When it comes to whales and dolphins, there is usually some confusion. Are they fish? Are they mammals? But does it even matter what group these animals are in? Well, one thing is for sure… it does matter! As much as they can seem similar, fish and marine mammals have a lot of different features.
Among other differences, the way they carry their babies and nurse them are among the main differences between these two large groups of marine animals.
Do whales and dolphins lay eggs like fish? Keep reading to find out!
Do whales & dolphins actually lay eggs?
They are marine mammals
Whales, dolphins, and porpoises are what we, marine scientists, call cetaceans. Basically, cetaceans are marine mammals that are fully adapted to life in the ocean. Thus, as mammals, cetaceans are closer to us than fish. Why? You’ll understand this in a bit.
As already said, fish and marine mammals do not share the way they carry their babies or their nursing behavior. But still, can whales and dolphins lay eggs? In fact, marine mammals carry their babies in the same way as we do, in an amniotic sac inside the mother’s womb.
Indeed, whales and dolphins have internal fertilization, carry their babies, and give birth to live young. Besides, marine mammals take care of their babies once they are born, and they breastfeed on the mother’s milk, whose compounds are a bit different from those of humans.
Did you know
Cetaceans are uniparous, meaning they give birth to a single newborn, though twins and even triplets can occur.
On the other hand, most fish species carry their babies in eggs that they lay in specific spots. Once the eggs are hatched, the larvae will emerge and live on their own.
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Differences between fish & marine mammals
However, there are not only differences between fish and marine mammals regarding their reproductive features, but also in their respiration and diving physiology.
Cetaceans have lungs, which means they cannot breathe underwater and must come up to the surface to take in oxygen from the air. So yes, they are great freedivers! Now, the time they can last underwater depends on the species and size of the individuals, ranging from 5 to 60 minutes. Unlike cetaceans, fish have skin folds called gills that they use to capture oxygen from the water, which is why they don’t have to go to the surface to breathe.
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Gestation process
As said before, cetaceans get pregnant with one baby, and once it is born, they breastfeed it with the mother’s milk. The duration of pregnancy, as well as the times they get pregnant throughout their lives, will vary by species. In the next table, several species of whales and dolphins, ordered from largest to smallest, are shown with their respective calving, gestation, and weaning times.
| Species | Common name | Gestation | Calving | Weaning |
| Balaenoptera musculus | Blue whale | 10 – 12 months | 2 years | 6 – 8 months |
| Megaptera novaenglidae | Humpback whale | 11 – 11.5 months | 2 years | 10 – 12 months |
| Physeter macrocephalus | Sperm whale | 14 – 16 months | 5 – 6 years | 2 years |
| Balaenoptera acutorostrata | Minke whale | 10 – 12 months | 1 year | 4 – 6 months |
| Orcinus orca | Orca | 15 – 18 months | 3 – 8 years | 1 – 2 years |
| Globicephala macrorhynchus | Short-finned pilot whale | 14 – 15 months | 3 – 5 years | 2 – 3 years |
| Tursiops truncatus | Bottlenose dolphin | 12 – 12.5 months | 3 – 6 years | 1.5 – 2 years |
| Delphinus dolphis | Common dolphin | 10 – 11.5 months | 1 – 4 years | 10 – 19 months |
On average, cetacean pregnancies last around one year, and once born, calves will stay with their mothers between one and eight years, as is the case with common dolphins, and 8 years, being the maximum time orca calves can stay with their parents. As marine mammals, calves feed on the mother’s milk for a period that will again depend on the species.
Birth & first few hours of the newborn
The fetus is developed inside the mother’s womb for around one year, folded to the right or to the left side with the tail placed near the cranium. In fact, once they are born, they get called fetal folds; some marks of the folding position that are visible in the newborns from weeks to months after birth. We often see this on our whale and dolphin watching trips.
When in labor, it has been seen that sometimes the mother can suffer from severe pains, and the birth can last for a few hours. The newborn’s first projection through the mother’s vulva is the caudal fin. As mammals, cetaceans have a pregnancy similar to that of humans.
The fetus will be fed with the nutrients coming from the mother through the umbilical cord.
Baleen whales’ mothers, such as blue whales, do not eat the placenta nor bite the umbilical cord once the calf is born, unlike it occurs in other mammal species. Instead, the calf can be connected to the mother for some days after birth, as it may happen with some dolphins, too. However, in some dolphins, the separation occurs right after birth, once the mother has continued swimming for some meters. And yes, once the umbilical cord falls off, they get the belly button, too.
Once the babies are in the giant blue, they need to learn how to breathe. And at least the first breath of the newborn is induced by the mother and other members of the group, as they push it towards the surface.
What do calves feed on & how
As mammals, cetaceans’ calves will feed on the mother’s milk for a period that, as said before, will depend on the species – check table 1, weaning column for specific species information. Now, cetacean milk is quite different from cow’s or human milk. Let’s think about it: if it were as liquid as human milk, it would easily disperse in the water; therefore, cetacean milk has a larger amount of fats than human milk. Thus, milk thickens to a yoghurt-like consistency, preventing rapid spread through the water.
Moreover, cetaceans have their own mechanisms to avoid the dispersion of their milk. For example, mammary glands are ‘hidden’ in folds in the ventral part of the animal, so that for most species, with the exception of baleen whale species, calves have to insert their lower jaw into these folds in order to drink the milk.
Did you know
Cetaceans are born in the opposite direction from humans; they first emerge with the tail and last, the head.
Unlike human babies, who grow slowly during the lactation period, cetacean calves rapidly grow in size from birth to weaning, as they must be strong enough to face the many dangers of the wild. That is why the amount of milk produced by the mother must be extraordinary, as well as highly nutrient-rich.
In conclusion, whales and dolphins do not lay eggs because they are marine mammals. Therefore, they give birth just like we humans. However, gestation varies by species. But the concept stays the same. In other words, humans and cetaceans have more things in common than you could imagine.
References
- Berta, A., Sumich, J. L., & Kovacs, K. M. (2006). Marine mammals: evolutionary biology (Elsevier (ed.); 2nd ed.).
- Oftedal, O. T. (1997). Lactation in whales and dolphins: Evidence of divergence between baleen- and toothed-species. Journal of Mammary Gland Biology and Neoplasia, 2(3), 205–230. https://doi.org/10.1023/A:1026328203526
- Slijper, E. J. (2021). On Some Phenomena Concerning Pregnancy and Parturition of the Cetacea. Bijdragen Tot de Dierkunde, 28(1), 416–448. https://doi.org/10.1163/26660644-02801049
- West, K. L., Oftedal, O. T., Carpenter, J. R., Krames, B. J., Campbell, M., & Sweeney, J. C. (2007). Effect of lactation stage and concurrent pregnancy on milk composition in the bottlenose dolphin. Journal of Zoology, 273(2), 148–160. https://doi.org/10.1111/J.1469-7998.2007.00309.X
