Up to What Age Do Rats Grow?

Understanding Rat Growth

The Lifecycle of a Rat

Neonatal Stage

The neonatal period in rats begins at birth and lasts approximately 21 days. During this time, pups are altricial: eyes remain closed, fur is sparse, and thermoregulation depends on the dam. Weight gain is rapid; average birth mass of 1.5–2 g increases to 15–20 g by the end of the third week, representing a ten‑fold increase.

Key physiological changes in the neonatal stage include:

Sensory development: Pinnae open around day 10, while eyelids open between days 13 and 15, enabling visual and auditory processing.
Thermoregulation: Brown adipose tissue activity peaks in the first week, allowing non‑shivering heat production.
Nutrition: Milk intake supplies essential antibodies and growth factors; weaning typically occurs at day 21, transitioning to solid food.

Musculoskeletal growth is evident through elongation of long bones and increased muscle fiber diameter. By the conclusion of the neonatal phase, skeletal ossification reaches a level comparable to that of juvenile rats, setting the stage for subsequent rapid growth during the post‑weaning period.

Weaning Stage

The weaning period marks the transition from maternal milk to solid food and is a pivotal phase in the development of laboratory rats. Occurring around post‑natal day 21, the process involves the gradual reduction of suckling and the introduction of a nutritionally balanced pellet diet. During this interval, the gastrointestinal tract matures, enzymatic activity shifts toward carbohydrate and protein digestion, and the immune system adapts to environmental antigens.

Key characteristics of the weaning stage:

Age: approximately 21 days after birth, with slight variation among strains.
Dietary change: complete replacement of milk with laboratory chow, providing essential nutrients for continued somatic growth.
Physiological adjustments: increased intestinal villi length, up‑regulation of digestive enzymes, and stabilization of body temperature regulation.
Growth impact: weight gain accelerates as solid food supplies higher caloric density; skeletal growth continues, but the rapid growth phase peaks shortly after weaning and tapers by 8–10 weeks of age.

Understanding the timing and conditions of weaning is essential for accurately determining the overall growth span of rats, which typically concludes near the end of the adolescent period.

Juvenile Stage

Rats enter the juvenile phase shortly after weaning, typically between post‑natal day 21 and day 35. During this interval body length and mass increase rapidly; average weight rises from roughly 30 g at weaning to 100–150 g by the end of the period. Skeletal growth accelerates, with the femur and tibia lengthening at a rate of 0.3 mm per day, while dental eruption completes the transition to adult dentition.

Physiological milestones in the juvenile stage include:

Maturation of the hypothalamic‑pituitary‑gonadal axis, leading to the first estrous cycles in females and detectable testosterone in males.
Development of adult‑type locomotor patterns; coordination improves as myelination of spinal tracts reaches 80 % of adult levels.
Onset of reproductive behavior, although full fertility is not achieved until early adulthood (approximately 6–8 weeks of age).

These changes define the window in which rats cease most of their linear growth and begin to approach their ultimate adult size, establishing the upper limit of their growth trajectory.

Adulthood and Senescence

Rats achieve skeletal and body‑size maturity within the first two months of life; after this point weight gain plateaus and the animal enters the adult phase. Sexual maturity appears at 5–6 weeks, confirming the transition from juvenile growth to functional adulthood.

The adult stage extends from approximately 8 weeks to 12–18 months, during which organ systems operate at peak efficiency. Key characteristics include stable body mass, fully developed dentition, and maximal reproductive output. Physiological parameters such as basal metabolic rate, immune competence, and locomotor performance remain relatively constant throughout this interval.

Senescence commences around the end of the first year and accelerates after 18 months. Age‑related changes encompass:

Gradual loss of muscle mass and strength
Decline in regenerative capacity of tissues (e.g., liver, skin)
Reduced fertility and altered hormone profiles
Increased incidence of neoplasia and chronic inflammation

The median lifespan of laboratory rats ranges from 24 to 30 months, with mortality sharply rising after 20 months as senescent processes dominate. Understanding the boundaries between adulthood and senescence is essential for experimental design, welfare considerations, and interpretation of age‑dependent data.

Factors Influencing Rat Growth

Genetics

Rats reach their final body size during a defined post‑natal period that ends roughly between six and eight weeks of age, depending on strain and environmental conditions. Genetic control of this trajectory is evident from the rapid growth phase driven by hormonal cascades and the subsequent plateau when growth‑plate closure occurs.

