Experimental Animal Numbering, Marking, Grouping, and Depilation: Standardized Procedures and Practical Considerations
Experimental Animal Numbering, Marking, Grouping, and Depilation: Standardized Procedures and Practical Considerations
Individual identification, grouping, and depilation are core elements of quality control in animal experiments. Standardized numbering and marking ensure full-process traceability and reduce operational errors; appropriate grouping strategies improve between-group comparability and statistical efficiency; and suitable depilation protocols support surgical exposure, sampling, and imaging while minimizing skin irritation and potential interference with experimental endpoints. This article summarizes commonly used numbering/marking methods, mouse grouping strategies, and key depilation practices, and provides combined recommendations for typical application scenarios together with relevant product catalog information.
Keywords: laboratory animals; individual identification; numbering; marking; mouse grouping; randomization; matching; depilation; animal marking dye; depilatory agent
I. General Principles: Quality Control for Numbering, Marking, and Depilation
1.1 Traceability and Error Prevention
(1) Individual identification should be unique and stable throughout the entire study lifecycle, ensuring consistency among cage card information, individual marks, procedure records, sample labels, and data file naming.
(2) A structured numbering convention (e.g., batch-group-sequence) should be established, finalized before study initiation, and supported by formalization of the numbering plan and personnel training.
(3) At critical checkpoints, two-person verification or dual-channel cross-checking (three-way consistency among cage card, individual marking, and record sheet) should be implemented to keep mismatch risk within an acceptable range.
1.2 Animal Welfare and Compatibility with Experimental Endpoints
(1) The selection of marking and depilation methods should be based on an integrated assessment of pain and stress, risks of skin/tissue injury, infection risk, and potential impacts on primary endpoints (e.g., inflammation, immunity, metabolism, behavior, imaging).
(2) Marking approaches involving tissue injury or invasive manipulation should be performed under institutional SOPs and within the ethical approval framework, ensuring operator qualification and procedural consistency.
(3) Depilation should follow a minimum-irritation principle to avoid introducing additional variables that could affect endpoint interpretation.
II. Numbering and Marking Methods for Experimental Animals
2.1 Long-Term or Permanent Identification Methods
(1) Tagging Method
Numbers are embossed on metal tags, or encoded by group and fixed onto collars or straps worn around the neck; this method is suitable for large animals such as dogs.
[Advantages] Good readability and efficient long-distance identification; convenient maintenance.
[Limitations] Risks of friction injury, entanglement, and detachment; not suitable for rodents.
[Key points]
① Collar tightness should allow normal swallowing and locomotion, while preventing chronic friction injury.
② Wearing status should be checked regularly and maintenance should be documented.
③ The numbering system should be consistent with cage cards and source records, avoiding parallel numbering schemes.
(2) Number Punching (Ear Tag Pliers/Numbering Needle)
A numbering needle is used to imprint numbers on the pinna; a developing solution may be used when necessary to improve readability. This method is applicable to animals with relatively large auricles (e.g., rabbits, dogs).
[Advantages] Clear numbering with relatively long persistence; convenient for routine inspection.
[Limitations] Requires standardized practice to control bleeding and infection risk; readability is limited in animals with small pinnae.
[Key points]
① Clean the pinna before marking and select an area that avoids prominent blood vessels.
② Observe bleeding and local inflammation after the procedure, and record the number and location.
③ Within a given study, the marking location and coding rules should be standardized.
(3) Ear Punching or Notching
Holes are punched or notches are cut at fixed positions on the ear to represent codes; this is commonly used for large-scale breeding management and lifelong identification.
[Advantages] Low cost, high stability, suitable for high-throughput management.
[Limitations] A tissue-injury marking method; healing of notches or positional deviation may affect interpretation.
[Key points]
① A position/code reference chart for holes/notches should be established before study initiation, with unified training.
② Positioning templates should be used to ensure inter-operator consistency.
③ Post-procedure healing and infection should be monitored; local care should be provided when needed and records retained.
(4) Tattooing and Electronic Microchips (Supplement)
Tattooing (e.g., tail or footpad) and electronic microchips (RFID/UID) are used for long-term tracking and high-reliability identity assignment.
[Advantages] Tattoos have relatively strong readability; microchips provide unique identification and support automated reading.
[Limitations] Require equipment and training; microchips are relatively costly and require evaluation of implantation effects on local tissue and endpoints.
[Key points]
① Compatibility with imaging-, inflammation-, and other endpoint assessments should be evaluated during protocol design.
② Implantation site, anesthesia/analgesia requirements, and postoperative monitoring requirements should be clearly defined.
③ A scanning and data-integration workflow should be established to ensure closed-loop documentation.
2.2 Short-Term or Auxiliary Marking Methods
(1) Needle-Prick Method
A fine needle dipped in a small amount of carbon ink is inserted subcutaneously at the ear, limbs, or tail to generate point marks; applicable to mice, rats, guinea pigs, and related species.
