Thylacine Population issues / Thoughts?

[obsidian351]

I like no doubt most of you here hope against hope the a Thylacine is still alive and hiding, but concede that 70 or so years is a long time to have not captured one, sightings aside the real proof would be in a trapped specimen that could then be tagged and released to potentially give away its friends hiding spots (or even a credible non blurry video) this got me to thinking a few things

* what was the Thylacines lifespan?

* how many generations would have come to pass in the last 70 years?

* how many individuals would there need to be for a breeding population?

the following links give me a little hope that a small number of survivors could sustain a population but also raises some points about genetic diversity

 

www.pelicannetwork.net/elephant.seal.history.htm

genetics.thetech.org/ask/ask113  

 

[Thylacoleo Gal]

I seem to recall Eric Guiler thought thylacines were relatively short lived animals. Maybe less than 10 years?

Sorry - can't look stuff up just now. Pressed for time and on a "foreign PC". The erudite Molloch may have knowledge to share about genetics and viable population sizes?

[greatdane]

I think you´re right, Debbie
From memory I seem to remember something like an estimate of only 6-7 years. I don´t personally put a lot of creedence in that, since not only is the Thylacine (probably) extinct, but our knowledge of it´s life in the wild is rudimentary at best. Only some snippets of anecdotal evidence have survived

[saggitarius]

Hi obsidian351.
By the look of your avatar we could be brothers!!
My files are some 100km away from me at the moment but I think I have an article there that puts average life span of thylacine at 7-8 years and from memory if says something about them not breeding until their third year and normally having 2 pups, occasionally three.
In the back of my mind there is also something about them only breeding every second year. Can anyone help in that regard??

If that's true then a healthy adult female would only have a maximum three litters, adding an average six pups, many of which might not see it through to adulthood.

[youcantry]

Eastern quolls live about 4 years. Five years is old. Compare that to a domestic cat, at about 16. Devils live about 6 years. Thylacines were estimated to live about 8 years, but some estimates put it just over 10 years, perhaps 12.

Most people agree the thylacine had incredibly limited genetic diversity before extinction. There was a (US?) study a few years ago where they looked at the DNA from thylacines taken from different locations in Tas, decades apart, and concluded the diversity was incredibly small. Some authors then say the species was on the way to extinction anyway, and we just knocked it over the edge, but others say that the species had done well for thousands of years *despite* the genetic diversity. I believe there are other species with incredibly limited diversity that seem to do well too. My feeling is that the limited genetic diversity was not too big an issue for it. (With most catastrophes, by the way, they usually result not from a single catastrophic failure, but from the accumulation of many small issues that could have been prevented - regardless of whether the catastrophe is a car crash, fire, or anything else.)

Tigerman (pseudonym) argues the breeding population could be as small as 40. He bases this on a claim that he has seen two thylacines in the wild, and that he believes there are (or were) 3 populations - a north-eastern population (ie, in Tas) of about 40, and a western population split north and south into groups of about 60 and 100 respectively. He felt the western populations migrated and intermixed, but the north-eastern would get pushed back by human "civil"isation each time it tried migrating west.

Interestingly, when I spoke to someone in a government position in Tas recently, who was based in the north-east, their sentiment was that thylacine sightings from that region seemed to push west until they hit Launceston, and that they were being pushed by the clearing of forest in order to put in plantation forests - until the native forest was basically gone, and then the sightings stopped.

(It's always interesting when two separate accounts describe a similar pattern).

As to how many generations had passed ... consider this:

The last verified individual died 1936 in the zoo. The last verified thylacine in the wild was captured in 1933. I know of a sighting from about 1932 on the west coast. Nobody argues that the sighting was a mistake because everyone magically accepts it because it was before 1936. However, expeditions on the west coast and in central Tas found evidence of tigers - including footprints and vocalisations, at least through to David Fleay's expedition of 1945-46.

If the last wild thylacine was seen in 1933 and if the 1946 records truly reflect the presence of at least one thylacine, and if the maximum age span is 12 years (which is conservative - 8 years in the wild seems more likely), then the 1945-46 records are 12 to 13 years after the last accepted wild presence. That makes the 1945-6 record highly likely to be an animal that was born in the wild after 1933. In other words, (again - if all the evidence is true), thylacines were breeding in the wild after 1933.

Apart from the logic of just the above, many authorities now tend to agree the thylacine persisted in the wild through until about the 1950s at least.

