Food, Not Time, Sets the Pace: Food-Limited Growth in Twenty-Three Recorded Tribble Outbreaks on Federation Vessels and Stations, 2268–2289

1. Introduction

Tribbles are small, furred animals that eat almost anything and breed at a rate out of proportion to their size. They were known in Federation space before 2268, but it was traders who first spread them widely. In 2268 a trader sold a few of them on Deep Space Station K-7, which was then holding a shipment of quadrotriticale grain destined for Sherman's Planet. Within days the animals had spread through the station and a visiting starship, and had filled the station's grain compartments (Deep Space Station K-7 Station Records, 2268). The grain turned out to have been deliberately contaminated; the tribbles that ate it died, which exposed the contamination. The outbreak entered Federation lore, and the trader was ordered to remove every tribble from the station.

Most readers know that outbreak through an estimate made on board the visiting starship. It assumed that each tribble produced a litter of ten every twelve hours and that food was unlimited, and on that basis projected well over a million animals within three days from a single individual. The estimate was a sound calculation under its assumptions. It was never tested, and its assumption about food was plainly not true of most tribble outbreaks, in which animals had access to whatever food a vessel happened to leave within reach.

During the K-7 outbreak the starship's medical officer observed that tribbles bred in proportion to how much they ate and advised against overfeeding them (Deep Space Station K-7 Station Records, 2268). That observation implies that food, not a fixed biological clock, sets the pace of population growth. This paper tests it with the full outbreak record. We write in the mid-2370s.

2. Methods

After 2268 the Federation Agricultural Quarantine Service kept a register of tribble outbreaks on Federation vessels and stations, for the practical reason that tribbles eat stored food (Federation Agricultural Quarantine Service, 2290). The register covers outbreaks arising from trade and transport and runs to 2289, after which it records none. It lists 27 outbreaks. We excluded four whose records lacked the number of animals introduced or the time of introduction, which left 23, including K-7. An infestation on Deep Space 9 in 2373, by animals brought forward from 2268 in a temporal incident, falls under the Department of Temporal Investigations and is outside the register.

For each outbreak the register gives the number of animals introduced, the time from introduction to detection, the count or estimated count at detection, and a description of the food the animals could reach. From the descriptions we classed food access in three levels: open, where the animals had reached stored food or unsecured galley or cargo stocks; rationed, where they had reached only scraps or food left out in small quantities; and withdrawn, where all food had been secured before detection. Two of us classed each outbreak independently from the descriptions and agreed on all 23.

Where counts at detection were estimated from the mass of animals removed, we used the Service's standard conversion of about 0.25 kilograms per adult (Marrow-Ansel & Tovic, 2279). Our main outcome was the population's per-hour growth rate: the natural logarithm of the ratio of count at detection to number introduced, divided by the hours between. Because a two-point estimate of this kind is biased when counts are small, we applied the correction described by Sepvek (2371). This is a continuous measure, and we fitted a linear regression of it on food access, open against rationed (20 outbreaks), adjusted for the ambient temperature class of the vessel or station, which the register records. We converted growth rates and their confidence limits to doubling times for presentation.

For the three withdrawn outbreaks, food had been secured at some point before detection, so their growth from introduction to detection mixes periods with and without food. We report them descriptively, using a second measure the register also records: the count at detection against the count at final removal.

3. Results

The 23 outbreaks began with between 1 and 14 animals and were detected between 11 hours and 9 days after introduction. Food access was open in 12 outbreaks, rationed in 8 and withdrawn in 3. Counts at detection ranged from 30 to over a million; the largest was K-7, where the count was estimated from the mass removed from the grain compartments at roughly 0.8 to 1.5 million.

Where food was open, populations doubled about every 4.1 hours (95% CI 3.6–4.7). Where it was rationed, they doubled about every 19 hours (95% CI 15–24). Rationed populations thus grew at about 0.22 times the rate of those with open food (95% CI 0.16–0.29). Ambient temperature made no detectable difference.

All three withdrawn outbreaks grew before detection, at rates between those of the rationed and open groups, which is what a period of access followed by securing the food would produce. After detection none of them grew: in two the count at removal matched the count at detection, and in the third it had fallen by about a tenth, which the register attributes to animals escaping into inaccessible spaces.

K-7 sat at the fast end of the open-food group, with an estimated doubling time of about 3.8 hours, uncertain because the count itself is. The contemporary estimate, a litter of ten every twelve hours, implies a doubling time of about 3.5 hours. That figure lies just outside the confidence interval for open-food outbreaks and is faster than any single outbreak in the register, but only modestly so. It was an upper bound, reached nowhere, and approached only where food was abundant.

4. Discussion

What tribbles eat sets the pace of an outbreak. Rationed populations grew about a fifth as fast as those with open access to food, and no population whose food had been secured grew further after it was found. The simplest reading is that reproduction tracks intake: a well-fed tribble breeds quickly, a poorly fed one slowly. Our counts measure net growth, however, and cannot separate births from deaths or escapes, so this remains a hypothesis about the mechanism. It fits the medical officer's observation on K-7, and it means that the famous estimate was a statement about unlimited food, not about tribbles as such.

For quarantine practice, the finding changes the order of operations. Early detection matters, but it matters less than securing food. A population detected late on a vessel whose food is locked away is a smaller problem than one detected early beside an open grain store. The Service's guidance has put food control first since its circular of 2272 (Federation Agricultural Quarantine Service, 2272; Ostrander, 2374), and the outbreak record supports it.

Our results also bear on the tribble's disappearance from the register after 2289. Klingon accounts describe a campaign in which tribbles were hunted and an armada destroyed their homeworld; its date is uncertain (Keth-Arvane, 2361). We cannot say from our data whether, before that campaign, tribble numbers on the homeworld were held down by food, by predators or by both. A species whose growth follows its food would be expected to have evolved where food, not time, was scarce, and to show explosive growth only when it reached an environment unusually rich in food, such as a grain store or a galley. That is a hypothesis about an ecology that can no longer be observed.

5. Limitations

Twenty-three outbreaks is a small sample, and the register records only outbreaks that were detected and reported. Small outbreaks controlled before anyone reported them are missing, and very large ones are over-represented. Counts at detection were often estimated from mass, and the mass-to-count conversion assumes adult animals. It therefore undercounts populations dominated by young, which biases growth estimates downward, most of all in the fastest outbreaks; if anything, the open-food rate is understated. Our food-access classes are coarse, built from descriptions written for other purposes, and they cannot capture the quantity eaten.