[Advantages] Rapid operation with simple consumables; suitable for short-term differentiation.
[Limitations] Complex point schemes are prone to misreading; local irritation may influence skin-related endpoints.
[Key points]
① Point schemes should be simplified as much as possible and standardized diagrams should be provided.
② Critical regions planned for sampling, modeling, or imaging should be avoided.
③ A point-to-ID correspondence table should be established and updated synchronously with cage cards.
(2) Chemical Fur-Dye Marking (Common)
Color markings are applied to the fur surface to distinguish individuals or groups, suitable for short-cycle verification and rapid differentiation of multiple animals housed in the same cage.
[Advantages] Noninvasive and efficient; suitable for high-frequency operational workflows.
[Limitations] Markings may fade due to fur growth, grooming, or friction; may introduce background interference in skin models or optical imaging.
[Key points]
① Avoid contact with mucosal surfaces (eyes, mouth, nose) and damaged skin.
② Avoid applying dye to the planned imaging ROI, scoring sites, or local administration regions.
③ Dedicated animal marking dyes are recommended to improve stability and consistency.
(3) Fur-Clipping Numerals
Numerals or symbols are clipped into dorsal or lateral fur to achieve short-term identification; applicable to medium-to-large animals such as dogs and rabbits.
[Advantages] High visual clarity and efficient on-site verification.
[Limitations] Rapid loss of readability as fur regrows; operator variability may reduce consistency.
[Key points]
① The position, size, and font style of numerals should be standardized.
② Cage cards and record sheets should be updated synchronously, with a review step in place.
③ Avoid clipping within planned surgical fields or key skin observation regions.
III. Common Mouse Grouping Methods and Implementation Considerations
3.1 Randomization-Based Grouping
(1) Simple Randomization
Mice are allocated to different groups with equal probability using a random number table or a random number generator; suitable for small sample sizes without pronounced stratification features.
[Advantages] Effectively reduces selection bias; implementation is relatively straightforward.
[Limitations] With small sample sizes or high inter-individual variability, imbalance in sex, body weight, or baseline metrics may occur between groups.
[Key points]
① Individual numbering and baseline data collection should be completed before randomization.
② Randomization methods, tools, and parameters (including random seeds when applicable) should be retained to ensure traceability.
③ Baseline balance checks should be performed after randomization, and results should be documented.
(2) Block Randomization
When sex, age, body weight, baseline glucose, or other factors substantially affect endpoints, mice should first be stratified into homogeneous blocks based on key factors, followed by random allocation within each block.
[Advantages] Improves between-group balance and reduces the influence of known confounders; typically enhances statistical efficiency.
[Limitations] Higher upfront measurement and management costs; overly fine stratification may create small blocks and increase operational complexity.
[Key points]
① Blocking variables should focus on major confounders, avoiding excessive stratification.
② Strict within-block randomization should be implemented, and block information should be fully recorded.
③ Block definitions should remain unchanged during the study, avoiding mid-study rule modification.
3.2 Matching and Adaptive Grouping
(1) Matched Pairing (Based on Specific Metrics)
Animals with similar body weight, baseline glucose, tumor volume, or other metrics are paired, and individuals within each pair are then randomized into experimental and control groups.
[Advantages] Reduces variance attributable to individual differences and improves power to detect treatment effects, particularly in small studies.
[Limitations] Dropouts may disrupt paired structures and increase statistical complexity; inappropriate matching metrics reduce control effectiveness.
[Key points]
① Matching metrics should have a clear relationship to the primary endpoint and demonstrate measurement stability.
② Dropout handling and statistical analysis strategies should be predefined (paired analysis vs conversion to unpaired analysis).
③ Pairing rules and random allocation procedures should be auditable and retained in records.
(2) Nested Matching Designs
Matching is performed under hierarchical factors, for example, matching by sex first and then matching by body weight within sex.
[Advantages] More refined control of multiple confounders and stronger between-group comparability.
[Limitations] High implementation complexity and stringent requirements for sample size and data management; excessive hierarchy may hinder matching feasibility.
[Key points]
① Nested levels should be designed around the core hypothesis, avoiding overdesign.
② The priority of matching at each level and rules for resolving conflicts should be clearly defined.
③ Data management and quality control should be strengthened to ensure traceability of matching relationships.
(3) Adaptive Grouping
Dynamic stratification or group adjustment is performed based on responses or performance during the experiment, for example, classifying animals into sensitive and tolerant groups based on early drug response.
[Advantages] Enables characterization of biological heterogeneity and supports mechanistic exploration and stratification research.
[Limitations] If rules are not predefined, selection bias may be introduced and inference reliability weakened; requires real-time monitoring and robust statistical design.
[Key points]
① Stratification thresholds, adjustment time points, and statistical analysis methods should be predefined.
