Weddell seal pups are born on top of the sea ice in Antarctica, and spend a lot of time with their moms in their first 45 days or so. They’re nursing, getting fat on their moms' milk, and learning how to swim with their moms' help. And they’re probably learning how Weddell seals behave by watching their moms and other seals interact around the colony.
But when the pups are around 50 days old, their moms leave them on their own to survive with what they’ve learned and the weight they’ve gained from all that nursing. The fact is that only around 20% of the female pups born in a cohort—born in the study area pupping colonies during the same pupping season—will survive to around 7 or 8 years old, which is when most females start having pups. During that time they will grow in size from a small adorable pup to an animal the size of a horse, and must be healthy enough to gain enough weight to be able to produce a pup and do the really big job of caring for that pup (like giving up half of their body weight during the pup-nursing period).
What makes the difference between surviving, growing, and being healthy enough to have a pup themselves—or dying before they’re old enough to do that? It turns out that who a pup's mom is can make an important difference in whether that pup survives to reproduce, or doesn't.
In one way, all Weddell moms are exceptional. They are among the relatively small group of females that live long enough to return to the colonies and reproduce. But within that small group of Weddell females that return to give birth, there are a lot of differences among those moms; and those differences can have an important influence on whether their pups survive to return, and have pups of their own. What scientists working over the years on this National Science Foundation funded Weddell seal population study have learned has come about as the result of a nearly 5 decade-long population study and its extensive multi-generation database.
The ways these project scientists have learned about what differences in Weddell seal moms can mean to their pups, is the result of decades of tagging nearly every new seal pup born in the study area to know how many pups are born each year, who their moms are, how much some of the pups weigh at birth, at the mid-nursing period, and then at weaning, as well as how much some of the moms weigh when they come in and give birth. The scientists are very interested in learning more about how much weight the moms lose during the nursing period.
Over the past 6 years researchers have also been using small temperature/time recording tags on some female pups to measure how much time those pups spend in the water, where they can develop swimming skills, learn to avoid larger animals, dive and search for food, and other important skills for life on their own.
For female pups born 10 years ago or more, the researchers will be able to assess which pups have survived to come back and produce pups of their own. This can be done because for this special population it is possible to tag and record every mom-pup pair every pupping season. The individual tags on the animals let the researchers know who’s who, how old they are, who their mothers are, and how many pups they’ve had to date. For female seals that received small temperature/time recording tags when they were pups (the small red tag in the photo above), the scientists will also know how much time those seals spent in the water when they were pups.
Using these data, the researchers apply powerful statistical methods to study what differences in moms can mean to their pups’ future survival and reproductive adulthood. Here’s some of what the scientists have learned about the ways in which moms differ, how they matter to their pups, and what makes a good mom.
First, a pup would want a mom that was born into a large birth cohort herself as such females typically experienced good environmental conditions early in life, making them prone to success later in life. Then, a pup would want a mom who came into the breeding colonies in the years before her first pup birth to possibly learn more seal behavioral skills from moms already having pups.
A pup would want a mom who, the year before that pup was born, had come into the colonies to select a good male to mate with, while she synchronized her breeding cycle with the other females. This way the pup would be born when the rest of the pups in that cohort were being born the next pupping season, which can be important for many reasons.
A pup would want its mom to have good marine conditions with plenty of food resources after she mated so that her pregnancy would begin. Weddell seal females are what are known as “delayed implanters”—the fertilized egg doesn’t implant in the mom’s womb until a month or longer after she mates. She somehow physiologically “chooses” whether the egg will implant to allow the embryo to begin to develop. Studies suggest that the availability of marine resources—food— and the female’s resulting physiological condition affect whether or not the fertilized egg will implant.
A pup would want its mom to be of prime age—somewhere in her early to mid teens—as that’s when females tend to have (1) experience at raising pups, (2) their maximal body weight, and (3) no obvious signs of age-related deterioration. The Weddell seal population scientists have learned over the course of this long-term study that Weddell females give birth to their first pups at different ages. Some give birth as early as 5 years old, but most females have their first pup around 7 or 8 years old. Those females that give birth earlier in life seem to be especially good at producing pups regularly over their lifetime and are more apt to produce pups that thrive.
The average mom will have around 7 pups over a typical lifespan of 16 years or so. She would have 1 to 2 pups about every 3 years. Yet some moms live much longer—into their 20s, and even 30s. Some females never do have a pup, while some have many pups over a longer lifetime. In the study area, there was one mom who was 31 years old and had 22 pups during her lifetime!
So moms differ a lot as to when they first give birth, how long they live, and how many pups they give birth to over their lifetimes. What project scientists have learned is that your ideal mom would likely be a mom that started having pups at an early age, has had lots of experience raising pups previously, and is of prime age.
A pup would also like its mom to have skipped having a pup the previous year so that she is in great body condition with lots of stored energy in the form of more body fat the year the pup is born. After it's born, the pup would want a mother who spends lots of time with her pup nursing—transferring lots of weight to her pup throughout the nursing period.
A Weddell pup would want its mom to have lots of experience raising pups so she is good at helping her pup learn to swim when the pup is around 1 to 2 weeks old, and will do a good job raking the sea ice with her front teeth to make ice ramps that can help her pup get into and, especially, out of the water. Since raking ice can wear down a mom’s teeth over the years, a pup would want a prime age mom with good, sharp teeth.
When a Weddell pup is born, it will weigh anywhere from 40 to 70 pounds. After the roughly 35 to 45 day nursing period, it will weigh anywhere from the low 100s up to 300 pounds, or more! The project scientists have learned that how much a pup weighs when it's weaned is related to characteristics of the pup's mom.
One difference among Weddell moms affecting how much weight their pups gain is the age of the mom. Pups of younger moms start out lighter and tend to gain less weight. Pups of the oldest moms also start out lighter, but have mothers near the end of their lives that invest heavily in their pups and wean their pups at similar weights to pups from prime-age moms. So, the scientists hypothesize that a pup should be better off with a mom of prime age or older and hope to test that idea in the coming years.
While with many large long-lived mammal species, scientists have learned that weaning heavier is better for offspring survival, this may be more complicated with Weddell seal pups. For Weddell pups the nursing period—when pups are closely interacting with their moms and learning new skills like how to swim and get in and out of the water, and other behavioral skills—is longer than in most other seals.
Project scientists are now studying whether there is some kind of tradeoff between how much weight a pup gains during the nursing period versus the pup swimming a lot and possibly learning more survival skills from its mom underwater, but gaining less weight due to all that activity. The scientists are using small temperature/time recording tags on some of the pups to measure how long and how often they are in the water, and weighing those pups to learn how much weight they gain, so that the researchers can later learn which pups went on to survive to have pups themselves, "recruit".
One of the most interesting aspects of differences among moms that the project scientists have recently discovered is that after all the different measurable characteristics of moms are accounted for, some moms are just better at having and raising pups than others, and this is a fixed characteristic of those moms. This fixed characteristic of just being better at having pups remains the case whether it is good year for pupping or a far more challenging year, such as when a massive iceberg blocked access to the pupping areas in the past decade.
Using all of these valuable data collected over the decades by researchers, coupled with newer, more powerful approaches to statistical modeling, project scientists are able to learn more about these characteristics of Weddell moms that influence how likely their pups are to survive and grow up to be moms themselves. And, we’ll gain an even better understanding of how maternal investment in pups relates to differences in Weddell moms and changes in the environment.
One thing is certain from this long-term population study to date: if you’re a Weddell seal pup, who your mom is can make important differences in whether you will be among the exceptional 20% of female pups to survive and return to the study area pupping colonies to have pups of your own.