Which Rat Breeds Live the Longest?

Understanding Rat Lifespan: General Factors

Genetic Predisposition and Longevity

Inherited Traits Affecting Lifespan

Inherited traits exert a decisive influence on rat longevity. Genetic variations that enhance cellular repair, regulate metabolism, and bolster immune function consistently correlate with extended lifespans across breeds.

Key hereditary factors include:

Telomere maintenance genes – alleles that preserve telomere length reduce age‑related chromosomal degradation.
Growth hormone/IGF‑1 axis – moderate expression limits excessive somatic growth, decreasing metabolic stress.
Oxidative stress resistance – elevated expression of antioxidant enzymes (e.g., superoxide dismutase, catalase) mitigates cellular damage.
Disease‑susceptibility loci – alleles conferring resistance to common rodent ailments such as lymphoma and renal disease prolong survival.
Metabolic efficiency genes – variants that favor balanced energy expenditure lower the incidence of obesity‑related complications.

Breed comparisons illustrate the impact of these genes. Long‑Evans rats often carry favorable telomere‑preserving alleles, resulting in median lifespans of 30–34 months. Sprague‑Dawley lines display moderate antioxidant gene activity, averaging 24–28 months. Wistar strains, with less robust disease‑resistance loci, typically reach 20–22 months.

Selective breeding programs that prioritize the above genetic markers have demonstrated measurable increases in average lifespan, confirming that inherited traits constitute the primary determinant of longevity among rat breeds.

Lineage and Health History

Lineage and health history provide the most reliable indicators of a rat breed’s potential lifespan. Breeds that have been developed through careful selection for vigor and disease resistance consistently outlive those derived from narrow, inbred programs.

Outcrossed lines that combine genetics from multiple foundation stocks reduce the frequency of recessive disorders.
Breeds with documented low incidence of respiratory infections, renal disease, and neoplasia show median lifespans of 3 – 4 years, compared with 2 – 2.5 years for breeds lacking such records.
Populations maintained under strict health‑screening protocols exhibit fewer cardiac anomalies and longer reproductive viability, extending overall longevity.

The longest‑living varieties—such as the Rex, Dumbo, Satin, and Standard Fancy—share several pedigree characteristics. Their breeding histories emphasize:

Multi‑generation health audits that identify and eliminate carriers of hereditary defects.
Introduction of genetically diverse sires every few generations to avoid bottleneck effects.
Selection criteria that prioritize robust immune response and moderate growth rates, limiting the prevalence of obesity‑related complications.

Health records reveal that breeds with minimal exposure to common pathogens and lower tumor rates achieve the greatest age benchmarks. Systematic veterinary monitoring, routine blood work, and genetic testing for markers linked to tumor suppression correlate directly with extended survival.

In practice, breeders who document lineage, perform comprehensive health screenings, and maintain genetic heterogeneity produce rats that routinely surpass the average lifespan expectations for the species.

Environmental Influences on Rat Lifespan

Diet and Nutrition

Dietary composition exerts a measurable impact on the lifespan of the longest‑living rat varieties. Caloric restriction, without malnutrition, consistently extends median survival across multiple strains. Protein intake should be calibrated to 18–20 % of total calories; excess protein accelerates renal load and reduces longevity, while insufficient protein compromises growth and immune function.

Key nutrients that support extended life expectancy include:

Omega‑3 fatty acids – improve cardiovascular health and reduce inflammatory markers.
Antioxidants (vitamin E, vitamin C, selenium) – mitigate oxidative damage to cellular membranes.
Fiber (soluble and insoluble) – maintains gut motility, supports microbiome diversity, and lowers post‑prandial glucose spikes.
Choline – essential for hepatic lipid metabolism and neural integrity.

Feeding schedules influence metabolic stability. Providing consistent portions at the same time each day prevents hormonal fluctuations that can accelerate aging. Water should be fresh and available ad libitum; dehydration accelerates renal decline, a common cause of premature death in rats.

Breed‑specific considerations arise from genetic variations in metabolism. For example, dwarf rats exhibit higher basal metabolic rates and benefit from modest caloric reduction, whereas larger breeds tolerate slightly higher energy density without adverse effects. Monitoring body condition scores and adjusting macronutrient ratios accordingly ensures optimal health trajectories for each lineage.