② Execution consistency should be maintained, avoiding post hoc rule changes driven by outcomes.
③ All adjustments and rationales should be fully documented to ensure consistency with ethics and protocol requirements.
IV. Depilation in Experimental Animals
4.1 Common Depilation Methods and Application Scenarios
(1) Scissor Trimming
Fur is moistened and trimmed close to the skin; removed hair should be collected to reduce contamination.
[Advantages] Relatively low skin irritation; suitable for experiments sensitive to skin barrier integrity; can serve as pretreatment before shaving or chemical depilation.
[Limitations] Substantial hair roots remain, which is suboptimal for imaging or patch application requiring high surface cleanliness.
[Key points]
① Avoid lifting the fur before trimming to prevent accidental skin injury.
② Collect removed hair and dispose of it appropriately.
③ Evaluate whether additional depilation is needed based on downstream disinfection or imaging requirements.
(2) Manual Plucking
Fur is manually removed from a small area, commonly used for rapid exposure of venous regions.
[Advantages] No instruments required; simple operation.
[Limitations] Relatively strong irritation and possible transient hyperemia; unsuitable for inflammation-sensitive endpoints or large-area depilation.
[Key points]
① Limit the indication and avoid repeated manipulation of the same region.
② If repeated exposure is required, switch to lower-irritation methods.
③ Monitor local skin responses and document findings.
(3) Shaving
Long fur is clipped short first, then softened (e.g., with warm soapy water) and shaved; suitable for surgical field exposure and larger operational areas.
[Advantages] High efficiency and broad applicability; relatively low chemical irritation.
[Limitations] May cause minor skin abrasions; residual hair roots may affect highly sensitive optical imaging.
[Key points]
① Blades or clipper heads should be clean and sharp, and replaced as needed.
② Control pressure and direction to minimize abrasions.
③ Inspect skin integrity after shaving and integrate with antisepsis procedures.
4.2 Key Control Points for Chemical Depilation and Formulation Notes
(1) Key Operational Considerations for Chemical Depilation
[Advantages] More complete hair removal with minimal root residue; suitable for imaging, patch application, transdermal administration, and fine surgical visualization.
[Limitations] Requires strict control of contact time and thorough residue removal; improper handling may cause irritation or skin damage.
[Key points]
① Clip the fur short first; apply a thin layer to the target area; avoid mucosa and damaged skin.
② Strictly control exposure time; rinse and remove residues immediately upon reaching the specified duration.
③ Assess erythema, abrasion, or other reactions after depilation; adjust the protocol when necessary and document the entire process.
(2) Traditional Self-Prepared Depilatory Formulations
For large animals such as dogs: sodium sulfide 10 g, quicklime 15 g, dissolved in 100 mL water.
For small animals such as rabbits and rodents:
① sodium sulfide 3 g, soap powder 1 g, starch 7 g, add an appropriate amount of water to form a paste;
② sodium sulfide 8 g, starch 7 g, sugar 4 g, glycerol 5 g, borax 1 g, add water to 75 mL;
③ sodium sulfide 8 g dissolved in 100 mL water.
[Notes] Sulfide-based systems are typically alkaline, difficult to control with respect to irritation, and often show pronounced batch-to-batch variability. In animal experiments, dedicated laboratory-animal depilatories are generally preferred and should be used according to product instructions under institutional SOPs and safety requirements to reduce skin irritation and potential interference with experimental endpoints.
V. Aladdin-Related Products
Catalog No. | Product Name | Grade & Purity |
Experimental Animal Depilatory Agent (Ⅰ) | BioReagent | |
Experimental Animal Depilatory Agent (Ⅱ) | BioReagent | |
Experimental Animal Depilatory Agent (Ⅲ) | BioReagent | |
Experimental Animal Depilatory Agent (Ⅳ) | BioReagent | |
Animal Stain Marker (Yellow) | BioReagent, Biological Stain | |
Animal Stain Marker (Red) | BioReagent, Biological Stain | |
Animal Stain Marker (Blue) | BioReagent, Biological Stain | |
Animal Stain Marker (Purple) | BioReagent, Biological Stain | |
Animal Stain Marker (Black) | BioReagent, Biological Stain | |
Animal Stain Marker (Brown) | BioReagent, Biological Stain |
The reproducibility and interpretability of animal research depend on standardized control of fundamental procedures. It is recommended to finalize the individual numbering and marking framework at study initiation, select grouping strategies compatible with the primary endpoints, and incorporate depilation methods, contact time, anatomical sites, and skin responses into auditable records. Establishing unified SOPs, strengthening personnel training, and implementing checkpoint verification can substantially reduce mismatch and bias, improve data quality, and provide a robust foundation for reliable and reproducible conclusions.
For more related articles, please see below:
[1] Laboratory animal grouping, labeling, and dyeing experiments