Are giraffes living on Calauit island an example of Island dwarfism?

Question

When I traveled to Calauit island I was stricken by the small size of giraffes living there. The giraffes are the descendants of Africa animals imported in the 1970s.

Like Noah and the biblical ark, a boatload of animals was shipped from Africa to remote Calauit Island in the Philippines in the 1970s

The descendants of these animals—20 giraffes, along with dozens of zebra and antelope—are now among the most intriguing legacies of the late dictator Ferdinand Marcos.

The following picture will give you an idea of how small they are (my wife's height is 160cm). I asked the local guides why they were so small (around 3 meters height, some even less than 3 meters), they told me because they are island born and they have become smaller compared to their ancestors.

To me this seems to be a perfect example of Island dwarfism. But when I google it I can't find any result of this. The only result I got was the dwarf giraffes found in Uganda. The science.org even used the title "World's first dwarf giraffes spotted in Uganda and Namibia", also check NYTimes article "Dwarf Giraffes Discovery Surprises Scientists" for detailed information of that finding.

So are Calauit giraffes an good example of Island dwarfism? If not, what is the explanation of their small sizes, e.g. does the NYTimes article's words "The condition, also known as skeletal dysplasia, affects bone growth, often resulting in short stature." apply here ?

PS. the original article NYTimes refers to was published at springer nature under a Creative Commons Attribution 4.0 International License, so you should be able to read it free.

Answer 1

Update

Updates to question and comments clarify:

• 3m high giraffes (small)
• locals say giraffes are smaller because they are "island born".

The first data point indicates that the giraffes are in fact smaller than usual (usually >4m).

Being "island born" would be considered as an environmental perturbation. This would indicate malnutrition or something similar, covered under my 4th hypothesis below. Granted, it is not clear to me what exactly the local folks meant.

Original answer

Island dwarfism is a natural macroevolutionary process that is presumed to take a very long time due to long-term ecological pressures. The example you provide appears to be one in which humans took some giraffes to the island 50 years ago, or ~5-10 generations of giraffes (estimates differed in their generation time).

The body size evolution of distinct island varieties is measured in the millions of years. Based on Table 1 from the linked paper, I think that I interpret that the expected rate is something like a 10% size change per million years.

In other words, if those giraffes were island dwarfs, that would be an astonishingly fast natural adaptation. I can think of several possible explanations other than island dwarfism as we generally think of it:

• The giraffes you observed were normal sized. Average height for a female giraffe is ~4-5m, or ~3m at the shoulder. Their shoulder height at least does seem a bit short in the picture, but it might be a weird perspective thing.
• The giraffes you encountered were small, but that's because they were immature and had not reached full stature. (No dwarfism.)
• The giraffes you encountered were derived from a variety with relatively small stature. (There is some variation, especially the females are not so tall, at 14 feet maximum height). Supposedly the animals were from Kenya which I believe are one of the bigger varieties, but who knows.
• The giraffes you encountered were dwarfed by poor nutrition due to a suboptimal habitat (developmental loss of stature, not evolutionary).
• There is genetically determined dwarfism / short stature, but it's due to genetic drift (the original giraffes had substantial dwarfism, as has been observed in natural populations, as you note).

I would probably want to exclude all of those hypotheses (and perhaps more) before I would say anything about island dwarfism. Evolution usually happens really slowly, unless it's acting on existing variation!

For a general description of the evolution of island dwarfism/giantism, see here.