Habitat and Living Conditions

Habitat quality and daily living conditions are decisive factors in the lifespan differences observed among rat breeds. Controlled temperature (18‑24 °C), moderate humidity (45‑55 %), and proper ventilation prevent respiratory and metabolic stress, extending average life expectancy.

Space: Minimum floor area of 0.5 m² per animal, with vertical enrichment, reduces aggression and promotes physical activity.
Bedding: Absorbent, dust‑free material (e.g., paper or wood shavings) maintains hygiene and lowers infection risk.
Diet: Balanced commercial rat chow supplemented with fresh vegetables, limited fruit, and occasional protein sources supplies essential nutrients without excess calories.
Enrichment: Rotating toys, tunnels, and foraging opportunities stimulate mental health, decreasing stress‑related illnesses.
Health care: Regular veterinary checks, prompt parasite control, and vaccination where applicable prevent chronic disease development.
Social environment: Cohabitation with compatible conspecifics fulfills social needs, reducing solitary stress.

Breeds such as the Long‑Haired, Dumbo, and Standard Fancy exhibit longer lifespans when these conditions are consistently met. Conversely, inadequate space, extreme temperatures, or poor nutrition accelerate aging processes across all varieties.

Optimal habitat management therefore constitutes the primary means of maximizing longevity among rat breeds, regardless of genetic predisposition.

Stress and Social Environment

Research on rat longevity consistently shows that stress exposure and social conditions shape lifespan differences among breeds. High‑intensity stressors—such as unpredictable noise, temperature fluctuations, or forced restraint—activate the hypothalamic‑pituitary‑adrenal axis, elevate corticosterone, and accelerate cellular aging. Breeds that exhibit lower baseline corticosterone responses, such as the Long‑Evans, tend to outlive more stress‑sensitive strains like the Sprague‑Dawley when both are kept under identical adverse conditions.

Social environment exerts comparable influence. Group housing that permits stable hierarchies reduces chronic anxiety, whereas solitary confinement or overcrowding provokes sustained social stress. Studies measuring telomere attrition reveal that socially isolated rats experience faster telomere shortening than those housed in compatible groups, regardless of genetic background. Breed‑specific social tolerance modulates this effect: the Wistar rat maintains relatively stable social bonds and shows modest lifespan reduction under isolation, while the Fischer 344 displays marked vulnerability, with isolation shortening median lifespan by up to 20 %.

Key mechanisms linking stress and social factors to longevity across breeds include:

Corticosterone‑mediated suppression of immune function.
Oxidative stress amplification via increased reactive oxygen species.
Dysregulation of sleep architecture, leading to metabolic disturbances.
Altered expression of longevity‑associated genes (e.g., Sirt1, FoxO1).

Practical implications for extending rat lifespan in research settings involve minimizing unpredictable stressors, providing enriched group housing with clear social structures, and selecting breeds with documented resilience to environmental challenges. Implementing these measures aligns physiological conditions with the innate longevity potential of each rat breed.

Exploring Rat Breeds and Their Lifespans

Common Pet Rat Breeds

Standard Rats

Standard rats, often referred to as the typical laboratory or pet variety, represent the baseline for longevity comparisons among rodent breeds. Average lifespan under optimal care ranges from 2.5 to 3.5 years, with documented cases reaching up to 4 years when health management is meticulous.

Key determinants of extended lifespan in standard rats include:

Balanced diet rich in protein, fiber, and essential vitamins
Regular veterinary examinations to detect and treat diseases early
Enriched environment providing stimulation and exercise opportunities
Genetic health screening to avoid hereditary defects

When contrasted with dwarf, hairless, or specialty breeds, standard rats consistently exhibit longer lifespans. Dwarf varieties often average 1.5 to 2 years, while hairless strains may experience increased susceptibility to skin infections, reducing average longevity to approximately 1.8 years. These differences stem primarily from genetic predispositions and the heightened care requirements of less robust phenotypes.

Optimizing nutrition, preventive health care, and environmental enrichment maximizes the natural lifespan potential of standard rats, positioning them as the longest-lived among common rat breeds.

Fancy Rats and Their Variations

Fancy rats represent a genetically diverse group of domesticated brown rats (Rattus norvegicus) that have been selectively bred for coat color, pattern, size, and temperament. The breeding process emphasizes aesthetic traits, yet health and lifespan remain critical factors for responsible owners.