Key genetic determinants include:

Growth hormone (Gh) and its receptor (Ghr): stimulate hepatic production of insulin‑like growth factor 1 (Igf1), which promotes linear bone growth.
Insulin‑like growth factor 1 (Igf1) and binding proteins (Igfbp): mediate the proliferative response of chondrocytes in the growth plate.
Parathyroid hormone‑related protein (Pthrp) and Indian hedgehog (Ihh): regulate the timing of growth‑plate senescence.
Myostatin (Mstn) and related signaling molecules: limit muscle hypertrophy, influencing overall body mass.
Sex‑determining genes (Sry, Sox9) and gonadal steroid pathways: accelerate growth cessation in males after puberty.

Different laboratory strains exhibit distinct growth curves because of allelic variation in these loci. For example, the Sprague‑Dawley rat typically attains adult weight by day 45, whereas the Long‑Evans strain may continue modest growth until day 60, reflecting differences in Gh promoter activity and Igf1 expression levels.

Understanding the genetic architecture of rat growth informs experimental design. Researchers must align the age of phenotypic assessment with the expected growth plateau for the chosen strain to avoid confounding developmental changes with treatment effects. Genetic manipulation of the listed pathways can extend or truncate the growth period, providing a controlled model for studying growth‑related disorders.

Nutrition

Protein Intake

Protein requirements in laboratory rats change markedly from birth through the end of skeletal growth, typically around 10–12 weeks of age. Adequate dietary protein supports muscle accretion, organ development, and the hormonal milieu that drives lengthening of long bones.

During the neonatal period (0–3 weeks), diets containing 20–25 % crude protein by weight supply the amino acids needed for rapid cell proliferation. Below this range, growth velocity declines, and mortality increases.

From weaning (3–4 weeks) to the cessation of longitudinal growth (approximately 10 weeks), protein levels of 18–20 % maintain lean‑mass gain while preventing excess adiposity. Excessive protein (>25 %) can accelerate growth but may induce renal stress and alter gut microbiota, compromising long‑term health.

In the post‑growth phase (after 12 weeks), protein intake can be reduced to 14–16 % without affecting body weight maintenance, as the anabolic demand for skeletal extension diminishes.

Key points for optimal protein management:

0–3 weeks: 20–25 % crude protein, high biological value sources (casein, whey).
3–10 weeks: 18–20 % crude protein, balanced amino‑acid profile.
12 weeks: 14–16 % crude protein, sufficient for maintenance and reproduction.

Monitoring feed consumption and body‑weight trajectories ensures that protein provision aligns with the rat’s developmental stage, maximizing growth efficiency while avoiding metabolic overload.

Caloric Intake

Caloric consumption directly influences the length of the growth phase in laboratory and pet rats. Growth proceeds rapidly during the first six weeks, slows after eight weeks, and generally ceases by the tenth to twelfth week, depending on strain and environmental conditions. Energy intake during this interval determines whether the animal attains its genetic size potential.

In the juvenile stage (approximately 3–6 weeks), rats require 12–15 kcal per 100 g of body weight each day. This level supports rapid tissue synthesis, skeletal elongation, and organ development. From weeks 7 to 10, the requirement declines to 10–12 kcal per 100 g, reflecting a shift from linear growth to tissue maturation. After the growth plateau (≥ 12 weeks), maintenance needs stabilize at 9–10 kcal per 100 g daily.

Excess calories before the growth plateau extend the period of weight gain without increasing skeletal length, leading to higher adiposity and potential metabolic disturbances. Conversely, caloric deficits delay the onset of the growth plateau, resulting in reduced final body mass and possible skeletal abnormalities.

Typical daily caloric intake by age

3 weeks: 13 kcal / 100 g
5 weeks: 14 kcal / 100 g
8 weeks: 11 kcal / 100 g
12 weeks and older: 9.5 kcal / 100 g

Accurate provisioning of these energy levels ensures that rats complete their growth trajectory within the expected timeframe and achieve optimal adult body composition.

Micronutrients

Micronutrients are essential for the physiological development of rats from birth through the cessation of skeletal growth, which typically occurs between 10 and 12 weeks of age in laboratory strains. Adequate intake of vitamins and trace elements supports cellular proliferation, bone mineralization, and metabolic pathways that drive weight gain and organ maturation during this period.

