does not show that heating a septic mammal helps (mammals make their own fever; forced external heating is a different intervention), it does not indict antipyretics in patients who generate fever internally, and it gives no temperature prescription for any species. What it establishes — and why this library keeps it — is that fever is an ancient, conserved, survival-linked response that predates endothermy itself.

Synthesis of Kluger MJ et al. Science. 1975. PMID: 1114347; Bernheim HA, Kluger MJ. Science. 1976. PMID: 935867; Grieger TA, Kluger MJ. J Physiol. 1978. PMID: 678329. Fenced: Vaughn LK, Bernheim HA, Kluger MJ. Nature. 1974. PMID: 4431469 (no abstract in the record). Registry IDs as on the individual cards. Historic pack — built at the developer's request, Nov 10, 2026.

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