Longevity among fancy rats varies according to lineage, coat type, and breeding practices. Studies of captive populations show that the median lifespan ranges from 2 to 3 years, with some lines consistently reaching 4 years or more. Factors that extend life expectancy include robust genetic stock, minimal inbreeding, and careful management of diet, environment, and veterinary care.

Typical variations and their observed lifespan ranges:

Standard (plain‑coated) rats: 2.5–3.5 years; stable health profile, low incidence of coat‑related skin issues.
Hairless (satin, rex) rats: 2–3 years; prone to temperature regulation problems and skin infections, which can reduce lifespan if not managed.
Dumbo (large‑ear) rats: 2.5–4 years; ear size does not inherently affect longevity, but enlarged ears may be more susceptible to trauma.
Miniature (dwarf) rats: 1.5–2.5 years; smaller body mass correlates with a slightly shorter lifespan, though proper nutrition mitigates risk.
Color‑patterned (agouti, hedgehog, harlequin) rats: 2–4 years; no direct impact on lifespan, but certain patterns are linked to specific genetic mutations that may influence health.

Longevity correlates more strongly with breeding ethics than with visual traits. Breeders who prioritize genetic diversity, health screenings, and balanced nutrition produce fancy rats that regularly exceed the average lifespan for the species. Selecting lines with documented health records and avoiding extreme phenotypic extremes maximizes the chances of a long, healthy life for these companion animals.

Specific Breeds and Their Longevity Potential

Factors Influencing Breed-Specific Lifespan

Genetic composition sets the baseline for a rat’s lifespan. Specific alleles influence growth rate, immune robustness, and susceptibility to age‑related disorders, creating measurable differences between breeds.

Metabolic rate determines how quickly physiological processes consume energy. Breeds with slower metabolism typically experience reduced oxidative stress, extending cellular function and overall longevity.

Nutrition directly affects health outcomes. Diets rich in balanced proteins, essential fatty acids, vitamins, and minerals support organ maintenance, while excess calories accelerate obesity‑related complications.

Environmental conditions shape survival prospects. Stable temperature, low humidity, and minimal exposure to toxins reduce chronic strain on organ systems.

Healthcare quality governs disease prevention and early intervention. Regular veterinary examinations, vaccinations, and prompt treatment of infections correlate with longer average lifespans.

Breeding practices influence genetic diversity. Controlled outcrossing limits inbreeding depression, preserving heterozygosity that buffers against hereditary ailments.

Reproductive status alters hormonal balance. Spaying or neutering eliminates reproductive‑system cancers and mitigates metabolic fluctuations, contributing to increased longevity.

Activity level affects cardiovascular and musculoskeletal health. Environments that encourage regular exercise maintain muscle tone and improve circulatory efficiency.

Stress exposure impacts endocrine function. Chronic stress elevates cortisol, impairing immune response and accelerating tissue degeneration.

Housing density governs social interaction and competition. Overcrowding increases aggression and disease transmission, whereas moderate group sizes promote stable social structures and reduced conflict.

These factors interact synergistically; optimizing each element creates conditions in which particular rat breeds achieve their maximum breed‑specific lifespan.

Anecdotal Evidence and General Observations

Anecdotal reports from laboratory technicians, pet owners, and small‑scale breeders consistently highlight certain rat strains that outlive others. The most frequently cited examples include the Long‑Evans, the Wistar, and the Dahl S. In multiple private breeding operations, Long‑Evans individuals have reached ages of 3.5 years, surpassing the typical 2‑year expectancy for most domestic rats. Wistar rats, documented in university colonies, occasionally live beyond 3 years when provided with enriched environments and a low‑fat diet. The Dahl S strain, originally selected for cardiovascular research, shows a modest lifespan extension under calorie‑restricted feeding regimes, with several cases of 3.2‑year-old specimens reported.

General observations from veterinary records reinforce these anecdotes. Rats that receive regular health checks, preventive dental care, and a varied diet of fresh vegetables, protein sources, and high‑quality rodent pellets tend to achieve the upper end of the species’ lifespan range. Environmental factors such as low stress, ample cage space, and social housing also correlate with longer lives across all breeds. Conversely, overcrowding, poor nutrition, and chronic respiratory irritation consistently appear in cases of premature mortality, regardless of genetic background.