Now add to this that the central and south west - where the 1932 and 1945 sightings come from - has only 2 major roads, is over 100km north-south, has stretches 80km x 20km that rarely see humans, and is almost identical to the environment it has been for thousands of years, and you have to ask why it isn't possible a small population persists there. Yes, we probably have more bushwalkers, but we also have sightings reports from bushwalking parties.

Finally, if you haven't read it, grab Col Bailey's second book, released this year, which includes a photograph of a thylacine manus (foot), coming from an animal allegedly shot in central Tasmania in 1990. It's quite a compelling photograph.

[molloch]

Thylacines having low genomic diversity is not an issue directly, as we have discussed before, but it is no good for long term survivability. It creates inbreeding and lowers adaptive response.

In this case, there could be very few breeding adults, and still maintain a population. The home range was most likely large, these are top level carnivores, and this alone would be enough to allow breeding between disparate individuals.

I think that there were probably still Thylacines left up until the 50's, probability alone suggests that Benjamin was not the last one. There are some excellent sightings in the 80s and 90s, and other evidence such as the foot mentioned above. Tasmania, though, is not what it was even 10 years ago. There are a lot more people, a lot less bush and a lot more food sources (road kill) and therefore more possibility for sightings. In the current environment, the Thylacine should have been able to breed almost unchecked. The population should be recovering and sightings should be more and more frequent. Given the number of cameras we all carry around now, it is ever more likely someone will get a decent pic or video - if the Thylacine is still out there.

They lived for about 7 years, we think, in the wild. If they were like most Dasyurids, they bred once a year and probably raised around 3 pups at a time (from historical records and bounties). They may have been able to reproduce in their first year, but possibly not. They would not have been semelparous like many small dasyurids.

If we take it that they reproduced at 2 years of age, had an average litter of 3, had a survival rate of 2/3 to reproductive age, bred yearly and that they had a reproductive lifespan of 4 years (between 2 and 5 years of age). If we start with 2 animals, and they bred at this very successful rate - which is probably possible for an animal recolonising an area if there is enough food. The potential population of Thylacines could now be (greatly simplified to a 30% population growth), if my very rough calculations here are correct, around 200,000,000 individuals. Ok, so maybe they struggled at first - there were a lot less pademelons around and the population dwindled until there were more cars and roadkill, in about the early 80's. This would still give the potential for around 10,000 individuals alive today. Maybe they only breed every second year - but the population can't be constrained by this alone. There would have to be some other pressure on them to prevent them from becoming more populous.

Habitat is restrictive, but only if you are being hunted or food is scarce - neither of which is now the case. For the population to remain unnoticed, the Thylacine would have to be walking a very fine line between keeping the population at a steady minimum, and becoming extinct. It would be very easy to trip over that line and be gone. As time goes on, we should be more and more likely to rediscover the Thylacine. That this hasn't happened yet, doesn't bode well for it's continued existence.

My argument here is that there must be some kind of pressure to keep the population small, or we would have noticed more Thylacines by now. Whatever that pressure is, it has prevented a theoretical population of a quarter of a billion Thylacines - that pressure must be significant, and if it is that significant it is just as easy to push the Thylacine into extinction. Could the population growth be much lower? Of course, if it was 5%, for example, there would only be 60 thylacines alive today from 2 founding individuals - BUT there would have been a period of 10 years where the population didn't get above 2 individuals (and 18 years before there was a population of 5). If there were 2 males born instead of 1 of each sex, or 1 more animal died before reaching reproductive age - that would have been it.

What is the limiting factor? Food - unlikely, they were not picky eaters. Home ranges - if so it is more likely that they would be infringing on farmland like at the end of the 1800's. Disease - but diseases don't really work like that, there would be herd immunity by now as all prone animals would have died out. Predation - they have no known predators - except humans, and we are not killing them anymore. Stress from humans - maybe, but this wasn't really the case back in the 1890s-1910s era, there are accounts of them following humans, and even being kept as pets.

Having said this, Quolls seem to have been able to hold on in the Grampians for 140 years without being spotted - but they are much smaller, have smaller home ranges, are under direct competition from cats and have nearby, extant populations with which to get reintroduced. We have no evidence that the recent quoll sighting came from a population that has remained hidden in the area for 140 years. It isn't impossible that it was transported from a nearby population in the Otways, etc.

[Deleted]

I read of some research by the National Geographic Society into pumas in the Rocky Mountains which showed that they maintained large territories, and small populations, by killing any competing puma that moved into their patch. Or being killed.

