Rat Pregnancy Duration

Understanding Rat Gestation

The Basics of Rodent Reproduction

Reproductive Cycle Overview

The reproductive cycle of rats comprises estrus, mating, fertilization, embryonic development, and parturition. Estrus lasts 4–5 days, with females entering receptivity for 12–14 hours each cycle. Successful copulation typically occurs during the first night of estrus, after which sperm travel through the oviducts to meet the ovum.

Gestation in rats averages 21–23 days, with embryogenesis progressing rapidly. By day 5, implantation is complete; organogenesis proceeds from days 6–14, and fetal growth accelerates during the final week. Near term, the uterus contracts rhythmically, preparing for delivery.

Key physiological markers include:

Rising progesterone levels throughout gestation, peaking at day 15.
Increased prolactin secretion to support mammary development.
Elevated estradiol during the late stage, triggering parturition.

Post‑partum, the lactation phase lasts 3–4 weeks, during which the female’s estrous cycle is suppressed until weaning concludes. Understanding each stage provides essential context for studies involving rat reproductive timing and experimental scheduling.

Factors Influencing Conception

Successful breeding in laboratory rats directly impacts the length of their gestation period, making the identification of variables that affect conception essential for reliable experimental timelines.

Key variables influencing conception include:

Genetic background – strains differ in fertility rates and litter sizes, altering the probability of successful mating.
Age of the female – sexual maturity peaks between 8 and 12 weeks; fertility declines markedly after 6 months.
Body condition and nutrition – adequate protein and caloric intake sustain ovarian function, while deficiencies lower ovulation frequency.
Health status – presence of pathogens, parasites, or chronic disease suppresses estrous cycles.
Photoperiod and lighting – exposure to 12–14 hours of light per day stabilizes estrous regularity; irregular lighting can delay or suppress cycles.
Stress levels – handling, cage crowding, and environmental noise elevate corticosterone, which interferes with hormone secretion.
Hormonal balance – endogenous levels of estradiol, progesterone, and luteinizing hormone must reach specific thresholds for ovulation and implantation.
Breeding protocol – timing of male introduction relative to the female’s estrus, and the male-to-female ratio, affect mating success.
Housing conditions – temperature (20–24 °C) and humidity (40–60 %) support optimal reproductive physiology; extremes reduce conception rates.

Controlling these factors standardizes the onset of pregnancy, thereby producing consistent gestation lengths across experimental cohorts.

Stages of Rat Pregnancy

Early Gestation

Initial Signs and Hormonal Changes

Early detection of pregnancy in laboratory rats occurs within the first five days after mating, aligning with the onset of endocrine activity that sustains gestation. The initial phase is marked by subtle physiological adjustments that precede visible abdominal enlargement.

Behavioral changes: increased nesting activity, reduced aggression toward cage mates, heightened grooming of the genital area.
Physical signs: slight weight gain (approximately 2–3 g), softened vaginal opening, occasional vaginal discharge containing spermatozoa.
Reproductive tract: enlargement of the uterus and ovaries detectable by palpation or ultrasonography after day 4.

Hormonal profiles shift dramatically during this period. Progesterone rises sharply, reaching peak concentrations by day 6 and remaining elevated throughout gestation to maintain uterine quiescence. Estradiol levels exhibit a biphasic pattern, with an early rise supporting implantation followed by a gradual increase that peaks near parturition. Prolactin secretion escalates concurrently, promoting mammary gland development and preparing for lactation. Luteinizing hormone declines after the luteal phase, reflecting the transition from ovulation to gestational maintenance.

Monitoring these early indicators enables precise timing of experimental interventions, reduces variability in developmental studies, and supports accurate assessment of reproductive health in rodent models.

Embryonic Development

The gestational period of laboratory rats averages 21–23 days, and embryonic development occupies the initial half of this interval.

Fertilization occurs within minutes of coitus; the resulting zygote undergoes rapid cleavage, reaching the morula stage by day 2. By day 3 the blastocyst forms and initiates implantation into the uterine epithelium.

Between days 5 and 10 the embryo establishes the three primary germ layers through gastrulation. The primitive streak appears, the notochord is laid down, and a nascent heart tube begins rhythmic contractions.

Days 11 through 15 witness the emergence of limb buds, closure of the neural tube, and differentiation of major organ systems. The brain vesicles expand, the retina develops, and the liver assumes hematopoietic function.

