What a breeding population would require

Whatever else a sasquatch would be, it would be a large-bodied mammal, and large-bodied mammals obey arithmetic. This page runs the arithmetic. It does not settle whether the animal exists. It sets out what would have to be true if it does, and it corrects two numbers that circulate widely in this argument and are wrong in opposite directions.

The number you need is not a small number

The oldest rule of thumb in conservation genetics is the 50/500 rule. Fifty breeding individuals holds off inbreeding depression over a few generations. Five hundred retains enough genetic variation for the population to keep adapting. Both numbers refer to effective population size, written Ne, and not to a head count. The rule is usually credited to Ian Franklin and Michael Soulé, both writing in 1980, though the 1980 chapters themselves were not read for this entry and the attribution here is secondhand.

That Ne/N distinction does most of the work. Richard Frankham reviewed 192 published estimates from 102 species and found that comprehensive estimates of Ne/N, the ones accounting for fluctuating population size, variance in family size and unequal sex ratios, averaged "only 0.10 to 0.11." DOCUMENTED An effective size of 500 therefore implies a census population on the order of 5,000 actual animals.

The thresholds have since been argued upward. Frankham, Corey Bradshaw and Barry Brook argued in Biological Conservation in 2014 that evidence accumulated since 1980 shows Ne = 50 is inadequate, that Ne of at least 100 is required to hold fitness loss to 10 percent or less over five generations, and that "even Ne = 500 is too low for retaining evolutionary potential for fitness in perpetuity; a better approximation is Ne of at least 1000." CONTESTED That revision is genuinely disputed. Pérez-Pereira, Wang, Quesada and Caballero ran simulations including purging and reported in 2022 that purging can pull the long-term requirement back "close to Ne = 500 for species with moderately large reproductive rates," while agreeing that low-reproductive-rate species need at least Ne = 1000. A great ape with a multi-year interbirth interval falls on the wrong side of that split.

The empirical check is Traill, Bradshaw and Brook's meta-analysis of 30 years of published minimum viable population estimates, which reported "a cross-species frequency distribution of MVP with a median of 4,169 individuals (95% CI = 3,577 to 5,129)." The full text of that paper was not accessible for this entry and the quoted figures are secondhand, so no chip is placed on them.

Three routes land in the same place. Persistence over centuries requires low thousands. Call it 5,000 animals as a working floor.

Set 5,000 against Washington state's 172,587 square kilometers of land and you get one animal per roughly 35 square kilometers. That is about a quarter of Washington's black bear density, where the state agency estimates approximately 22,000 bears, or one per about 8 square kilometers. DOCUMENTED Note that this is the arrangement most favorable to being found. Putting all 5,000 in one state maximizes density. Spread the same 5,000 across the whole claimed range and the animals get sparser, not denser.

The North Cascades grizzly gets recruited into this argument as proof that a several-hundred-kilogram mammal can go undetected in exactly this terrain. It proves the reverse. The US Fish and Wildlife Service states that "there are currently no known populations within the North Cascades or the Bitterroot Mountains of central Idaho and western Montana," and on April 25, 2024 the Park Service and Fish and Wildlife Service announced a Record of Decision to actively restore grizzlies there, with no translocations having taken place as of that agency's project page. DOCUMENTED Agencies searched, failed to detect, concluded the animal was gone, and are now proposing to put it back.

What it would have to eat

Kenneth Nagy compiled doubly labeled water measurements of field metabolic rate for 229 free-living terrestrial vertebrates and fitted, for eutherian mammals, FMR = 4.82 M^0.734 kilojoules per day with mass in grams, from 79 species. DOCUMENTED Body mass alone explained over 94 percent of the variance within each class.

Run the reported sasquatch mass range through it and you get about 41,100 kJ per day (roughly 9,800 kcal) at 227 kg, and about 58,100 kJ per day (roughly 13,900 kcal) at 363 kg. Those are the figures usually quoted here, and for a primate they are roughly double what they should be.