And I once heard an estimate of the current population of thylacines in southeastern Australia being something between 250 and 350, although that was before the Black Saturday bushfires.

[youcantry]

Oct 3, 2013 22:58:21 GMT 10 molloch said:

If we take it that they reproduced at 2 years of age, had an average litter of 3, had a survival rate of 2/3 to reproductive age, bred yearly and that they had a reproductive lifespan of 4 years (between 2 and 5 years of age). If we start with 2 animals, and they bred at this very successful rate - which is probably possible for an animal recolonising an area if there is enough food. The potential population of Thylacines could now be (greatly simplified to a 30% population growth), if my very rough calculations here are correct, around 200,000,000 individuals. Ok, so maybe they struggled at first - there were a lot less pademelons around and the population dwindled until there were more cars and roadkill, in about the early 80's. This would still give the potential for around 10,000 individuals alive today. Maybe they only breed every second year - but the population can't be constrained by this alone. There would have to be some other pressure on them to prevent them from becoming more populous.

Another way to look at this is that in the most ideal of situations - pre European invasion/colonisation/settlement - our estimates are a maximum population at any given time of 5,000 animals. Regardless of what factors are constraining the population I think it's entirely reasonable to suggest we cannot expect more than 5,000 individuals now. In fact, if you simply took the proportion of the state which is now occupied by human activity and assumed they now totally avoid humans (more on that below), the maximum we could expect will be less. For argument's sake, let's say the National Parks are more or less untouched. I believe 30% of the state is protected land? So at a rough calculation, let's assume a maximum state-wide population of 1,500. But let's take that further - if migration was important (and more than one pre-extinction source suggests it was) then you need contiguous untouched land. That's basically going to be only the west / south west. I don't know figures but let's assume 10% and now the state maximum can only be 500. I have no examples to hand, but surely there must be examples of other top-level predators persisting with numbers below 500 for several generations or years?

Oct 3, 2013 22:58:21 GMT 10 molloch said:

What is the limiting factor? ... Stress from humans - maybe, but this wasn't really the case back in the 1890s-1910s era, there are accounts of them following humans, and even being kept as pets.

Tim Flannery notes in his book "Throwim Legway", which is about his expeditions to Papua New Guinea in the 1980s searching for new mammal species, that on one occasion they collared 3 kangaroos by practically just walking straight up to them (ie the kangaroos had never seen humans and didn't flee). It was some *years* later they went back to locate the kangaroos and the kangaroos fled at the sound of their approach. That's a learned flight behaviour after just one interaction.

The short-eared dog of South America is described as "a very rare species from the Amazon rainforest that is largely unknown, even to naitive people". A fact sheet says "this species avoids developed areas, and there are no known cases of road kills, so the impact of vehicles on population numbers is probably minimal". It went for almost 20 years without any known sighting or roadkill or other evidence until rediscovery in the 1990s. With the thylacine we're not even claiming that level of absence - there are many people claiming sightings. See more here: www.wherelightmeetsdark.com/index.php?module=newswatch&NW_user_op=view&NW_id=463

Oct 3, 2013 22:58:21 GMT 10 molloch said:

Having said this, Quolls seem to have been able to hold on in the Grampians for 140 years without being spotted

That is a big assumption to make. More likely your later suggestion that this is a rogue visitor from a nearby population. But I guess this is the thing - while I might think a rogue visitor is more likely, we just don't know, do we? Perhaps they *have* sat there for 140 years undetected, just like Gilbert's potoroo sat there for 120 years undetected - and that despite at least 2 active searches for the species and all old-timers who were familiar with it saying there was no evidence anymore of it surviving. I think there were about 10 to 20 individuals when rediscovered? (Less than 40 now, at any rate). Let's see - they have 1 baby per year (with a second on hold in case the first fails). Females become sexually mature at the end of their first year and males may breed before the end of their second year. They live for 10 years. Can you do the maths for me and work out how many million should have been hopping around after 120 years without observation?

