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Why has the Bactrian camel evolved to have two humps instead of the one that the dromedary has?

I searched on Google and I found nothing about it.

All I found was that the that humps store fat, that the fat can be converted into energy and water, that this lets them survive in deserts. As far as I know this is true also for dromedaries.

I also found that the Bactrian is smaller than the dromedary, that Bactrians are less numerous and they are at risk of extinction.

No site explains why Bactrian camels have two humps instead of one hump, as in the dromedaries.

Sources:

https://www.google.com/search?channel=fs&client=ubuntu-sn&q=why+do+bactrian+camels+have+two+humps#bsht=CgRmYnNtEgQIBDAB

https://en.wikipedia.org/wiki/Bactrian_camel

https://en.wikipedia.org/wiki/Dromedary

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    $\begingroup$ Why do you think there needs to be a reason as opposed to say genetic drift of two different lines? $\endgroup$
    – John
    Commented Apr 19, 2023 at 0:03
  • $\begingroup$ @John Of course there was a genetic drift, but why dromedary was successful? I suspect that the fact that it's more small make it more adaptable to desert conditions. But I suppose this is another question. $\endgroup$
    – Marco Sulla
    Commented Apr 19, 2023 at 18:13

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First of all, it seems that the Bactrian is not smaller than the dromedary, and that dromedary and the Bactrian camel evolved from a common ancestor with two humps. So it seems it's the dromedary that evolved to have only one hump, and not the contrary. I quote Wikipedia:

The modern dromedary probably evolved in the hotter, arid regions of western Asia from the Bactrian camel, which in turn was closely related to the earliest Old World camels.

It seems that the dromedary hump can store 80 pounds of fat: https://www.nationalgeographic.com/animals/mammals/facts/arabian-camel

I failed to find how much fat a Bactrian hump can contain. A male Bactrian weights 590-1000 kg. Dromedary weights 400-690 kg.

So it seems a dromedary has only one hump because on average its weight is less than the Bactrian.

We can assume that the ancestor of the dromedary was a smaller camel with two humps. Over time, probably the second hump became smaller and smaller, until it disappeared. From Wikipedia:

the dromedary foetus has two humps, while in the adult male an anterior vestigial hump is present.

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    $\begingroup$ Be careful with conclusions. Does it only have one hump because its weight it lower or is its weight lower because it has one hump only? I tend to the second possibility. $\endgroup$
    – Chris ♦
    Commented Apr 18, 2023 at 20:32
  • $\begingroup$ @Chris good observation. I think you're right. I don't know if we have dromedary fossils, but I suppose a big camel with only one hump have smaller opportunity to survive than a smaller camel with two humps. I'll edit the answer. $\endgroup$
    – Marco Sulla
    Commented Apr 19, 2023 at 17:51

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