Key micronutrients influencing growth include:

Vitamin D: facilitates calcium absorption, enhances bone density, and regulates endocrine signals that modulate growth plate activity.
Vitamin A: required for cell differentiation, immune competence, and visual development; deficiency can delay somatic growth.
Zinc: cofactor for DNA synthesis, protein translation, and hormone production; low levels correlate with reduced body mass and delayed puberty onset.
Iron: critical for hemoglobin synthesis and oxygen transport; insufficient iron impairs muscle development and aerobic capacity.
Selenium: contributes to antioxidant defenses, protecting rapidly dividing tissues from oxidative damage.

Optimal concentrations of these nutrients must be maintained in rodent chow throughout the pre‑pubertal phase. Adjustments in diet formulations are often based on age‑specific requirements: neonates receive higher relative amounts of vitamin A and zinc, while older juveniles benefit from increased calcium and vitamin D to support the final phase of epiphyseal closure. Monitoring serum markers and growth curves enables precise supplementation, ensuring that rats achieve maximal adult size within the expected growth window.

Environment

Stress Levels

Rats achieve most of their somatic growth within the first six weeks after birth. By eight weeks, skeletal length and body mass plateau, and further increases are limited to modest tissue remodeling.

Elevated stress hormones interfere with this developmental trajectory. Cortisol spikes suppress appetite, decrease nutrient absorption, and redirect energy toward gluconeogenesis rather than tissue accretion. Consequently, rats exposed to chronic stress often weigh less than unstressed counterparts of the same chronological age.

Stress also influences skeletal maturation. Persistent activation of the hypothalamic‑pituitary‑adrenal axis delays epiphyseal plate closure, extending the period of longitudinal bone growth but producing weaker bone architecture. In laboratory settings, stressed juveniles display:

Reduced daily weight gain (average 10–15 % lower)
Delayed fur coloration and whisker development
Lower bone mineral density measured by DXA

Acute stress events produce transient hormonal surges that may temporarily slow growth without lasting effects, provided recovery occurs before the fifth week. Chronic stress persisting beyond this window can permanently alter the growth curve, resulting in adult rats that remain smaller than genetically typical individuals.

Understanding the interaction between developmental timing and stress levels is essential for interpreting experimental outcomes that involve rat models of nutrition, endocrinology, or neurobehavioral research.

Social Interaction

Rats reach adult size by roughly one year of age, with the most rapid increase occurring during the first six months. After weaning, juveniles form stable groups in which dominance hierarchies develop quickly.

Play fighting, allogrooming, and nest sharing dominate interactions among post‑weaning pups. These behaviors stimulate locomotor activity, enhance muscle development, and promote efficient nutrient absorption, thereby accelerating somatic growth.

Social rank influences access to food and shelter. Subordinate individuals often experience slower weight gain and delayed skeletal maturation compared to dominant peers, especially when group density exceeds optimal levels.

In laboratory settings, housing rats in mixed‑sex pairs or small groups that allow regular contact reduces stress‑induced growth retardation. Providing enrichment objects encourages natural social play, supporting normal growth trajectories up to the species‑specific growth ceiling.

Health and Disease

Parasitic Infections

Rats attain skeletal maturity between 10 and 12 weeks of age; body mass plateaus shortly thereafter, with most individuals reaching maximal size by eight months under optimal conditions. Growth curves flatten as growth hormone levels decline and metabolic rate stabilizes.

Parasitic infections interfere with this trajectory. Energy diverted to immune responses reduces feed conversion efficiency, delaying weight gain and extending the period of linear growth. Chronic infestations can suppress appetite, impair nutrient absorption, and trigger inflammatory catabolism, resulting in lower adult body mass and increased mortality before the typical growth plateau.

Common parasites affecting laboratory and wild rats include:

Hymenolepis nana (dwarf tapeworm): induces malabsorption, stunting.
Nippostrongylus brasiliensis: causes pulmonary inflammation, reduces aerobic capacity.
Cysticercus fasciolaris: hepatic cysts impair protein synthesis, slowing somatic growth.
Protozoan Giardia duodenalis: disrupts intestinal brush border, leading to weight loss.

