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Milk flag
This journal brought to you by:

  • Braden Hendry in 5th grade at Crosswind Elementary

A Weddell Seal’s Milk

What would it take for a Weddell seal pup to go from this…

New born pup
Take a look at this cute nearly, new born pup. Photo credit: Alex Eilers.

to this…

Older pup
This pup is several weeks older. Photo credit: Alex Eilers.

in only 6 weeks?

What makes a seal pup grow so big?
What makes a seal pup grow so big? Image credit: Lindsey Newton.

Human babies usually triple their body weight in the first year of life, which is pretty remarkable, because it is the fastest healthy growth period of our lives. But it is nothing compared to the first 6 weeks of a Weddell seal pup’s life! During that time, Weddell seal pups multiply their weight around 4 times. That means if a seal pup weighs about 50 pounds at birth, it could weigh 200 pounds after just 6 weeks. That means the pups can gain 150 pounds! Yikes! That’s some serious weight gain! Let’s find out how and why this happens.

Let's take a look and a mom nursing her pup.

MILK!

That’s some pretty fast weight gain, so I have to ask, ’What’s in that milk?’

Milk
Compare the milk of a Weddell seal mother and a human mother.

Both Weddell seal milk and human milk contain four main parts:

  • Water
  • Fat
  • Protein
  • Sugar

But it’s the percentage of fat that makes the biggest difference. Weddell seal milk is 54% fat compared to only 3.8% fat in human milk. That’s over 14 times more. This means more growth and faster growth!

Protein

Another difference between Weddell seal milk and human milk is not just fat, though! The graph also shows us that Weddell seals have 10% protein in their milk, while humans have only 1% protein. While most protein goes towards new lean tissue growth, another reason seals need more protein is that their bodies are covered in fur, and protein is needed to help grow that fur. Seal pups start out life with a really soft, fluffy, brownish coat of fur. That coat is called the lanugo, and it insulates the pup by trapping air around the fuzzy fur, acting like a down blanket to keep the seal warm. Why do pups have this lanugo?

During the first few weeks of life, pups do not have much blubber, so the lanugo is their primary source of insulation. As the pups grow, they build their blubber layer, which then becomes their primary source of insulation. They then shed their lanugo for their new, short, sleek, silvery-gray coat. Protein is the key to this new fur coat growth!

Amount

Another reason Weddell seal pups grow so fast is that they consume so much more milk. Let’s look at how much milk a seal pup and a human infant need to drink each day.

An average human infant consumes ~25 fluid ounces (~750 mL) of milk each day. But seal pups are born much larger than human infants, and they grow a lot faster, so they need more milk. They consume ~3.75 quarts (~3549 mL) each day. That’s almost 5 times as much in volume as a human baby consumes!

Milk amounts
Why do Weddell pups need so much more milk? Image credit: Lindsey Newton.

Caloric Density

All these differences add up, and it comes down to calories. A calorie is a measure of how much energy is in food. You’ve probably noticed that a lot of your food comes with a nutrition label on the container. That label tells you what is in your food, including fats, protein, carbohydrates and CALORIES!

So let’s calculate the difference in calorie intake of the two mammal infants:

First, we need to know the caloric density of both types of milk:

Caloric density
Caloric density here is measured in kilocalories per 100 milliliters. Image credit: Lindsey Newton.

Next, let’s multiply those numbers by the volume of milk, in milliliters, each infant drinks per day:

Math
Let's do the math! Image credit: Lindsey Newton.

Now, we can divide to find out how many times more kilocalories Weddell pups get than human babies:

Math
Let's do the math! Image credit: Lindsey Newton.

That’s quite a bit more kilocalories per day! It’s no wonder these seal pups are growing and changing so quickly!

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