[youcantry]

Oh, and in about a dozen trips to Tasmania for a whole range of reasons, spanning work to visiting friends to field trips looking for tigers, I have only seen one single Tassie devil in the wild. The sighting lasted less than one second as it ran across a walking track in front of Michael, who was walking in front of me. Neither of us saw it well enough to be confident of what we saw - we had to exchange the details we each picked up before we came to the conclusion it was a devil. My earliest visit was a motorbike tour around the island in 1993 - about the time when DFTD broke out and devil numbers were estimated as high as 120,000 across the state. Even at their present low estimate of 20,000, that's a heck of a lot more than 500 thylacines. Tigerman, who has spent well over a decade searching and considering this issue, estimates a population size of 200 - at least about 10 years ago. That's 1% of the present devil low-estimate. Based on my sighting rate of devils, it should take me 100 times as many trips, or 1,200 trips to the Apple Isle before I see my first tiger. That's about consisted with the rate of sightings for people like Col Bailey and Tigerman who spend years searching. Or look at it another way - how many people see devils in the wild? (Probably not many, given how long it took me to see one). Well, approximately 1% of that number should see tigers.

Now - for the differences: we pick up devils on camera and in traps all the time. Here's where you come back to the contiguous land I mentioned, and the south-west in particular being most suitable and most likely. Recall the 1932 sighting was from the south west and I have 3 other pre-extinction accounts from that region. Now reduce the population size to 200 - or even just 120 per Tigerman's estimate for that region, put them in the 80x20km which sees about 4 Homo Sapiens every 2 years, and there's your answer. Natural land barriers in the form of large rivers and you've got the population more or less constrained. Yes, they're bound to wander out of that region from time to time, just like a quoll is bound to wander into the Grampians from time to time - resulting in an occasional sighting in areas still dependent on relative human inactivity.

That's basically as far as my theory can take me. Col obviously found his tiger on the outskirts of human habitation to the south of Hobart, and I believe that is dependent on the Weld Valley running from south-east to north-west - across one of the two roads in the south west near Maydena, and then up through the guts of the state where most of the bounty captures and most of the last tigers were caught and, incidentally, close to where Tigerman saw his tigers too. It's still south-west, but the habitat is more suitable than the west coast stuff I've been speculating on.

[molloch]

The top level predator is almost always population constrained by food availability. Although it is unclear as the the Thylacine's main prey species, it is most likely the Tasmanian Pademelon, but it would have also taken bandicoots, possibly Red-necked Wallaby, young wombats, devils etc. Territorial behaviour, reduced fecundity and restricted home ranges are all subsets of food availability. Before human settlement, the Thylacine population would have been constrained in this way.

Who says that pre-European settlement was the most optimum conditions for the Thylacine? Since European settlement, the prey species have widened and their numbers increased - one of the few places on Earth where this has happened. We have introduced more prey species - rabbits, chickens, ducks, sheep - and land clearing has increased the preferred habitat for the natural target prey species. Since the 1950s, hunting pressure has been removed on the prey species by a reduction in humans trapping them. More recently, easy meals have been created in the form of road kill, and even more recently the availability has increased due to the reduction of devil numbers.

Habitat loss, by itself, is not a direct constraint unless the species is so dependant on the habitat that it can no longer survive. In the case of koalas, tree kangaroos or possums, this is a constraint as they are obligatory tree dwellers. Larger kangaroos are obligatory grazers, so they need open grasslands, some wallabies are browsers, they need shrubs and bushes. In the case of the Thylacine, it is not. We know, from the fossil record, that Thylacines once lived in deserts, the Nullarbor plain, the tropics, grasslands and Alpine environments. We know from historical records that they frequented farms and approached human dwellings. They existed in a wide range of environments, and ate a wide range of prey.

So we need some mechanism to constrain the population. If it is habitat loss - why is this a constraint? Nesting sites? Hiding from humans isn't enough of a reason, and neither is preference. The reason is that in evolutionary terms, these things do not have a pay off. If an individual is born that is less "scared" of humans, or less picky about where it lives, it is going to have a greater reproductive potential - therefore the population will move towards being less scared and less picky - I talked about this a while ago looking at flight or fight responses and ESS (Evolutionarily stable strategies). If humans were actively killing Thylacines still, this would be a constraint - but the are not. And in the time since they were, the population should have recovered enough to be more visible.

So here is a question as a bit of a thought exercise and brainstorm: Can you come up with other mechanism that might constrain the population?

[mingle]

What a great discussion - some fascinating points are being raised.

Keep it up!

[obsidian351]

I've made a quick excel spreadsheet from 1933 to the present that starts with 2 animal's, it breeds pups and kills them off ( old age and random death ) every time you refresh it the data changes and almost every time it refreshes the population dies out around 1960, the points you raised about constraints will need to be also factored in and fine tuned, I also suspect I've over looked some other things because when the do make it to 2013 there are over 2000000 individuals

[tygeresque]

,,,

[tygeresque]

...

