There is a world somewhere between reality and fiction. Although ignored by many, it is very real and so are those living in it. This forum is about the natural world. Here, wild animals will be heard and respected. The forum offers a glimpse into an unknown world as well as a room with a view on the present and the future. Anyone able to speak on behalf of those living in the emerald forest and the deep blue sea is invited to join.
Like the Dodo and Passenger Pigeon before it, the predatory marsupial Thylacine (Thylacinus cynocephalus), or ‘Tasmanian tiger’, has become an iconic symbol of anthropogenic extinction. The last captive animal died in 1936, but even today reports of the Thylacine's possible ongoing survival in remote regions of Tasmania are newsworthy and capture the public's imagination. Extirpated from mainland Australia in the mid-Holocene, the island of Tasmania became the species' final stronghold. Following European settlement in the 1800s, the Thylacine was relentlessly persecuted and pushed to the margins of its range, although many sightings were reported thereafter—even well beyond the 1930s. To gain a new depth of insight into the extinction of the Thylacine, we assembled an exhaustive database of 1237 observational records from Tasmania (from 1910 onwards), quantified their uncertainty, and charted the patterns these revealed. We also developed a new method to visualize the species' 20th-century spatio-temporal dynamics, to map potential post-bounty refugia and pinpoint the most-likely location of the final persisting subpopulation. A direct reading of the high-quality records (confirmed kills and captures, in combination with sightings by past Thylacine hunters and trappers, wildlife professionals and experienced bushmen) implies a most-likely extinction date within four decades following the last capture (i.e., 1940s to 1970s). However, uncertainty modelling of the entire sighting record, where each observation is assigned a probability and the whole dataset is then subject to a sensitivity analysis, suggests that extinction might have been as recent as the late 1980s to early 2000s, with a small chance of persistence in the remote south-western wilderness areas. Beyond the intrinsically fascinating problem of reconstructing the final fate of the Thylacine, the new spatio-temporal mapping of extirpation developed herein would also be useful for conservation prioritization and search efforts for other rare taxa of uncertain status.
Quote:Scenarios and sensitivity analysis for the Thylacine's extinction
Our analysis is predicated, in large part, on assumptions about which records are true and which are false (whether from misidentification, illusion or deception). In terms of possible scenarios, the most conservative assumption is to accept only those records based on kills or captures, in which a body, a photograph of a body, or a live animal was produced, i.e., physical specimens. We first used standard EDEs to model these records, using either island-wide or regional collections. We then experimented with the remaining dataset of uncertain observations, to assess the impact of applying different inclusion/exclusion criteria, in concert with the probabilistic weightings, on the extinction-year estimate (the physical records are included in all other scenarios because they are certain and should never be rejected). For example, one possibility is to also include unconfirmed kills and captures (where a kill was reported but the body was left behind, or an animal was trapped but then released or it accidentally escaped during handling). Another is to set a plausibility threshold on uncertain sightings, accepting only those that meet rigorous quality standards, and rejecting all others, or to only consider sightings that were made by two or more witnesses. Similarly, a threshold could be applied to a date, by accepting records prior to a given year and rejecting those reported afterwards (in reality, after the true extinction year, all sightings are axiomatically false; however, this year is of course not known, a priori, being the variable under question). Finally, a mixed-certainty EDE could be applied, using some (e.g., only ‘expert-rated’ sightings), or all records, with each sighting assigned a probability of being true, and scenarios constructed based on different relative weightings. We tried examples of all these approaches herein, while acknowledging that there is essentially no limit to the alternative interactions and dependencies among assumptions that might be imagined.