From day 16 to parturition the fetus experiences exponential growth. Skeletal elements ossify, pulmonary alveoli mature, and the gastrointestinal tract prepares for milk ingestion.

Key embryonic milestones:

Day 3 – blastocyst implantation
Day 5 – gastrulation onset
Day 7 – heart tube beating
Day 12 – limb bud emergence
Day 15 – neural tube closure
Day 19 – lung surfactant production

These stages define the embryonic trajectory that precedes the final fetal phase and culminates in birth at the end of the typical rat gestation.

Mid-Gestation

Fetal Growth and Organogenesis

Fetal development in laboratory rats follows a tightly regulated timeline that spans approximately 21‑23 days from conception to birth. Early embryogenesis occupies the first five days, during which the blastocyst implants into the uterine wall and the inner cell mass differentiates into the three germ layers.

Organogenesis commences around day 12 and proceeds rapidly, establishing the functional architecture of all major systems. By day 14, the neural tube has closed, the heart begins rhythmic contractions, and the primitive circulatory network expands. Lung buds emerge, and the forebrain shows distinct regionalization. By day 16, the kidneys develop nephrons, the liver attains metabolic activity, and the gastrointestinal tract forms recognizable segments. Completion of skeletal ossification and the appearance of hair follicles occur near day 20, preparing the fetus for extra‑uterine life.

Key developmental milestones:

Day 12‑13: Neural tube closure; initial cardiac activity.
Day 14‑15: Formation of primary lung buds; forebrain segmentation.
Day 16‑17: Nephrogenesis; hepatic enzyme expression.
Day 18‑19: Limb ossification; onset of whisker (vibrissa) development.
Day 20‑21: Hair follicle emergence; maturation of gastrointestinal motility.

The precise timing of these events provides a reliable framework for experimental manipulation and comparative studies. Variations in gestational length, influenced by strain, maternal age, and environmental factors, can shift the onset of organogenesis by ±1–2 days, underscoring the need for accurate staging in reproductive research.

Maternal Physiological Adaptations

Maternal physiological adaptations are essential for supporting the gestational period in rats, which typically spans 21–23 days. These adaptations enable the dam to meet the increasing demands of the developing embryos and to prepare for parturition and lactation.

Cardiovascular adjustments include a 30–40 % rise in cardiac output, driven by elevated heart rate and stroke volume. Peripheral vasodilation reduces systemic vascular resistance, facilitating increased uterine blood flow. Plasma volume expands by approximately 20 %, enhancing tissue perfusion and nutrient delivery.

Respiratory changes involve a 15–20 % increase in tidal volume and minute ventilation, improving oxygen uptake to match fetal requirements. Arterial blood gas analysis during late gestation shows a modest rise in arterial oxygen tension and a slight decrease in carbon dioxide tension, reflecting enhanced gas exchange efficiency.

Endocrine modulation is characterized by progressive elevation of progesterone and estrogen concentrations, which sustain uterine quiescence and stimulate mammary gland development. Prolactin levels increase markedly in the final days of gestation, priming the secretory apparatus for milk production.

Metabolic shifts encompass heightened insulin sensitivity early in pregnancy, followed by a controlled insulin resistance in the later stage to prioritize glucose availability for the fetuses. Lipid mobilization intensifies, providing additional energy substrates for both maternal and fetal tissues.

Uterine remodeling includes hypertrophy of smooth muscle layers and proliferation of endometrial glands, resulting in a 2–3‑fold increase in uterine mass. These structural changes accommodate fetal growth and facilitate efficient nutrient transfer.

Mammary gland adaptation features ductal branching and alveolar development, driven by estrogen, progesterone, and prolactin. By the end of gestation, the glandular tissue is prepared for immediate milk secretion upon delivery.

Collectively, these coordinated physiological responses ensure that the dam can sustain embryonic development throughout the relatively brief gestation of rats and transition smoothly to the lactational phase.

Late Gestation

Rapid Fetal Development

The gestation period of laboratory rats spans approximately 21–23 days, during which fetal growth proceeds at a markedly accelerated pace compared to larger mammals. Embryonic organogenesis completes within the first 10 days, establishing the fundamental body plan, while subsequent days focus on tissue differentiation and functional maturation.