Pontzer and fourteen colleagues compared doubly labeled water measurements of total energy expenditure across 17 primate species against other placental mammals and reported in PNAS in 2014 that primates "use remarkably little energy each day, expending on average only 50% of the energy expected for a placental mammal of similar mass," a difference not explained by physical activity. DOCUMENTED Apply that correction and a 227 to 363 kg ape needs roughly 4,900 to 6,900 kcal a day. Kleiber's law, at about 70 M^0.75 kcal per day, puts basal rate at roughly 4,100 and 5,800 kcal for those masses, so the corrected field figure sits at about 1.2 times basal. Low, and consistent with how little primates burn.

Blueberries run 57 kcal per 100 g, so the daily requirement is roughly 9 to 12 kg of berry-equivalent food. The Smithsonian's National Zoo states that adult male gorillas eat about 45 pounds (32 kilograms) of food per day, with females eating about two-thirds of that. That is a zoo fact sheet for a different gorilla subspecies rather than a field measurement, so take it as an order-of-magnitude check and nothing finer. Against a real ape's intake, the sasquatch figure is unremarkable.

The aggregate needs stating carefully. Five thousand animals at 4,900 kcal a day is about 9 billion kilocalories a year, on the order of 16,000 metric tons of berry-equivalent biomass, and that removal should leave feeding sign. Two honest gaps there. I found no published documentation of browse lines, feeding platforms or systematic sign attributed to a large unrecognized omnivore in this region, and I found no Pacific Northwest berry and mast productivity figure to measure 16,000 tons against. So the claim here is only that the consumption should be visible, not that anyone has published a search for it.

The comparison

The mountain gorilla row should give both sides pause. A 120 to 191 kg ape that moves about half a kilometer a day inside a 3 to 15 square kilometer range, at roughly 1.5 animals per square kilometer, is trivially findable, and was found, named, individually photographed and counted to the animal. A sasquatch would need the opposite ecology in every respect: enormous range, low density, high mobility, no habituation. A large omnivore can have that ecology. Bears do.

Lozier, Aniello and Hickerson built ecological niche models from georeferenced sasquatch reports and compared them against a black bear model in the Journal of Biogeography in 2009, and the distributions matched. Their stated purpose was methodological: to demonstrate how "convincing environmentally predicted distributions of a taxon's potential range can be generated from questionable site-occurrence data." From the match they propose that "many sightings of this cryptozoid may be cases of mistaken identity." CONTESTED Read the verb. They propose. The paper is routinely cited as though it settled the question, and its authors were careful not to say that.

Winter

The popular skeptical line runs: apes do not hibernate, the Cascades are cold, therefore no sasquatch. The first premise is wrong, the second is weak, and the argument survives anyway for a third reason.

Kathrin Dausmann, Julian Glos, Jörg Ganzhorn and Gerhard Heldmaier reported in Nature in 2004 that the fat-tailed dwarf lemur of Madagascar hibernates in tree holes for up to seven months. DOCUMENTED It is the first tropical mammal and the only primate in which hibernation has been demonstrated. So a primate does hibernate, and the blanket claim that primate physiology forbids dormancy is wrong.

The counterexample transfers badly, which the version of this argument you usually see leaves out. The fat-tailed dwarf lemur weighs 4 to 10 ounces, roughly 113 to 283 grams, three orders of magnitude below the reported sasquatch mass. It does not regulate its body temperature while hibernating. If the tree hole is poorly insulated, its body temperature tracks the outside air. A 200-gram animal going passively poikilothermic in a Malagasy dry season tells you very little about a 227 kg endotherm in a Cascade January.

Cold alone is still not the constraint. Mountain gorillas are given an elevation range of 2,200 to 4,300 meters, where freezing is routine, and a 300 kg body has a favorable surface-to-volume ratio. Thermally, a large hairy ape in winter is the easy part.

The hard part is January calories. Every large mammal in this habitat solves winter one of three ways. Black bears den and do not eat. Elk migrate downslope and shed body mass, which is why Yellowstone's summer count of 10,000 to 20,000 elk in six to seven herds falls below 2,000 in the park in winter, though those are a different population in a different range from the Olympic Roosevelt elk. Gorillas do neither, because their habitat supplies green herbaceous biomass year-round. A snow-covered conifer forest at 1,000 meters does not.