[Deleted]

Oct 4, 2013 13:15:16 GMT 10 molloch said:

So here is a question as a bit of a thought exercise and brainstorm: Can you come up with other mechanism that might constrain the population?

I recall hearing somewhere that Tasmanian Devils were known to prey on young Thylacines, so that could act as a brake on population expansion. The opinion has even been expressed that the current epidemic of Devil Facial Tumor Disease could result in a resurgence of the Tasmanian Thylacine population, as the predation pressure eases from this direction.

I've also heard that Tasmanian Aboriginal people hunted Thylacines, and customarily gave the meat to their women to eat, it being considered unfit for general consumption.

So while neither of these factors applies on mainland Australia today, they may have been a factor on Tasmania in the past.

[Deleted]

I agree that the discussion raised here has been most thought-provoking and interesting.
Just one question; going right back to the start of the thread it is assumed that Thylacines have been extinct since 1936.
Surely, though, Naarding's sighting of 1982 is reliable (it was certainly taken to be a sighting by the Primary Industries Dept at the time)?
If so, then we could assume a breeding population had survived at least until 1975-ish?
Does this alter result of a breeding model?

One comment about genetic diversity: apparently all cheetahs are as genetically similar as twins! Thus, the population survives on extremely limited genetic diversity and has done so for some time. There are problems with fertility (many sperm are defective) and disease resistance (if one succumbs; most of them do), yet they live on...

Cheers,

[obsidian351]

Oct 4, 2013 13:15:16 GMT 10 molloch said:

Can you come up with other mechanism that might constrain the population?

I imagine extreme weather events would play a part?

[Thylacoleo Gal]

This is a most excellent thread Questers! Many thanks to Molloch & YouCanTry for your very thoughtful comments. Obsidian gives us great insights with his spreadsheet population model.

Yes, it's well known that populations can soar skywards when resources are unlimited. They can crash just as quickly when resources run out. Some species are adapted to boom-and-bust cycles of repeated population busts but I'm pretty certain that this so-called R/k selection payoff only benefits physically small, fast breeding animals. Like rodents, for example. Molloch may correct me in this? I don't believe large large mammals, like say thylacines, would be able to bounce back from just a few breeding pairs after a series of population wipeouts.

Repeated cycles of inbreeding tends to reduce the litter size: ultimately, small litters will raise the odds that either no offspring will survive to adulthood or that a generation of same sex offspring will be born. Either case means the end of the line for that population. Thus inbreeding might be a mechanism to bring about extinction in the case of the thylacine.

However, I have an idea repeated cycles of inbreeding drastically reduces diversity only when the population is kept artificially low: that is, when the death rate is maintained unrealistically high over many generations. So high that, in realistic wild conditions, such a population would be killed off. All animals' genomes contain a large reserve of suppressed variability and if their numbers are free to surge to hundreds of thousands within 10 generations or so after a population crash, then all is well. If inbreeding in the case of small, founder populations were always lethal, then how do rabbits in Australia come to number in the hundreds of millions? How is it that cheetahs today are an apparently healthy population even though they passed through a "population bottleneck" only about 10,000 years ago, so I believe and as DrTom mentions? By the way, humans too are apparently a "bottleneck species" having been decimated globally not so long ago.

For 200 years mathematicians have a had a lot of fun devising equations to model populations:
en.wikipedia.org/wiki/Population_model

The best known, perhaps, is the Lotka-Volterra model. This consists of coupled equations that describe the dynamics of predator-prey population, where the "prey" is loosely defined as any sort of resource that a "predator" consumes. The important thing is that the "resource" is both finite and dynamic: the predators can kill it off, whereupon the predators themselves mostly starve to death. But then, once the predators numbers are few, the resource itself can grow back. The whole thing generates cycles of boom and bust, which are easy to visualise on the computer screen.
See "LotkaVolterra_2D.png" below.

A 3D phase space representation shows the oscillatory swings of boom and bust clearly. See "LotkaVolterra_3D.png" below.

I have an idea that all dynamically interacting populations would show such oscillations: that is, the difference between boom-and-bust fast breeding "r" critters and slower reproducing "K" animals is only a question of degree. The amplitude of population gyrations, the depths of crashes, is much less for elephants and humans than it is for mice or lemmings, and so we tend to think of the former as enjoying "big K" stability. But everything is relative in time and space.