Key patterns emerging from the collected stories:

Breeds selected for research purposes (e.g., Long‑Evans, Wistar) often receive superior husbandry, contributing to extended longevity.
Owners who monitor weight and adjust food intake in response to seasonal activity levels report fewer age‑related health declines.
Social interaction with compatible conspecifics reduces stress‑induced illnesses, a factor noted by both amateur breeders and veterinary clinics.

These anecdotal and observational data suggest that, while genetics influence potential lifespan, optimal care practices are decisive in realizing the longest‑possible lives for any rat breed.

Health Considerations for Extended Lifespan

Common Health Issues in Rats

Rats commonly face respiratory infections such as Mycoplasma pulmonis and Streptococcus pneumoniae, which can progress rapidly without prompt treatment. Early signs include nasal discharge, audible wheezing, and reduced activity; veterinary intervention typically involves antibiotics and supportive care.

Dental overgrowth is frequent due to continuously erupting incisors. Uneven wear leads to malocclusion, causing difficulty eating, weight loss, and facial deformities. Regular inspection, provision of chewable materials, and occasional trimming are essential preventive measures.

Neoplastic growths appear in various forms, with mammary tumors prevalent in females and pituitary adenomas common across sexes. Detection relies on palpation and imaging; surgical removal combined with monitoring offers the best chance for extended survival.

Skin conditions, notably dermatitis and fungal infections, manifest as hair loss, redness, and itching. Maintaining dry bedding, controlling humidity, and applying topical antifungals reduce incidence.

Parasites, including mites, lice, and intestinal worms, contribute to anemia, gastrointestinal distress, and secondary infections. Routine fecal examinations and prophylactic treatments help keep infestations under control.

Metabolic disorders such as diabetes mellitus and fatty liver disease emerge in older or obese individuals. Symptoms include polyuria, polydipsia, and lethargy; dietary management and insulin therapy are standard responses.

Cardiovascular disease, though less common than in larger mammals, presents as heart murmurs and arrhythmias, often linked to genetic predispositions in certain breeds. Echocardiography and medication can mitigate progression.

Kidney dysfunction, characterized by polyuria and increased blood urea nitrogen, may result from chronic infections or age-related decline. Fluid therapy and dietary adjustments support renal function.

Understanding these health challenges clarifies why some rat breeds achieve greater longevity; breeds with lower susceptibility to respiratory and neoplastic conditions typically outlive those prone to such ailments. Effective monitoring, preventive care, and timely veterinary intervention collectively enhance lifespan potential.

Preventive Care and Veterinary Support

Effective preventive care and veterinary oversight are essential factors in extending the lifespan of long‑living rat varieties. Regular health monitoring enables early detection of common conditions such as respiratory infections, dental overgrowth, and tumors, which are prevalent in many rodent species. Timely intervention reduces morbidity and supports sustained vitality.

Key components of a preventive program include:

Routine physical examinations every three to six months, focusing on weight, coat condition, and respiratory sounds.
Scheduled dental checks and, when necessary, trimming to prevent malocclusion and associated pain.
Vaccination against common pathogens where available, combined with strict biosecurity to limit exposure to infectious agents.
Environmental management: temperature control (18‑24 °C), low humidity, and enrichment items that encourage natural foraging and exercise.
Nutritional planning: balanced commercial diets supplemented with fresh vegetables, limited fruit, and occasional protein sources, avoiding high‑fat treats that predispose to obesity.

Veterinary support should extend beyond examinations. Preventive medication protocols, such as monthly anti‑parasitic treatments and periodic blood work, provide baseline data for each individual. Establishing a health record that tracks growth curves, reproductive history, and any interventions allows practitioners to tailor care to the specific needs of each breed, recognizing that larger or hairless varieties may have distinct health profiles.

When health issues arise, prompt veterinary attention—preferably from a practitioner experienced with small mammals—ensures accurate diagnosis and appropriate therapeutic options. Access to diagnostic imaging, such as radiography or ultrasound, facilitates early identification of internal abnormalities, while pain management and supportive care improve quality of life during chronic conditions.

In summary, a structured regimen of regular examinations, environmental optimization, targeted nutrition, and proactive veterinary involvement forms the foundation for maximizing the longevity of rat breeds known for extended lifespans.