Could a sasquatch den? The arithmetic does not forbid it, but it is expensive. Paying full basal rate, 4,100 kcal a day for 150 days is about 615,000 kcal, roughly 68 kg of fat at 9 kcal per gram, about 30 percent of a 227 kg body. Bears do not pay that. Tøien and colleagues reported in Science in 2011 that hibernating black bears suppress metabolism to 25 percent of basal while regulating body temperature between 30 and 36 degrees Celsius in multiday cycles, with heart rates reduced 55 percent to as few as 9 beats per minute, and concluded that most of the metabolic suppression is independent of lowered body temperature. DOCUMENTED A quarter of basal turns a 68 kg fat budget into something closer to 17 kg.

What survives, then, is a narrower objection than the one usually made. No ape has any of that machinery. And the report literature contains winter sightings describing an animal that is awake, though this page has not counted them and does not know what fraction of the corpus they are, so treat that second point as an impression of the literature rather than a measured result.

Parasites, and the ones nobody has described

Real primate populations come with a parasite fauna, and when anyone looks, they find it. Kouassi and colleagues examined 3,142 fecal samples from seven monkey species in Taï National Park, Côte d'Ivoire, collected between November 2009 and December 2010, and identified 23 parasite species: nine protozoa, thirteen nematodes and one trematode. DOCUMENTED One park, one survey, fourteen months.

No helminth, protozoan, louse or mite has been formally described from a sasquatch, and no candidate scat has produced published ova attributable to an undescribed primate parasite. This cuts less deeply than it looks. I found no published parasitological examination of a purported sasquatch scat in a peer-reviewed venue. Claims that such examinations happened in the 1960s and 1970s circulate in the popular literature, and no named source for them could be verified here, so they are not cited and no weight is placed on them in either direction. The absence of described parasites is at least partly an absence of anyone looking with a taxonomic key.

Disease is the other half. Stochastic epidemic risk is one of the standard reasons minimum viable population thresholds sit in the thousands rather than the hundreds. A small, genetically depauperate ape population sharing a temperate zone with humans, dogs and bears is a poor candidate for surviving four centuries of introduced pathogens.

The bones that were counted

The North American primate record ends a long time ago. Ekgmowechashala, a roughly 2.3 kg lemur-like animal known from South Dakota, Nebraska, Oregon and Texas and dated to about 30.8 to 26.3 million years ago, is the youngest primate in the North American fossil record before humans arrived. DOCUMENTED Its affinities were debated for decades. CONTESTED Rust, Ni, Tietjen and Beard described new fossils from Nebraska and from Guangxi, China in the Journal of Human Evolution in 2023 and placed the genus within a radiation of southern Asian adapiforms, concluding that its arrival was a colonization from Asia and "an example of the Lazarus effect, whereby a taxon (in this case, the order Primates) reappears suddenly in the fossil record after a lengthy hiatus." On that reading it arrived from Asia rather than descending from the earlier North American primate radiation, so the common description of it as the last native North American primate lineage is wrong on the word native.

The Pleistocene record is not thin. The 1913 to 1915 excavations at La Brea alone extracted 750,000 specimens at 96 sites, and radiometric dating gives an age of 38,000 years for the oldest known material from the seeps. DOCUMENTED No ape.

The Pacific Northwest archaeological record is often cited here, and it needs handling more carefully than it usually gets. Iain McKechnie and Madonna Moss compiled fine-screened zooarchaeological data from 222 sites between Oregon and southeast Alaska, encompassing 513,605 fish remains. DOCUMENTED At Ozette on the Olympic coast, a mudslide sealed a Makah village, and excavation over eleven years beginning in 1970, by Washington State University archaeologists working with the Makah, recovered over 55,000 artifacts, roughly 30,000 of them wooden. Much of that material is now held at the Makah Cultural and Research Center.

Read those two numbers carefully, because both get misused. McKechnie and Moss analyzed fish, and a fish assemblage would not be expected to contain ape bone whether or not an ape existed. The Ozette figure counts artifacts, most of them wooden, rather than fauna. Both appear in Bigfoot arguments as though they were exhaustive faunal inventories of the coast. Neither is. What they do establish is that enormous, fine-screened, systematically quantified assemblages from this coast exist and have been published, which makes the region a place where the question could in principle be asked. I did not find a published survey asking specifically whether any Northwest Coast faunal assemblage has produced a non-human primate element other than human. Treat that negative as my reading of the literature and not as a cited result.