Key characteristics of this rapid development include:

Early neural tube closure by day 9, enabling swift central nervous system formation.
Lung maturation reaching the saccular stage by day 15, preparing the neonate for immediate post‑natal respiration.
Muscle fiber formation intensifying between days 12 and 18, resulting in functional locomotor ability at birth.
Skeletal ossification initiating around day 14, providing structural support for the newborn.

Hormonal regulation drives this speed. Elevated progesterone levels maintain uterine quiescence, while a surge of placental estrogen around mid‑gestation stimulates fetal growth factors such as IGF‑1. Placental efficiency ensures continuous nutrient transfer, matching the high metabolic demands of the developing fetus.

By the final trimester, fetal weight increases from roughly 0.1 g at implantation to 5–6 g at birth, reflecting a growth rate exceeding 50 mg per day. This exponential increase underscores the necessity for precise timing of maternal nutrition and environmental conditions to avoid developmental abnormalities.

Overall, the condensed gestational timeline in rats demands tightly coordinated cellular proliferation, differentiation, and organ readiness, making the species an optimal model for studying accelerated fetal development mechanisms.

Preparation for Parturition

The gestation period for laboratory rats averages 21–23 days, culminating in a rapid transition from fetal development to parturition. Effective preparation reduces neonatal mortality and supports maternal health during this critical phase.

Key actions for breeders and researchers include:

Environmental control: Maintain temperature between 20–26 °C and humidity at 45–55 %. Provide a quiet, low‑light area to minimize stress.
Nesting material: Supply 2–3 cm of shredded paper, cotton, or aspen shavings at least 24 hours before the expected delivery date. Adequate material encourages nest building and promotes proper positioning of pups.
Nutrition: Increase dietary protein to 20 % and ensure constant access to fresh water. Supplement with calcium and vitamin D to meet the heightened metabolic demand of late gestation.
Health monitoring: Conduct daily visual checks for signs of labor, such as abdominal swelling reduction, increased grooming, and vocalizations. Record any deviations from normal progression for immediate veterinary assessment.
Sanitation: Clean cages 48 hours prior to parturition, removing waste while preserving nesting material. Disinfect surfaces with a mild, non‑toxic solution to limit pathogen exposure.

Implementing these measures aligns husbandry practices with the physiological timeline of rat gestation, facilitating a smooth transition to the postpartum period and optimizing outcomes for both dam and offspring.

Variances and Influencing Factors

Genetic Predisposition

Breed-Specific Differences

Gestation length varies noticeably among rat strains, influencing experimental timing and breeding management.

In Sprague‑Dawley rats, the average gestation period ranges from 21 to 23 days, with most litters delivered on day 22. Wistar rats exhibit a similar span, typically 21–22 days, but occasional early deliveries occur as early as day 20. Long‑Evans rats tend toward the upper end of the range, often reaching day 23, and some individuals extend to day 24.

Fancy or pet breeds display broader variability. For example, the Dutch dwarf rat commonly delivers between 20 and 22 days, while the larger Himalayan breed may extend to 24 days. These differences reflect genetic influences on uterine contractility, fetal growth rates, and maternal hormonal cycles.

Key comparative points:

Average gestation: Sprague‑Dawley ≈ 22 days; Wistar ≈ 21.5 days; Long‑Evans ≈ 23 days.
Maximum reported: Long‑Evans and Himalayan breeds up to 24 days.
Minimum reported: Dutch dwarf and some Sprague‑Dawley individuals as early as 20 days.
Standard deviation: Laboratory strains typically ±0.5 day; pet breeds may show ±1 day or more.

Understanding these breed‑specific timelines is essential for precise scheduling of prenatal interventions, litter monitoring, and developmental studies. Adjusting protocols to accommodate the documented range reduces the risk of premature or delayed assessments.

Individual Genetic Factors

Genetic variation significantly influences the length of gestation in laboratory rats. Studies using inbred strains reveal heritability estimates between 0.3 and 0.5, indicating that roughly one‑third to one‑half of the observed variance is attributable to inherited factors.

Key loci identified through quantitative trait mapping include:

PrlR (prolactin receptor) – allelic differences correlate with earlier parturition.
Esr1 (estrogen receptor α) – polymorphisms affect uterine receptivity and timing of fetal maturation.
Ghr (growth hormone receptor) – variants modulate fetal growth rate, indirectly altering gestational span.
Pit1 (pituitary-specific transcription factor) – impacts hormonal cascades that regulate implantation and placental development.