Maximizing Your Rat's Lifespan

Optimizing Diet for Longevity

Nutritional Requirements for Rats

Rats with extended lifespans require diets that meet precise macro‑ and micronutrient demands. Protein should constitute 18–20 % of total calories, sourced from high‑quality animal or soy proteins, to support tissue repair and immune function. Fat levels must remain between 4–6 % of calories, emphasizing omega‑3 fatty acids (e.g., fish oil) to reduce inflammation and promote cardiovascular health. Carbohydrates provide the remaining energy, preferably complex grains and vegetables that supply steady glucose release and fiber for gastrointestinal motility.

Key micronutrients include:

Vitamin A (800–1 200 IU/kg feed) for vision and epithelial maintenance.
Vitamin D₃ (1 000–1 500 IU/kg) to regulate calcium metabolism.
Vitamin E (50–100 mg/kg) as an antioxidant protecting cellular membranes.
B‑complex vitamins (B₁, B₂, B₆, B₁₂) at levels recommended by rodent nutrition standards to support metabolic pathways.
Minerals: calcium 0.8–1.0 %, phosphorus 0.6–0.8 % (Ca:P ratio ≈ 1.5:1), magnesium 0.05 %, zinc 50 ppm, selenium 0.2 ppm, and copper 6 ppm to sustain skeletal integrity, enzymatic activity, and immune competence.

Water must be continuously available, free of contaminants, and maintained at 20–30 °C to encourage adequate intake. Feeding frequency of 2–3 small meals per day prevents overconsumption and mimics natural foraging behavior, reducing obesity risk—a factor that shortens lifespan in many rat strains.

Supplementation guidelines:

Probiotic blends containing Lactobacillus and Bifidobacterium strains improve gut flora balance, enhancing nutrient absorption.
Prebiotic fibers (inulin, oligosaccharides) support microbial diversity.
Antioxidant extracts (green tea catechins, curcumin) may mitigate oxidative stress linked to age‑related decline.

Breed‑specific adjustments are advisable. Long‑living varieties such as the Dahl and the Sprague‑Dawley benefit from slightly higher omega‑3 ratios and reduced caloric density (≈ 12 kcal/g) to delay metabolic aging. Monitoring body condition scores weekly ensures diets remain aligned with health objectives and maximizes longevity potential.

Supplements and Their Role

Research on longevity in various rat strains indicates that targeted nutritional support can modify age‑related decline. Supplements influence metabolic pathways, oxidative stress, and immune function, all of which affect lifespan potential.

Commonly evaluated compounds include:

Omega‑3 fatty acids – improve membrane fluidity and reduce inflammatory markers.
Coenzyme Q10 – supports mitochondrial electron transport, enhancing cellular energy production.
Vitamin E and C – act as antioxidants, limiting lipid peroxidation.
Probiotics – maintain gut microbiota balance, promoting nutrient absorption and immune modulation.
N‑acetylcysteine – replenishes glutathione stores, protecting cells from oxidative damage.

Effective dosing requires adjustment for body weight and strain‑specific metabolism. Over‑supplementation may lead to toxicity, particularly with fat‑soluble vitamins and minerals. Regular monitoring of blood parameters ensures safety and allows fine‑tuning of regimens.

Integrating these agents into a balanced diet aligns with observed longevity trends in the longest‑lived rat breeds. Evidence suggests that consistent, moderate supplementation, combined with proper husbandry, yields measurable extensions in average lifespan without compromising health status.

Creating an Ideal Environment

Cage Size and Enrichment

Adequate housing directly influences the lifespan of rats, regardless of breed. Space that permits natural movement reduces stress, supports musculoskeletal health, and promotes robust immune function.

Recommended cage dimensions provide at least 1 ft² (0.09 m²) floor area per adult rat, with a minimum height of 12 in (30 cm) to allow vertical exploration. Multi‑level enclosures increase usable space without expanding footprint. Wire mesh should have gaps no larger than ½ in (12 mm) to prevent injuries while maintaining ventilation.

Enrichment items must be varied, safe, and regularly refreshed. Effective options include:

Chewable wooden blocks or untreated hardwood toys to maintain dental health.
Climbing ladders, ropes, or hammocks that encourage vertical activity.
Hideaways such as PVC tunnels or fabric shelters that allow retreat and reduce anxiety.
Foraging puzzles that disperse food, stimulating problem‑solving behavior.
Nesting material (e.g., shredded paper) for building nests and promoting natural instincts.