Oral tradition is a different kind of record, handled in the Indigenous record and not summarized here. The traditions of this coast belong to many distinct nations, with distinct beings, names and obligations, and collapsing them into one legend is the standard error that entry exists to correct. Nothing on this page speaks for any nation, and nothing here treats any tradition as zoological evidence in either direction.

What this establishes

A viable sasquatch population needs roughly 5,000 animals, and concentrating all of them in one state is the arrangement most favorable to being found. Each animal needs something like 4,900 to 6,900 kilocalories a day once a primate correction is applied, an ordinary figure for an ape and about half what a generic mammal equation predicts. Cold is not the obstacle. The winter obstacle is January calories and dormancy machinery no ape has, and the hibernating-primate counterexample, though real, comes from an animal weighing a fifth of a kilogram. The parasite argument is weaker than it sounds because nobody has looked properly. The zooarchaeological argument is weaker than it is usually stated, because the large published counts from this coast are fish bones and wooden artifacts.

None of that is proof of absence. All of it is a bill the hypothesis has to pay, and several line items on it are smaller than skeptics usually claim.

Sources

• Frankham, R. "Effective population size/adult population size ratios in wildlife: a review." Genetical Research 66, no. 2 (1995): 95–107. Abstract read via Crossref: 192 estimates, 102 species, comprehensive Ne/N averaging only 0.10 to 0.11. doi:10.1017/S0016672300034455
• Frankham, R., Bradshaw, C.J.A., and Brook, B.W. "Genetics in conservation management: Revised recommendations for the 50/500 rules, Red List criteria and population viability analyses." Biological Conservation 170 (2014): 56–63. Abstract read via Europe PMC. doi:10.1016/j.biocon.2013.12.036
• Pérez-Pereira, N., Wang, J., Quesada, H., and Caballero, A. "Prediction of the minimum effective size of a population viable in the long term." Biodiversity and Conservation 31, no. 11 (2022): 2763–2780. Abstract read via Crossref. doi:10.1007/s10531-022-02456-z
• Traill, L.W., Bradshaw, C.J.A., and Brook, B.W. "Minimum viable population size: A meta-analysis of 30 years of published estimates." Biological Conservation 139, no. 1–2 (2007): 159–166. Bibliographic record confirmed via Crossref; the full text was not accessible, and the median of 4,169 (95% CI 3,577–5,129) is quoted from the paper as reproduced at Minimum viable population, Wikipedia. The number of species covered is not asserted here because it could not be verified. doi:10.1016/j.biocon.2007.06.011
• Nagy, K.A. "Field metabolic rate and body size." Journal of Experimental Biology 208, no. 9 (2005): 1621–1625. Full text read for the eutherian equation FMR = 4.82 Mb^0.734, N = 79. journals.biologists.com
• Pontzer, H., Raichlen, D.A., Gordon, A.D., et al. "Primate energy expenditure and life history." PNAS 111, no. 4 (2014): 1433–1437. Abstract read via Europe PMC: 17 primate species, "only 50% of the energy expected for a placental mammal of similar mass." doi:10.1073/pnas.1316940111
• Kleiber's law, Wikipedia, for BMR ≈ 70 M^(3/4) kcal/day.
• Dausmann, K.H., Glos, J., Ganzhorn, J.U., and Heldmaier, G. "Physiology: Hibernation in a tropical primate." Nature 429 (2004): 825–826. Bibliographic record confirmed via Crossref; the paywalled text was not read, and the seven-month duration, tree-hole site, absence of thermoregulation and the "first tropical mammal and only primate" framing are taken from Fat-tailed dwarf lemur, Wikipedia, which cites it, and which is also the source of the 4 to 10 ounce body mass. doi:10.1038/429825a
• Tøien, Ø., Blake, J., Edgar, D.M., Grahn, D.A., Heller, H.C., and Barnes, B.M. "Hibernation in black bears: Independence of metabolic suppression from body temperature." Science 331, no. 6019 (2011): 906–909. Abstract read via Crossref and OpenAlex. doi:10.1126/science.1199435