One curious thing is that the cycles of boom-and-bust in the models depicted above are not stable over the long term. When they are run for 10,000 "seasons", one observes a slow slow decline: the population after each crash is ever so slightly less than its immediate predecessor. Ultimately, these boom-and-bust computer populations do die out. The parameters one plugs into a Lotka-Volterra model must be extremely finely tuned to yield long term stability. Question is, does the same thing apply in the real world?

Whatever. Population dynamics indicate that thylacines may very well have died out, post-1910s, through natural cycles of boom times followed by attrition: if their numbers fell below a critical threshhold then, sadly, they're all gone by now. On the other hand, these L-V models imply that natural selection cannot be constant over time: selection for or against a breeding pairs' reproductive chances will depend upon what stage of the boom-bust cycle the population is at.

In the thylacine's case, there are all those post-1940s sightings to explain away. We can only hope selection against them is relaxed just a little, in some hidden valley ... somewhere?

Images: LotkaVolterra_2D.png & LotkaVolterra_3D.png

[obsidian351]

here is the spread sheet I did up, don't crucify me it took 20 minutes and I'm sure its riddled with problems, feel free to point them out so I can fix it up

[Deleted]

Here is my crude Thylacine population model. I’m sure there are much “fancier” models available.
I started it in 1974 and it runs to this year.
Why 1974? Because I don’t doubt Naarding knew what he was looking at and it must have had parents, so I assumed there was a small but viable population at that time. I started it at 24. I'm sure 24 thylacines could live happily undisturbed in the vast forests of Tasmania.

I have assumed that they do not breed in the first 2 years of life.
So, M1 and F1 are the pups from the breeding females from previous season (F3, F4, F5, F6 and F7.)
I introduced a mortality coefficient that varies from year to year. In year one it is 0.25, thus, 75% survive their first full year.
Then M2 and F2 are M1 and F1 from the previous season, with a mortality coefficient of 0.33. That is, 67% survive to become M3’s and F3’s, and so on. (I did not introduce them from T=0 because the numbers are so small, but of course in real life they would be working.)

I have assumed that mortality rates are higher for pups and for the oldest (years 6 and 7) and somewhat lower in between, when they are “in their prime”. I did not consider reasons for mortality; the net loss from all causes is what is important.

If we assume that the peak population was somewhere around 4,000, then the population produced by the model is still well below that level.
If the mortality rates were higher (they may well have been) or fecundity rates were lower, then the total would be even lower.

Of course, we can model numbers all day and none of it proves a thing! But it’s fun to do and it shows that it is possible for thylacine numbers to at least partially recover, even from a population of only 24. Let's hope they have done so.

Attachments:

Thylacine Population Model.xls (34 KB)

[obsidian351]

That is a valid point, its a sort of lottery, it does show potential for survival of the species but no amount of number crunching (in excel anyway) can accurately reproduce nature,
Can anyone help me upload this?

[obsidian351]

More questions, this time on convergent evolution,
Did the thylacine sweat? Or pant like a dog?
I'm under the impression that marsupials sweat, can anyone clarity?
Sorry for the short replies im on my phone most of the time

[Deleted]

Kangaroos certainly don't sweat; they lick their forearms and the saliva evaporates, cooling the blood under the skin.
Not sure about thylacines.
Given the close physiology to canines, it may well have employed panting to cool itself.

[tygeresque]

thylas pant like a dog

[Thylacoleo Gal]

Oct 6, 2013 17:16:34 GMT 10 @drtom said:

Here is my crude Thylacine population model....

Got it, thanks Dr Tom. Will take a look.
There's one: Obsidian was having trouble with attachments, I think. Let's know?

[obsidian351]

Oct 7, 2013 6:36:54 GMT 10 Thylacoleo Gal said:

Oct 6, 2013 17:16:34 GMT 10 @drtom said:

Here is my crude Thylacine population model....

Got it, thanks Dr Tom. Will take a look.
There's one: Obsidian was having trouble with attachments, I think. Let's know?

yeah its being a pain, I'm running windows 8.1 beta so that may be causing the issues for me

[Deleted]

Just watching ABC 24 (I have it on in the background) and they ran a short item about Col Bailey and Nick Mooney.
NM admitted the thylacine "may still be ticking along" and showed a recent footprint cast that he judged to be "either a thylacine or a hoax".