Quote:Database metrics
The final database comprised 1237 entries (99 physical records, 429 expert sightings, 709 other sightings), with observations from all years except 1921, 2008 and 2013. Many records from 1910 to 1936 (the year the last captive specimen died—a male captured in 1931 (Sleightholme et al., 2020), see photograph in Fig. 2) were of confirmed kills or live captures, although 56.6 % (128) of the 226 entries dating from this period were unverified sightings. The last fully documented wild animal (with photographs) was shot in 1930, but there is little reason to doubt the legitimacy of two bodies noted from 1933, nor two other capture-and-releases from 1935 and 1937. Thereafter, over the course of eight decades, a further 26 deaths and 16 captures were reported (but not verified), along with 271 sightings by ‘experts’ (e.g., former trappers, bushmen, scientists, or officials). The other 698 sightings from Tasmania were made by the general public.
There were notable spikes in reporting rates in 1937 and 1970, the former following legal protection and the latter arising from media attention linked to a well-publicized expedition: these are examples of the framing and recency biases noted above (Iftekhar and Pannell, 2015). There are also many examples of discrete spatio-temporal sighting clusters with closely matching visual descriptions, the interrelationships of which would not have been apparent at the time the reports were submitted to authorities. Overall, the annual number of reports in the six decades spanning 1940–1999 were relatively constant (x̅ = 14.9 year−1, σx̅ = 1.15), but fell substantially (x̅ = 3.6 year−1, σx̅ = 0.60) from 2000–present. A breakdown of observations by type and quality, and time-series plots, are reported in the Supporting Information (Appendix S4).
Quote:The most vexing difficulty with a scientific mystery like this—trying to decide which sightings are correct, and which are false—is quantifying the risk of ascertainment bias. During the post-bounty period, Thylacine encounters were noted as being rare, but because there were still occasional kills and captures, the species was known with certainty to persist. During this time, unverified sightings were made regularly (128 reports between 1910 and 1936), and there seems little reason to doubt their authenticity, there being no general perception at the time that an unproven sighting was anything particularly remarkable (Sleightholme and Campbell, 2016). However, in the years following the death of the last captive Thylacine, when zoos sought new specimens (offering substantial remuneration) and yet none could be secured, interest in proving the species' ongoing existence steadily rose, such that by the 1960s it was recognized as a puzzling quandary (Griffith, 1972; Guiler, 1966). As recognition of this evidence gap grew, there was a greater incentive to falsely report sightings (for notoriety), or even a subconscious desire to want to see a live Thylacine, leading to inflated misidentification errors. Without the reassurance of an occasional ‘ironclad’ (physical) record, the time point at which there was a switch from some sightings being true, to all being wrong (i.e., after extinction), is left inevitably shrouded in the conservation-biology equivalent of a ‘fog of war’. To try and cut through this, we are left with an inescapable reliance on observer credibility and the associated sighting details (much as is done when weighing up the usefulness of eyewitness testimonies in law courts; Wechsler et al., 2015), the specifics of which are listed for each record in the multivariate meta-data of the TTSRD. An extended discussion of this topic is given in Appendix S2 (Supporting Information).
It is the science of probabilities.
It suggests insights into what science as science does not assume is measurable. Incurring the “most palatable sightings”, even without any physical evidence of the animal after the 1930s is to turn empirical evidence into hypothetical science.
Possibly the most salutary aspect of the article is the issue of mapping the latest sightings and producing a sketch to demonstrate the speed and spatial data of their contraction throughout the 20th century.
Every study on Thylacine makes headlines… I'm sure that even 30 years from now genetic science will still not be able to produce an animal similar in morphological terms. Still, being a Thylacine is much more than looking like him, it has to do with instinctive and associated behavior, sociability, nutrition, metabolic rate, ecology, biotic connection, etc. The most that science will be able to do in the coming decades is to collect X number of genes (some of the most important ones) and inject them into the format of a new animal. The complete set of genes will NEVER be returned. That is why many question: why so many genomic studies are receiving millionaire investments while the conservation of our surviving faunal wealth is, in many cases, agonizing.
One of the best examples we have of human irrationality and harmful behavior. This example should remain questioning and provoking the popular imagination.