Effective control measures—regular deworming, sanitation, and quarantine—maintain the expected growth timeline. Monitoring parasite load through fecal examinations allows early intervention, preserving the standard age of maximal size and reducing experimental variability.

Chronic Illnesses

Rats reach skeletal maturity between 5 and 7 weeks of age, after which growth slows and body weight stabilizes. Chronic diseases can interrupt this trajectory, extending the period of weight gain or causing premature plateau.

Common chronic conditions that alter rat growth include:

Chronic kidney disease – leads to reduced appetite, muscle wasting, and delayed weight gain.
Diabetes mellitus – causes hyperglycemia, polyphagia, and irregular growth patterns.
Respiratory infections (e.g., chronic bronchitis) – impair oxygen delivery, limiting tissue development.
Cardiovascular insufficiency – reduces perfusion to growth centers, slowing somatic expansion.
Neoplastic disorders – divert metabolic resources toward tumor growth, compromising normal tissue growth.

These illnesses affect hormonal regulation, nutrient absorption, and metabolic rate, resulting in extended growth periods, lower adult body mass, or uneven development. Researchers must account for such variables when interpreting age‑related data, as chronic pathology can mask or exaggerate normal growth curves.

Effective management—regular health monitoring, early diagnosis, and appropriate therapeutic interventions—helps maintain expected growth timelines and ensures experimental reliability.

Growth Patterns Across Rat Species

Domestic Rats «Rattus norvegicus domestica»

Domestic rats (Rattus norvegicus domestica) reach full physical size within a few months after birth. Growth proceeds through distinct stages that are well documented in laboratory and pet populations.

The first three weeks after birth constitute the neonatal period. During this time, pups double their birth weight and develop functional eyes and ears. By the end of week 3 they are weaned and begin to consume solid food independently.

Rapid somatic growth continues until approximately the tenth week. Between weeks 4 and 10, average body weight increases from 30 g to 250–300 g, while total length (head‑to‑tail) extends from 12 cm to 25 cm. Skeletal maturation, including closure of growth plates, largely completes by week 12.

Typical growth milestones for domestic rats are:

Week 1: Birth weight 5–7 g; length 6–7 cm.
Week 3: Weaning weight 30–40 g; length ≈12 cm.
Week 6: Weight 120–150 g; length ≈20 cm.
Week 10: Weight 250–300 g; length ≈25 cm.
Week 12–14: Adult weight 300–500 g; length 25–30 cm; growth plates closed.

Growth rate depends on genetics, sex, nutrition, and housing conditions. Males typically achieve greater final mass than females, while high‑protein diets accelerate weight gain. Overcrowding or chronic stress can suppress growth and delay maturation.

In standard laboratory or pet environments, domestic rats cease measurable growth by the end of the thirteenth week, after which weight fluctuations reflect changes in body composition rather than linear size increase.

Wild Rats «Rattus norvegicus»

Wild Norway rats (Rattus norvegicus) reach their final body size within a few months after birth. Neonates weigh 5–7 g and measure about 5 cm in total length. By the third week they are weaned, attaining 30–45 g and a length of 12–15 cm. Sexual maturity appears at 5–6 weeks, when individuals weigh 80–120 g and are capable of reproduction.

Growth slows markedly after the fourth month. Most wild rats cease linear growth by 12–14 weeks, attaining a body mass of 250–350 g and a head‑body length of 20–25 cm. The skeletal framework is fully ossified at this stage; further weight gain reflects fat deposition rather than additional length.

Factors that limit the ultimate size of wild R. norvegicus include:

Availability of high‑quality food; scarce resources reduce both growth rate and final mass.
Seasonal temperature; colder months often delay maturation and lower adult size.
Population density; high competition for nesting sites and food suppresses growth.
Parasitic load and disease; health challenges divert energy from somatic development.

In natural settings the growth period ends well before the species’ typical lifespan of 12–24 months. After reaching adult dimensions, rats allocate physiological resources to reproduction and survival rather than further somatic expansion.

Black Rats «Rattus rattus»

The black rat (Rattus rattus) is a small rodent native to tropical and subtropical regions. Newborn pups weigh 1–2 g and measure about 2 cm in head‑body length. Their eyes open at 12–14 days, and they are fully weaned by three weeks of age.