When combined with optimal cage size, these enrichment strategies extend the average longevity observed in longer‑living rat breeds, such as the Fancy and Dumbo varieties. Consistent provision of space and stimulation mitigates breed‑specific health risks, resulting in measurable gains in survival years.

Temperature and Humidity Control

Temperature stability directly influences the lifespan of laboratory and pet rats. Maintaining an environment within the optimal thermal range reduces metabolic stress, slows the progression of age‑related diseases, and supports immune function across all long‑living breeds.

Ideal ambient temperature: 68–74 °F (20–23 °C).
Preferred relative humidity: 40–60 %.
Daily temperature fluctuation: no more than ±2 °F (±1 °C).
Seasonal humidity shift: limited to ±5 % relative humidity.

Consistent monitoring prevents excursions beyond these limits. Digital thermostats paired with calibrated hygrometers provide real‑time data; alarms should trigger when values exceed set thresholds. Supplemental heating or cooling devices, such as ceramic heat emitters or low‑noise fans, adjust conditions without creating drafts that could stress the animals.

Ventilation must exchange air without lowering humidity below the target range. HEPA‑filtered systems with adjustable flow rates maintain air quality while preserving moisture levels. Regular cleaning of cages and bedding eliminates excess moisture accumulation, which can raise humidity and foster pathogen growth.

When breeding or housing rat strains known for extended longevity, researchers report increased median lifespans of 10–15 % when environmental parameters remain within the specified ranges. Deviations, especially prolonged exposure to temperatures above 80 °F (27 °C) or humidity below 30 %, correlate with accelerated aging markers and higher mortality rates.

Implementing precise temperature and humidity control constitutes a measurable factor in extending the healthspan of rat breeds distinguished by their longevity potential.

Promoting Health and Well-being

Regular Health Checks

Regular veterinary examinations are essential for extending the lifespan of pet rats, regardless of breed. Early detection of disease, management of chronic conditions, and preventive care directly influence longevity metrics.

Key components of a comprehensive health assessment include:

Dental inspection: check for overgrowth, malocclusion, and signs of pain.
Weight monitoring: record body mass weekly; deviations indicate metabolic or nutritional issues.
Skin and fur evaluation: look for lesions, parasites, or alopecia.
Respiratory check: listen for abnormal sounds; rats are prone to respiratory infections.
Behavioral observation: note changes in activity, grooming, or social interaction as indicators of discomfort or illness.
Reproductive health: assess for uterine or testicular abnormalities in breeding colonies.

Veterinarians recommend a baseline exam at 6 weeks of age, followed by quarterly check‑ups for adult rats. Breeders and owners should maintain detailed health logs to track trends over time. Prompt treatment of identified problems, such as dental trimming or antibiotic therapy for respiratory infections, correlates with increased average lifespans across all rat varieties, including those recognized for exceptional longevity.

Exercise and Mental Stimulation

Regular physical activity extends the lifespan of rats by strengthening cardiovascular function, preserving muscle mass, and reducing obesity‑related disease. Daily opportunities for running, climbing, and swimming stimulate heart rate variability and improve metabolic efficiency, which correlates with longer survival in breeds such as the Long‑Haired, Dumbo, and Standard Fancy.

Cognitive enrichment prevents age‑related decline in neural plasticity. Interactive toys, maze navigation, and scent puzzles maintain synaptic connections, lower stress hormone levels, and delay onset of neurodegenerative conditions. Rat breeds that receive consistent mental challenges exhibit delayed cognitive impairment and higher longevity metrics.

Practical guidelines for owners of long‑lived rat varieties:

Provide a minimum of 30 minutes of unrestricted wheel running or tunnel exploration each day.
Rotate puzzle feeders and scent trails weekly to introduce novel problem‑solving tasks.
Incorporate vertical elements (rungs, platforms) to encourage climbing and balance training.
Schedule short, varied play sessions with human handling to reinforce social cognition.
Monitor weight and activity patterns; adjust enrichment intensity if lethargy or hyperactivity appears.

Evidence from longitudinal studies shows that rats combining aerobic exercise with regular mental stimulation outlive sedentary counterparts by 20‑30 % on average. Implementing these protocols maximizes the health potential of the longest‑living rat breeds.