Why Forecast a Colony
The mistake
Most labs scale up colonies "by feel." You set up 5 pairs in January, glance at the rack in March, and panic in June when you need 200 mice for an experiment but only have 60. Or worse: you panic because you have 800 mice and no room.
The fix
A discrete-time projection. Run the math forward from your real founder count, your real litter size, and your real retention rate. The output is a month-by-month curve and a generation table — exactly the surface you can present to a facility manager or PI to justify cage allocation.
The math
- weanlings_g = active_pairs_g × litter_size × retention_rate
- new_pairs_g = floor((weanlings_g × female_ratio) / 2)
- active_pairs_(g+1) = active_pairs_g + new_pairs_g
- live_colony_(g+1) = live_colony_g + weanlings_g (clipped at facility cap)
Geometric growth dominates the first ~6 generations. After that, either the facility cap or breeder retirement flattens the curve.
Facility Capacity & The Cull Decision
The trade-off
Every facility has a hard cage limit. When the colony hits the cap, you cull weanlings — usually the surplus males and the late-litter pups. Most labs do this *unintentionally* (oops, no clean cages this week) instead of *intentionally* (here is the plan).
Use the cap field to model both worlds
1. Run with no cap → see the unconstrained biological ceiling. This is what your colony would do in a perfect rack.
2. Run with your real cap → see the month the cull starts and the steady-state population the cap supports.
3. Compare the two curves to budget cull rates and to justify a facility expansion if needed.
The cost
A capped colony is also a *managed* colony — you are committing to cull every generation past the cap, which has IACUC, ethical, and per-diem implications. The 3Rs Justification Builder (coming in Phase 26) helps articulate the trade-off in writing.