Growth proceeds rapidly during the first month. By the end of the fourth week, body length reaches roughly 8–10 cm and weight approaches 30 g. Sexual maturity appears at 8–10 weeks; males may begin breeding at 60 days, females at 50 days. After sexual maturity, growth decelerates and plateaus.

Growth cessation: occurs between 12 and 16 weeks
Adult size: 15–20 cm head‑body length, 150–250 g weight
Maximum lifespan: 2–3 years in the wild, up to 5 years under laboratory conditions

The cessation of linear growth coincides with the attainment of adult body mass. Subsequent life stages involve maintenance of size, reproductive cycles, and gradual senescence.

Indicators of Mature Growth

Weight Milestones

Rats experience rapid weight increase during the first two months of life, after which growth plateaus. Typical weight milestones are:

Birth: 5–7 g. Newborn pups rely on maternal milk and show minimal variance between sexes.
Weaning (≈21 days): 20–30 g. Transition to solid food triggers a sharp rise in daily gain.
Juvenile (≈35 days): 40–55 g. Muscular development accelerates, preparing for sexual maturity.
Early adult (≈8 weeks): 150–200 g. Most laboratory strains reach their adult body mass within this window; males often exceed females by 10–15 %.
Mature adult (≈12 weeks onward): weight stabilizes, with occasional increments up to 250 g in large‑bodied strains.
Senior (≥12 months): slight weight decline or maintenance around 200–250 g, depending on health status and diet.

Growth rate declines sharply after sexual maturity, typically reached between 6 and 8 weeks. Subsequent weight changes reflect metabolic shifts rather than linear growth. Monitoring these milestones assists in experimental design, health assessment, and breeding management.

Length Milestones

Rats exhibit a predictable pattern of body‑length development from birth to adulthood. Neonates measure approximately 6–8 mm from nose to base of the tail. Within the first week, length increases to 15–20 mm as skeletal growth accelerates. By the third week, most individuals reach 40–50 mm, coinciding with the onset of sexual maturity. At five weeks, average length stabilizes around 60–70 mm; further growth becomes minimal. Full adult size, typically 70–90 mm, is attained by eight weeks, after which growth plate closure prevents additional length increase.

Key length milestones:

Birth: 6–8 mm
7 days: 15–20 mm
21 days: 40–50 mm
35 days: 60–70 mm
56 days (adult): 70–90 mm

These measurements apply to common laboratory strains such as Sprague‑Dawley and Wistar; variations may occur with different genetic lines or environmental conditions. Once the adult length is reached, weight continues to rise due to fat deposition rather than skeletal elongation.

Sexual Maturity

Rats reach sexual maturity well before the end of their overall growth period. In laboratory strains, males typically exhibit first signs of puberty between 45 and 55 days of age, while females mature slightly earlier, usually between 35 and 45 days. The onset of reproductive capability coincides with the emergence of spermatogenesis in males and the first estrous cycle in females.

Key physiological markers of sexual maturity include:

Testicular enlargement and the appearance of spermatozoa in the epididymis (males).
Vaginal opening and cyclic changes in vaginal cytology (females).
Surge in circulating gonadotropins and sex steroids, notably luteinizing hormone, follicle‑stimulating hormone, testosterone, and estradiol.

Sexual maturation does not halt somatic growth. Rats continue to gain body mass and increase skeletal length until approximately 90 days for most strains, with some larger breeds extending growth to 120 days. Consequently, sexual maturity represents a developmental milestone within a broader growth trajectory rather than the final stage of physical development.

Environmental variables such as nutrition, photoperiod, and housing density can shift the timing of puberty by several days. Adequate protein intake accelerates gonadal development, whereas caloric restriction delays it. Researchers must account for these factors when designing experiments that involve reproductive status or age‑dependent physiology.

Bone Development

Rats achieve skeletal maturity through a sequence of ossification events that culminate in the closure of growth plates. The distal femur, tibia, and vertebral bodies typically fuse between the fourth and sixth week of life, marking the end of longitudinal bone growth. After epiphyseal closure, bone mass continues to increase through remodeling, but length remains constant.

Key milestones in rat bone development:

Post‑natal day 1–7: Primary ossification centers form in long bones; cartilage matrix begins mineralization.
Day 14–21: Secondary ossification centers appear; growth plates are active, allowing rapid lengthening.
Week 4–6: Growth plates in major long bones close; longitudinal growth ceases.
Week 8–12: Peak bone mass is reached; cortical thickness and trabecular density stabilize.

