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Sea turtles are miraculous.
First, they've been around since the late Jurassic,
roughly 150 million years ago.
Cohorts of the dinosaurs, sea turtles have survived through the challenges of eons,
existing still today, where many others have ended their evolutionary run.
Second, throughout the centuries and up til today,
every living adult sea turtle has overcome the odds,
existing as a consequence of chance,
skill and capability.
The gauntlet each sea turtle faces in the course of its lifetime goes thus:
First, deposited as a clutch of leathery, ping pong ball-sized eggs
into a nesting pit dug by its mother high on the beach,
Of the 50 to 200 eggs laid,
roughly 20 percent will never hatch.
Roughly a month and a half after having been laid,
the surviving eggs hatch
and the young turtles, each small enough to fit in the palm of your hand,
Squirm to the surface, emerging from the sand en masse
and making their desperate dash for the sea.
Along the way, debris, pitfalls, crabs,
gulls, raccoons and other threats
will claim roughly 50 percent of those
who rose from the sand.
For those that actually reach the surf,
they trade one set of threats for another,
as they first face the repelling force of the waves,
and then find a whole new host of predators awaiting them.
various fish, dolphins, sharks and sea birds,
as the young turtles come to the surface for air.
For their first few days of life,
should they count themselves amongst the living,
the vulnerable turtles swim frantically forward.
Ultimately, they will often look to settle in a patch of flotsam.
preferably a patch of floating seaweed.
Now, for the next several months, they will seek to avoid those that would eat them.
find that which they might eat themselves,
and not fall to the pressures of challenging weather or unfortunate currents.
In this phase, roughly 50 percent of those who reach the surf
will perish.
Ultimately, with the passage of years,
the survivors will increase in size,
from that of a dinner plate at year one to that of a dinner table,
in the case of one species at least, the Leatherback,
a decade or so later.
With size comes some measure of protection.
The only truly worrisome predators now are some of the larger shark species --
bulls, tigers and whites --
and the occasional killer whale.
At approximately two decades of age,
the survivors will be old enough themselves to breed,
and continue the cycle which their very existence heralds.
Of those that began as eggs on a distant beach,
now less than 10 percent remain,
at least, those were the odds prior to significant human interference.
Over the past century, and in particular in the last several decades,
Human endeavors, from beach development
to plastic refuse to poaching, long lines, nets,
and even noxious chemicals, including oil,
have upped the ante for sea turtles,
causing their survival rate to drop to around one percent or less
from each nesting cycle.
It is this added human pressure which has pushed each of the eight sea turtle species
to either a threatened or endangered state.
For while they have evolved to overcome a host of obstacles,
the most recent has arisen so quickly
and at such scale that the species find themselves
overwhelmed.
So let's quickly recap this cycle of odds.
using a hypothetical nesting season,
for females may nest multiple times in a single year,
of 1,000 eggs, for sake of ease.
One thousand eggs laid.
Eight hundred hatch.
Four hundred make it to the water.
Two hundred progress toward adulthood.
Twenty survive to breeding age -- that is, without human interference.
Two survive to breeding age with human interference.
So a breeding adult sea turtle is the very embodiment
of a longshot. It is the exception, not the rule.
A jackpot. It is, in a very real sense,
a miracle.
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【TED-Ed】海龜的生存之道 (The survival of the sea turtle - Scott Gass)

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VoiceTube 發佈於 2013 年 3 月 21 日
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