Genome‑wide association studies have pinpointed additional single‑nucleotide polymorphisms in genes governing circadian rhythms, metabolic pathways, and placental angiogenesis. These variants interact epistatically, producing strain‑specific gestational profiles.

Experimental manipulation of candidate genes via CRISPR‑Cas9 editing demonstrates causality: knock‑out of PrlR extends gestation by 1–2 days, whereas overexpression shortens it by a comparable margin. Such interventions confirm that individual genetic makeup directly shapes reproductive timing.

Understanding these genetic determinants refines experimental design, improves predictive modeling of developmental outcomes, and informs selective breeding programs aimed at standardizing gestational intervals for reproducible research.

Environmental Impacts

Nutritional Considerations

Rats complete gestation in approximately three weeks, and maternal nutrition directly influences embryonic development, litter size, and neonatal viability.

Key nutritional components include:

Protein: 18‑20 % of diet dry matter; high‑quality sources such as casein or soy support fetal tissue synthesis.
Energy: 3000–3400 kcal kg⁻¹; balanced carbohydrate and fat levels prevent maternal weight loss or excessive gain.
Calcium and phosphorus: Ratio of 1.2–1.5 : 1; essential for skeletal mineralization of pups.
Vitamin A: 4 000–5 000 IU kg⁻¹; facilitates organogenesis, but excess leads to teratogenic effects.
Vitamin D3: 1 200 IU kg⁻¹; aids calcium absorption.
B‑complex vitamins: Particularly folic acid (2 mg kg⁻¹) to reduce neural tube defects.

Feeding protocol should provide ad libitum access to fresh, pelleted diet formulated for breeding females. Supplementation with soft, moist food during the final days of gestation encourages adequate intake when abdominal volume limits consumption. Continuous access to clean water is mandatory; dehydration compromises milk production postpartum.

Common errors involve reducing protein or caloric density in an attempt to limit litter size; such practices increase embryonic mortality and result in underweight neonates. Maintaining the specified nutrient levels throughout the entire gestation period ensures optimal reproductive outcomes and supports the health of both dam and offspring.

Stress and Housing Conditions

Stressful environments shorten the gestation period in laboratory rats. Elevated corticosterone levels, triggered by chronic noise, predator cues, or frequent handling, suppress luteal progesterone production, leading to earlier parturition. Studies report a reduction of 1–2 days in total gestation when rats experience unpredictable stressors throughout pregnancy.

Housing density and bedding quality also modulate gestational length. Overcrowding (more than three females per 200 cm²) increases aggression, raises maternal cortisol, and accelerates delivery. Conversely, spacious cages with nesting material provide thermal comfort and allow normal maternal behavior, maintaining the typical gestation span of 21–23 days. Adequate ventilation prevents ammonia buildup, which otherwise irritates respiratory pathways and contributes to hormonal imbalance.

Practical recommendations for reproducible gestation outcomes include:

Maintaining a maximum of two females per standard cage.
Providing at least 5 cm of clean, absorbent bedding.
Limiting external disturbances to scheduled, brief handling sessions.
Monitoring ambient temperature (20–22 °C) and humidity (45–55 %).

Age and Parity

Impact of Maternal Age

Maternal age exerts a measurable effect on the length of gestation in laboratory rats. Younger females (approximately 8–12 weeks old) typically exhibit gestational periods averaging 21.5 days, whereas older breeders (≥ 10 months) show a modest increase, with averages ranging from 22.0 to 22.5 days. The extension correlates with reduced uterine contractility and delayed implantation timing observed in histological examinations.

Key observations from controlled breeding experiments include:

A linear trend of gestational lengthening of 0.1–0.2 days per additional month of maternal age after sexual maturity.
Increased variability in delivery timing among aged females, reflected in standard deviations up to 0.8 days versus 0.3 days in younger cohorts.
Higher incidence of post‑implantation embryonic loss in older dams, contributing to the observed shift in average gestation.

These patterns suggest that age‑related physiological changes—such as altered hormone profiles, decreased myometrial responsiveness, and cumulative reproductive wear—directly influence the timing of parturition in rats. Researchers planning timed‑mating protocols should adjust expected delivery dates according to the breeder’s age to maintain experimental precision.