Factors influencing the timing of growth‑plate closure include strain genetics, sex hormones, and nutritional status. For example, male Sprague‑Dawley rats typically complete epiphyseal fusion by the end of week five, whereas females may do so one week earlier. Caloric restriction delays plate closure, extending the period of length increase, while excess calcium accelerates cortical thickening without affecting the closure date.

Understanding the precise age at which rat bones stop elongating is essential for experimental designs that rely on skeletal maturity, such as pharmacological studies of bone density or biomechanical testing. Researchers should schedule interventions after the sixth post‑natal week to ensure that longitudinal growth will not confound results.

Distinguishing Growth from Weight Gain

Healthy Growth vs. Obesity

Rats reach skeletal maturity between eight and ten weeks of age, after which linear growth ceases and body weight stabilizes under normal dietary conditions. During this period, nutrient intake, metabolic rate, and activity level determine whether growth proceeds healthily or shifts toward excess adiposity.

Healthy growth is characterized by steady weight gain that mirrors the increase in lean tissue and bone mass. Energy consumption aligns with basal metabolic demands plus the requirements of spontaneous locomotion. Hormonal regulation—particularly growth hormone, insulin‑like growth factor‑1, and leptin—maintains a balance between anabolic and catabolic processes, ensuring efficient nutrient utilization.

Obesity develops when caloric intake consistently exceeds expenditure after the growth plateau. Excess calories are stored primarily as visceral fat, disrupting insulin signaling and elevating leptin resistance. The resulting metabolic shift accelerates the onset of comorbidities such as hepatic steatosis, hypertension, and reduced fertility. Key indicators of the transition from healthy growth to obesity include:

Persistent weight gain beyond the expected plateau range
Increased adipose tissue proportion measured by body composition analysis
Elevated fasting glucose and insulin levels
Reduced physical activity observed in open‑field tests

Monitoring these parameters from the end of the growth phase through adulthood enables early detection of abnormal weight trajectories and informs dietary or environmental interventions to restore metabolic equilibrium.

Muscle Development

Rats exhibit rapid muscle growth during the early post‑natal period, aligning with overall body length increase that ceases around two months of age. Muscle fiber number is largely determined within the first three weeks; subsequent growth relies on hypertrophy of existing fibers.

By post‑natal day 7, myoblast proliferation peaks, establishing the primary myotube framework.
Between days 14 and 21, satellite cell activity intensifies, supplying nuclei for fiber enlargement.
From weeks 3 to 8, muscle cross‑sectional area expands approximately threefold, driven by protein synthesis and mitochondrial biogenesis.
After week 10, fiber size stabilizes; further mass gain is minimal unless stimulated by exercise or hormonal manipulation.

The transition from hyperplastic to hypertrophic growth coincides with the closure of the growth plates in long bones, marking the end of linear growth. Consequently, maximal muscle mass in laboratory rats is typically reached between 10 and 12 weeks of age, after which maintenance rather than expansion dominates.

Fat Accumulation

Rats achieve skeletal maturity around five to six weeks of age, and their overall body mass stabilizes by ten to twelve weeks. After this period, growth in length ceases, but weight can continue to increase due to adipose tissue expansion.

Fat accumulation in adult rats follows two primary processes.

Adipocyte hypertrophy: existing fat cells enlarge as they store additional triglycerides.
Adipocyte hyperplasia: new fat cells develop, increasing the total cell count.

Both processes are driven by hormonal shifts, including reduced growth hormone and altered leptin signaling, as well as by dietary excess, particularly high‑fat or high‑carbohydrate feeds.

When growth stops, basal metabolic rate declines modestly, while caloric intake often remains unchanged. The resulting energy surplus promotes visceral fat deposition, especially around the epididymal, retroperitoneal, and mesenteric depots. Older rats (beyond three months) commonly display a higher proportion of body fat relative to lean mass, even when body weight appears stable.

For experimental designs that involve obesity, insulin resistance, or metabolic studies, selecting rats at or just beyond the plateau phase (approximately 12 weeks) minimizes confounding effects of ongoing somatic growth and isolates age‑related adiposity as the variable of interest.