[molloch]

According to Tyndale-Biscoe, Kangaroos can and do sweat, they also pant and lick their arms to wet them as Dr Tom suggests above.

The smaller desert dwelling Dasyurids, such as Mulgara, do not sweat in order to retain moisture in very dry climates. Tasmanian devils do sweat according to research, I would assume that Thylacines could too, but there is really no way to tell. Sweating is a general (plesiomorphic) trait of mammals.

Panting does not have to be open mouthed. An increase in respiratory rate is enough to create a cooling effect, especially in an animal with large nasal passages, like a kangaroo. Panting is essentially another form of evaporative cooling, like sweating, and just requires a mouth and lungs, probably every mammal can do it. I have seen Spotted tail quolls panting (at a wildlife park).

References: Tyndale-Biscoe, The Life of Marsupials

[youcantry]

Oct 6, 2013 19:43:01 GMT 10 obsidian351 said:

Did the thylacine sweat? Or pant like a dog?

Aha! Someone else has come up with the same question!

There is a tribe of native North Americans known for their running prowess. I mean these guys run 100km without stopping, barefoot, even when they're aged 80. I'm not joking. But one interesting tidbit I learned is that they used running to hunt down deer. The logic is that humans perspire, and running creates a breeze over the perspiration which cools us. Deers pant, but you can't pant while you run. So if you chase a deer long enough (about 20 minutes), it overheats and then collapses, unable to run any more. You run up and claim your prize. You don't have to be fast, you just have to be persistent.

Of course, this kind of persistence is something the thylacine has often been described as having - which is what led me to this question.

So here's what I found out when I researched it. There is a 1972 paper by A. J. Hulbert and R. W. Rose studying similar questions about the Tasmanian devil. The abstract sums it up:

1. The cutaneous water loss, respiratory rate and rectal temperature of the Tasmanian devil (Sarcophilus harrisi) was measured before and during heat stress.
2. There was a sevenfold increase in respiratory rate under heat stress indicating panting, whilst cutaneous water loss increased by only 50 per cent and remained below levels considered indicative of thermal sweating.
3. No marsupials are now known to use sweating as a thermoregulatory response when at rest. This response appears to be restricted to the primates and some ungulates.

I also found that it seems thermoregulation was never studied in the thylacine. Although the belief is "no marsupials use sweating for thermoregulation" I had been hoping that primarily herbivores had been studied, or that most carnivores were so much smaller than the thylacine that it might not be fair to extrapolate the same conclusion to the thylacine. However the devil is the next largest carnivore after the thylacine and it, too, is known for its persistent, long-ranging habits (eg. 50km per night). So in the devil we have the next largest carnivorous marsupial exhibiting similar long-distance, persistent running capacity and a paper showing that panting increased 3.5x more than perspiration increased.

I guess if you wanted to take the question one step further you'd have to ask: how *efficient* is panting/perspiration in regulating devil temperature? For example, even though panting increases 7-fold, what if panting contributes only a small percentage to thermoregulation? Then increasing sweating by 50% might have the more significant effect. There must be some maths we can propose to speculate on what percentages are required in order to make the observations feasible?

[youcantry]

Regarding other points in this thread:

You're right - who's to say the most ideal conditions were pre European settlement? It's my assumption that European disturbance negatively affects thylacine population size but it's true other species increase in number with the same disturbance.

While cheetahs persist, haven't things like wolves died out in places like the UK? This is really not my area so maybe someone can enlighten me but my question is basically - why did they die out there?

The boom and bust cycle concept leads to some interesting questions: 1) having had sightings in the late 1980s and earliest 1990s now seem to largely disappear through the 2000s and early 2010s, are we just on a bust cycle, and can we expect another boom? 2) Where on that cycle were thylacines in the late 1700s when we estimate their population size at about 5,000? On a boom? Or a bust? Your chart suggests a majority of time is spent in the "bust" half - so do we have a better than 50% chance that "5,000" is an underestimate of the number of thylacines Tasmania could have supported pre European settlement?

This discussion warrants mention of Professor Henry Nix's 1990s research in which he classified habitats as optimally, sub-obtimally and marginally suited to the thylacine (based on bounty payouts), then analysed credible sighting reports against (then) present day habitats and found that his computer predictions for (then) present day thylacine presence matched credibly sightings reports almost perfectly. See: www.naturalworlds.org/thylacine/history/extvssurv/extinction_vs_survival_7.htm