Effects of Previous Pregnancies

The typical gestation period for laboratory rats averages 21‑23 days. Research indicates that a female’s reproductive history modifies several parameters of subsequent pregnancies.

Repeated breeding cycles tend to shorten gestation by 0.5‑1 day in later litters. This acceleration correlates with increased progesterone production and enhanced uterine receptivity. Conversely, extended intervals between pregnancies restore gestation length to baseline values.

Previous parturitions also influence litter characteristics:

Litter size generally rises after the first two pregnancies, stabilizing at a species‑specific maximum.
Neonatal weight improves with maternal experience, reflecting more efficient nutrient transfer.
Maternal mortality declines after the initial gestation, provided that breeding intervals exceed 4 weeks.

Hormonal profiling shows that multiparous females maintain higher circulating estradiol during early gestation, which supports faster embryonic implantation and reduced gestational duration. Uterine remodeling, measured by increased vascular density, persists across successive pregnancies, contributing to the observed physiological adjustments.

Overall, a rat’s reproductive record exerts measurable effects on gestational timing, offspring viability, and maternal health, underscoring the importance of parity considerations in experimental design and colony management.

Managing Pregnant Rats

Nutritional Requirements

Diet Adjustments

Dietary management during the gestational phase in laboratory rats directly influences fetal development and litter outcomes. Energy requirements increase as the gestation progresses, demanding adjustments to macronutrient composition and feeding frequency.

Key adjustments include:

Increase caloric density by 10‑15 % after day 7 of gestation, using a balanced mix of carbohydrates, fats, and proteins.
Ensure protein content remains at 18‑20 % of total diet, with high‑quality sources such as casein or soy isolate.
Supplement with essential fatty acids (omega‑3 and omega‑6) at 2‑3 % of total fat to support membrane formation.
Provide vitamin E (≥ 50 IU/kg) and vitamin C (≥ 250 mg/kg) to mitigate oxidative stress.
Offer mineral enrichment, specifically calcium (1.2 % of diet) and phosphorus (0.8 %) with a Ca:P ratio near 1.5:1.
Divide daily rations into two feedings after day 14 to reduce gastrointestinal overload and maintain stable glucose levels.

Monitoring body weight weekly and adjusting feed quantities accordingly prevents excessive maternal weight gain, which can compromise litter size and health. Maintaining consistent water availability, preferably with electrolytes during the final trimester, supports hydration and nutrient transport.

Supplementation

Supplementation can modify the gestational timeline in laboratory rats by influencing maternal metabolism, fetal development, and uterine environment. Studies that administer nutrients before mating or during early gestation report measurable shifts in the average length of pregnancy, typically ranging from 21 to 23 days in standard strains.

Key supplements evaluated in rodent reproductive research include:

Folic acid – 2–5 mg/kg diet; associated with a modest reduction of gestation by 0.2–0.4 days and decreased incidence of neural tube defects.
Calcium carbonate – 1 % of feed; linked to stabilization of uterine contractility, resulting in gestational periods close to the species norm.
Omega‑3 fatty acids (EPA/DHA) – 0.5 % of diet; correlated with a slight extension of pregnancy by up to 0.3 days and improved placental vascularization.
Protein enrichment (casein) – 20–25 % of calories; produces consistent gestational lengths, with high‑protein diets preventing premature parturition observed in low‑protein controls.

Dosage timing influences outcomes. Pre‑conception supplementation establishes baseline nutrient stores, affecting embryo implantation speed and early placental growth. Initiation during days 5–7 of gestation targets the period of rapid organogenesis, where micronutrient availability directly impacts fetal growth trajectories and may alter the timing of parturition signals.

Experimental design must control for strain variability, litter size, and environmental factors such as temperature and lighting. Accurate recording of mating dates, daily weight gain, and parturition onset ensures that observed changes in gestation length can be attributed to the specific supplement regimen rather than confounding variables.

Housing and Environment

Nesting Material Provisions

Nesting material directly influences the environment a pregnant rat experiences during the roughly three‑week gestation period. Adequate provision supports maternal comfort, reduces stress, and promotes successful parturition.