• Lozier, J.D., Aniello, P., and Hickerson, M.J. "Predicting the distribution of Sasquatch in western North America: anything goes with ecological niche modelling." Journal of Biogeography 36, no. 9 (2009): 1623–1627. Abstract read via Crossref. doi:10.1111/j.1365-2699.2009.02152.x
• Kouassi, R.Y.W., McGraw, S.W., Yao, P.K., et al. "Diversity and prevalence of gastrointestinal parasites in seven non-human primates of the Taï National Park, Côte d'Ivoire." Parasite 22 (2015): article 1. Abstract read via Europe PMC: 3,142 samples, November 2009 to December 2010, 23 parasite species. doi:10.1051/parasite/2015001
• Rust, K., Ni, X., Tietjen, K., and Beard, K.C. "Phylogeny and paleobiogeography of the enigmatic North American primate Ekgmowechashala illuminated by new fossils from Nebraska (USA) and Guangxi Zhuang Autonomous Region (China)." Journal of Human Evolution 185 (2023): 103452. Abstract read via OpenAlex. doi:10.1016/j.jhevol.2023.103452
• McKechnie, I., and Moss, M.L. "Meta-analysis in zooarchaeology expands perspectives on Indigenous fisheries of the Northwest Coast of North America." Journal of Archaeological Science: Reports 8 (2016): 470–485. Abstract read via OpenAlex: 222 sites, 513,605 fish remains. doi:10.1016/j.jasrep.2016.04.006
• Koehler, G.M., and Pierce, D.J. "Black bear home-range sizes in Washington: climatic, vegetative, and social influences." Journal of Mammalogy 84, no. 1 (2003): 81–91. Abstract read via OpenAlex for the finding that male home ranges were 3.8 times larger than female (P = 0.04); the full text was not accessible and no square-kilometer values from this paper are quoted here. doi:10.1644/1545-1542(2003)084<0081:BBHRSI>2.0.CO;2084%3C0081:BBHRSI%3E2.0.CO;2)
• Washington Department of Fish and Wildlife, black bear species page ("approximately 22,000 black bears in Washington across all land ownerships") and elk species page (Olympic National Park and surrounding forests, "about 5,000" Roosevelt elk).
• US Fish and Wildlife Service, grizzly bear species page, for at least 1,923 grizzlies in the contiguous 48 states, 727 in the Yellowstone demographic monitoring area, 1,092 in the Northern Continental Divide Ecosystem, and no known populations in the North Cascades; and North Cascades grizzly bear restoration, for the April 25, 2024 Record of Decision and the statement that no translocations have taken place.
• National Park Service, Elk, Yellowstone National Park, for 10,000–20,000 elk in six to seven herds in summer and fewer than 2,000 in winter.
• Smithsonian's National Zoo and Conservation Biology Institute, Western lowland gorilla, for adult male gorillas eating about 45 pounds (32 kg) of food per day and females about two-thirds of that. This is a zoo fact sheet for western lowland gorillas, not a field study of mountain gorillas.
• Wikipedia entries consulted for body mass, home range, density, denning and site figures, each of which carries its own citations: American black bear (masses, Long Island WA and Ungava home ranges, 85% vegetation, Great Smoky density), Grizzly bear (Alaska Peninsula coastal masses, 4,000 km² male territories, 80–90% plants), Roosevelt elk (bull and cow masses), Gorilla (mountain gorilla home range of 3–15 km² and about 500 m or less per day, citing McNeilage 2001; foliage-dominated diet), Mountain gorilla (masses, 2,200–4,300 m elevation), Bwindi Impenetrable National Park (321 km², 2018 census of 459 gorillas in 36 groups plus 16 solitary individuals), Ekgmowechashala (2.3 kg, 30.8–26.3 Ma, states), La Brea Tar Pits (750,000 specimens at 96 sites, 38,000-year oldest material), Ozette Indian Village Archeological Site (55,000 artifacts, ~30,000 wooden, eleven years from 1970, Washington State University, Makah Cultural and Research Center), Washington (state)) (land area 66,636 sq mi / 172,587 km²), Blueberry (240 kJ / 57 kcal per 100 g).