Elephants in Australia

As to whether elephants would abandon their long-life/fewer offspring strategy, I'm kind of feeling my way along with this, with no big commitment either way.

I feel like I'm arguing with you for the sake of arguing with you, and I don't feel terribly comfortable doing that. For what it's worth, you're allowed to make assumptions and draw conclusions from that. While I may be doctrinaire in my assumptions, they are simply that.

Let's take it from the Point of View of the Stegodons. They become dwarfed on whatever island they were on before the get to Australia/New Guinea.

At a guess, I would say that the most likely candidates are as follows:

Stegodon Trigonocephalus, found on the Island of Java. Weight up to 3500 lbs.

Stegodon Elephantoides, also found on the Island of Java. Weight unknown, but probably comparable to Trigonocephalus.

Stegodon Sondaari, 900,000 years ago, on Flores, Timor and Sulawesi. Weight about 660 lbs.

Stegodon Florensis, 850,000 years ago, also on the Island of Flores. Weight about 1900 lbs.

Stegodon Florensis Insularis, concurrent with Homo Florensis, and from Flores. Weight about 1200 lbs, described as intermediate in size between Sondari and Florensis.

Based on what I can determine, it looks like the originating species were Elephantoides and Trigonocephalus. They may well have been contemporaries, or at least, I haven't found a sequence which would put one first, and the second as a successor or replacement. Their presence in Java puts them throughout Sundaland, probably a million years ago or more.

I also don't have much of a clear idea as to which of them may have been the root stock which produced the first generation of Island Dwarfs, seen in Sondari

But we could assume that either or both of the full sized specimens, Trigonocephalus or Elephantoides (take your pick) was a strong enough swimmer to colonize outer islands like Flores, Timor and Sulawesi. Given that the Islands were relatively small, they might well have leapfrogged across Islands several times before Dwarfing set in. So you might have full sized stegodons reaching the Australian mainland. It would most likely be in the era of the full sized Stegodons when Australia's mainland is most accessible - this is when Sundaland was a single contiguous body, and the jumps across the Wallace line and Lydekker lines would be shortes.

Or you might have Sondari, the smallest of the Dwarfs, jumping from Flores. But I think this is unlikely. That small might have left them too small and weak to cross the divide to Australia. I count this as very unlikely.

Florensis which replaces Sondari isn't a Dwarf species, but it's dramatically reduced, about half the size of Tirgonocephalus, though still three times the size of Sondari. I think that the implication here is that Island Dwarfing was already taking place somewhat, when Florensis made it to Flores and pushed out its smaller cousin. That being the case, I see, Florensis as a very good candidate to hit Australia.

Finally, Florensis Insularis, the 1200 pound dwarf species, is probably the most recent candidate. But again, I think that the sequence of long term isolation makes it most difficult. The Lydekker line is wider and deeper.


Their main predator is a big monitor lizard. While their smaller size would allow them to get to adulthood, and start having offspring more quickly, less pressure from predators means they don't have to, and if they did, they would over-populate and kill off the plant they depend on.

One of the notes I looked up suggested that it was predation from Monitors that kept Insularis from reducing to an optimum Island size. They needed to be at least 1200 pounds as adults to ward off the Komodo dragons. The major threat, of course would be to the Juveniles, so you'd have seen a reinforcement of traditional behaviour models to protect the juveniles.


So, when they get to Australia, the Stegodons are still dwarfs, but they breed and grow at a rate that assumes they're invulnerable to predators as adults.

My thinking is that Trigonocephalus, Elephantoides, or Florensis might be the best candidates. And Sondari and Insularis are the worst candidates.

Of course, the bigger specimens might find it hard to sustain themselves, to perhaps the intermediate sized Florensis is the best bet?


[quote[When they get to Australia though, they probably wouldn't be invulnerable to predators as adults. Australia had some very large monitor lizards--up to twenty feet long. It also had marsupial lions (Thylacoleo). There is some controversy over how big Thylacoleo was, with some authors claiming it was actually panther-sized and others claiming lion-size. More recent studies tend toward lion-size. [/quote]

And of course, sea crocodiles, as big then as they are now, or bigger.
I think that even Florensis adults, particularly when organized socially, would be relatively immune from predation. The big challenge, of course, are the vulnerable juveniles.

Once they reached Australia/New Guinea, our stegodons would face increased pressure from predators. They could respond by getting bigger again or by faster reproduction.

Growth is probably related to available food supply and resources.

Bigger runs into a problem: Australia is resource poor,

How unstable is the Australian climate? Actually, in the sunken Sahul region, and northern Australia, there would probably be hospitable climate and sufficient resources to maintain a population. On the other hand, I'm not sure that Stegodons could survive in what is now the Great Desert. You might see Stegodons confined to being a northern species.


and New Guinea becomes an island too small to support elephant-sized animals during each interglacial. From old, and possibly faulty memory, the biggest Australian mammals were around a ton, and the biggest New Guinea mammals were less than half that size.

Tricky. Sri Lankan elephants are almost full sized. It's not necessarily the size of the Island, but the availability of suitable habitat within that size.


Depending on how early Stegodons arrived, they might develop into multiple species. Each interglacial would cut New Guinea and Tasmania off from Australia, isolating Stegodon populations on the islands.

I'd be skeptical of the range extending all the way to Tasmania. But possible.

Much would depend on the period of the colonizing event.

One thing to worry about though, is your starting pot. Every Island Hop reduces the genetic diversity of your starting population. So for instance, the initial population that spreads through Sundaland may represent 99% of the Genetic Diversity of the region. The population that makes it to Flores, may represent only 2 to 3 per cent of that diversity. The population that makes it to Australia from Flores represents maybe .02 to .03 per cent of that diversity.

Or put it another way - the Australian founder population may be only a half dozen animals, who are the inbred descendants of the half dozen animals who were the Flores founder population.

Lack of genetic diversity gives us all sorts of problems, including lack of phenotypical diversity and expression, much less mutation, much lower rate and chances of speciation.

So an open question as to how much diversity and speciation you'd see.

I suppose given a time frame of over a million years, you'd see some. If the time frame is less than 850,000 years you'd see less.

Something to think about.

Each Ice Age would reunite the islands with Australia. Typically, when animals with similar habits encounter one another, either one wins out or they subdivide the niche in some way. I don't know which way it would go in Australia/New Guinea, but given the probable superiority of Stegodons or Diprotodons (the big Australia/New Guinea herbivores) I could see multiple types of Stegodons, each taking over from a different species of Diprotodont.

I'm not sure that there were multiple species of Diprotodont. I think that the most recent research indicates that the variation of large and small forms may all be the same population, and that the Diprotodonts might have followed a bull/harem model of very big males and very small females.

On a related note - have you given any thought to the notion that Stegodonts as a cornerstone or keystone species might well affect their environment in a way that promotes or fosters other species. Diprotodonts for instance, might actually do better with Stegodonts around.

This is actually more significant when humans come along. It's now considered that human's use of fire to manipulate habitat may have been what killed much of the megafauna - habitat destruction. You might Stegodont activities counteracting human ones, and the survival of other Australian megafauna. Of course, Dogs would be the big game changer.
 
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