Soft, absorbent substrates such as shredded paper, tissue, or cotton pads
Structured materials like shredded cardboard or wood shavings for nest building
Non‑toxic, low‑dust options to maintain respiratory health

Guidelines for provision:

Offer at least 50 g of material per cage beginning on day 10 of gestation.
Replace soiled material daily after parturition to prevent contamination.
Position material in a secluded corner of the cage, away from food and water sources.

Studies demonstrate that dams with sufficient nesting supplies produce larger litters, exhibit fewer abnormal behaviors, and experience lower corticosterone levels during the late gestational phase. Conversely, scarcity of material correlates with increased nest‑building attempts outside the cage and higher pup mortality.

Optimal practice combines soft and structural components, monitors consumption, and adjusts quantity as the pregnancy advances. Consistent application of these measures aligns with established reproductive protocols for laboratory rats.

Temperature and Humidity Control

Temperature and humidity are primary environmental factors influencing the gestation period of rats. Precise regulation minimizes developmental delays and reduces the incidence of embryonic loss.

Optimal conditions are defined by narrow intervals:

Ambient temperature: 20 °C – 22 °C (68 °F – 72 °F)
Relative humidity: 45 % – 55 %

Deviations beyond these limits increase stress hormones, alter placental blood flow, and can extend gestational length by up to 24 hours.

Continuous monitoring requires calibrated sensors linked to automated climate-control systems. Data logging at 15‑minute intervals enables rapid correction of excursions. Calibration checks should occur weekly to maintain sensor accuracy within ±0.5 °C and ±2 % humidity.

Implementing these controls in breeding facilities standardizes reproductive outcomes, supports reproducible research, and aligns with animal‑welfare guidelines.

Veterinary Care and Monitoring

Recognizing Complications

During the gestational phase of laboratory rats, early detection of abnormal conditions can prevent loss of litters and protect animal welfare. Recognizing complications requires systematic observation and timely intervention.

Physical signs that may indicate trouble include:

Sudden weight loss or failure to gain expected mass after the second week of gestation.
Abdominal distension without proportional fetal growth, suggesting fluid accumulation or uterine torsion.
Vaginal discharge that is bloody, purulent, or malodorous, pointing to infection or placental separation.
Lethargy, reduced food intake, or abnormal grooming behavior, reflecting systemic distress.
Elevated body temperature (>39 °C) measured rectally, indicative of fever or inflammatory response.

Diagnostic measures to confirm suspicion:

Palpation under mild anesthesia to assess fetal viability and uterine tone.
Ultrasonography to visualize embryo development, detect resorptions, or identify abnormal fluid collections.
Hematology and serum biochemistry panels to reveal anemia, leukocytosis, or electrolyte imbalances.
Necropsy of non‑viable pregnancies for definitive pathology when other methods fail.

Management strategies depend on the identified issue:

Antibiotic therapy for confirmed bacterial infection, selected based on culture sensitivity.
Surgical correction for uterine torsion or severe abdominal adhesions, performed by experienced personnel.
Supportive care, including fluid therapy and analgesics, for systemic illness.
Early termination of pregnancy may be necessary when fetal viability cannot be restored, following institutional ethical guidelines.

Routine monitoring protocols—daily weight checks, bi‑daily visual inspections, and weekly ultrasonographic assessments—substantially reduce the incidence of unnoticed complications throughout the rat gestation period.

Post-Partum Care

After delivering a litter, a female rat enters a recovery phase lasting approximately one to two weeks. During this interval, her physiological demands increase, and the survival of newborns depends on precise maternal support.

The mother requires a stable nest environment. Provide a quiet cage, maintain ambient temperature between 22 °C and 26 °C, and ensure the nesting material remains dry and undisturbed. Observe the dam for signs of aggression toward pups; intervene only if severe harm occurs.

Nutritional support must match the heightened metabolic load. Offer high‑energy pelleted food, fresh water, and a supplemental source of protein such as boiled egg or soft cheese. Replace food and water daily to prevent contamination.

Health monitoring includes daily checks for:

Abnormal discharge or odor from the vulva
Lethargy, tremors, or loss of appetite in the dam
Pup weight gain of less than 1 g per day
Visible signs of infection in the nest

If any indicator appears, isolate the affected individual, consult a veterinarian, and consider antibiotic therapy based on culture results.

Weaning typically begins at 21 days of age. Gradually introduce solid food to the pups while reducing maternal contact. Complete separation occurs by 28 days, after which the dam can be returned to standard housing conditions.