Cat Population Simulator
Set up a suburban neighborhood, then watch a colony of outdoor cats grow, shrink, or stabilize over ten simulated years. Each monthly tick models reproduction, mortality, health, and, optionally, a trap-neuter-return program. Share the URL at the end to replay your exact starting conditions.
Total:
Monthly chance of death for cats under 6 months
Monthly baseline for cats 6+ months
Monthly chance an intact female near a male conceives
Purple = total population · Orange = intact · Gray = fixed
Frequently asked questions
What is logistic growth and why does the colony sometimes stabilize?
Exponential growth assumes unlimited resources. In practice, food availability and space create a carrying capacity: a ceiling the population can't sustainably exceed. As the colony approaches that ceiling, mortality rises and reproduction slows. The result is the S-shaped logistic growth curve. In this simulation, patch-level food values and the crowding penalty implement carrying capacity, so a well-fed medium neighborhood will often stabilize rather than grow without bound.
Why is the simulation stochastic, and what does that mean for results?
Stochastic means random variation is built into each event: whether a cat dies this month, whether a pregnancy occurs, how many kittens are born. Two runs with identical settings will produce different outcomes. This reflects real population dynamics, where small colonies are especially vulnerable to random extinction events even when conditions look favorable. Run the same configuration several times and compare the spread of peak populations. That range is the model's way of expressing uncertainty.
How does trap-neuter-return actually suppress population growth?
TNR works by removing intact cats from the reproductive pool faster than new cats enter it. Each month, a fraction of intact cats are fixed and can no longer reproduce, while natural mortality continues to reduce the total count. At a high enough TNR rate, the fixed-cat cohort grows faster than births replenish it, and the population declines. Research suggests TNR programs need sustained, high coverage (typically above 50–75% of new cats caught per year) to tip the balance toward decline rather than mere stabilization.
What is the effective population size, and why do kittens matter so much?
Effective population size refers to the number of individuals who actually contribute to the next generation. A colony with many older or fixed adults can have a small effective population even if the total count is high. Kittens carry high baseline mortality in this model (11% per month by default versus 2.5% for adults), but the ones that survive will reach sexual maturity at six months and immediately become reproductive. A large kitten cohort from one prolific season can double the effective population within a few ticks, which is why early intervention matters more than late-stage culling.
Why do coat colors shift over many generations?
Each kitten inherits its coat color randomly from one parent. In a small founding population, random sampling means some coats will appear more often in surviving offspring purely by chance, a process called genetic drift. Over many generations, one coat can dominate or even fix (become the only coat in the colony) without any selective advantage. The effect is strongest when the population passes through a bottleneck, such as a near-extinction event followed by recovery from a small number of survivors.
How is monthly mortality probability calculated?
Each cat faces a baseline death probability every tick: 11% for kittens under six months and 2.5% for adults by default, both adjustable in the Vital rates section of the setup panel. That base rate increases with cell crowding (2% per cat above three in a cell), with age past 72 months (1% per additional month), and with colony-wide overpopulation. Shelter value in each cell reduces it slightly. These are sampled independently per cat, so an unlucky cat in a crowded, low-shelter cell may die even at a young age. The compound effect of multiple modifiers is why high-food, low-density neighborhoods show dramatically longer lifespans.
Cat population facts
The default rates in this simulator are calibrated against published field research. Sources are listed below each entry.
Most feral kittens don't survive their first six months
A field study tracking free-roaming cats in North Carolina found that fewer than one in four kittens survived their first six months outdoors. Annual kitten mortality ran close to 75%, roughly ten times higher than the mortality rate for adults in the same population. That gap explains why intervening during the breeding season, before cohorts mature, has the largest effect on colony size.
Nutter, F.B., Levine, J.F., & Stoskopf, M.K. (2004). Reproductive capacity of free-roaming domestic cats and kitten survival rate. Journal of the American Veterinary Medical Association, 225(9), 1399–1402.
An unspayed female can produce two to three litters per year
Female cats reach sexual maturity between four and six months of age. Gestation lasts approximately 63–65 days (just over two months), and an unspayed female can produce two to three litters per year if conditions permit. Average litter size in free-roaming populations ranges from three to five kittens, with most litters landing near four. The same Nutter et al. study recorded a mean litter size of 3.7 kittens in its North Carolina sample.
Nutter et al. (2004), as above. Reproductive biology also documented in: Feldman, E.C. & Nelson, R.W. (2004). Canine and Feline Endocrinology and Reproduction (3rd ed.). Saunders.
With sustained effort, TNR programs substantially reduce colony size
A long-term study on a university campus in Florida tracked a free-roaming cat colony through 11 years of trap-neuter-return combined with adoption. The colony population declined by 66% over that period. The key factor was sustained program effort: earlier abandonment of TNR programs in other colonies in the study showed little lasting effect, suggesting that consistency matters more than intensity at any single point in time.
Levy, J.K., Gale, D.W., & Gale, L.A. (2003). Evaluation of the effect of a long-term trap-neuter-return and adoption program on a free-roaming cat population.Journal of the American Veterinary Medical Association, 222(1), 42–46.
Population modeling clarifies when TNR works and when it does not
Matrix population models (the same mathematical framework underlying this simulator) have been used to compare TNR against lethal removal as management strategies. The modeling work found that both approaches can suppress feral cat populations, but that TNR requires less ongoing effort once a high proportion of the colony is fixed, because the fixed cohort persists and blocks reproduction without continuous intervention. Lethal removal requires sustained effort because immigration and reproduction quickly refill the space.
Andersen, M.C., Martin, B.J., & Roemer, G.W. (2004). Use of matrix population models to estimate the efficacy of euthanasia versus trap-neuter-return for management of free-roaming cats. Journal of the American Veterinary Medical Association, 225(3), 395–400.