Human mothers have a relatively long gestation period compared to the rest of the animal kingdom, but a few animal mothers go even further. The size of the offspring is usually a determining factor, but not all lengthy gestations end with a big baby. Here's our list of the animals with the most prolonged gestation periods.

1 of 12 Manatees Liquid Productions, LLC / Shutterstock You might not know if a portly manatee is pregnant simply by looking at the animal, but this gentle giant carries its young for nearly 13 months. Lounging around in the water all day helps to relieve some of the extra weight, but a manatee mother is still owed plenty of respect for her patience.

2 of 12 Camels irabel8 / Shutterstock Camels are known for their stubborn and crabbish personalities, but consider this: camels have a gestation period of 13 to 15 months. The month in which conception occurred can shift the birthdate, with November conceptions pushing the birth 18 days longer than a May conception. Other camelids, such as llamas, also have a long gestation period — about 330 days.

3 of 12 Giraffes Coldimages / Getty Images Giraffes have gestation periods anywhere from 400 to 460 days. Despite being the tallest land animal in the world, the mother gives birth standing up — so the baby needs to be big enough to brace for a long fall. (Interestingly, the fall is typically what bursts the embryonic sac.) With lions and other predators abound, the world is a dangerous place for baby giraffes when they first come into the world — part of the reason for the long delay.

4 of 12 Velvet Worm Pedro Bernardo / Shutterstock Not all animals with long gestation periods are large mammals. There are some worm-like animals that bear live young, including the velvet worm. This bizarre-looking creature carries its young for as long as 15 months. Despite the name, they aren't true worms and they aren't made of velvet. Their bodies are covered with sensory hairs, which gives them a velvety appearance. They are considered close relatives of both arthropods (spiders and insects) and true worms (like the earthworm) — making them especially interesting to paleontologists.

5 of 12 Rhinoceros john michael evan potter / Shutterstock It might not come as a surprise that rhinos — because of their sheer size — are next on the list with a gestation period of about 15 to 18 months, depending on the species. This long gestation period is also an obstacle to replenishing the population. All five species of rhinoceros are endangered or considered vulnerable, and three of the five are considered critically endangered.

6 of 12 Walruses U.S. Geological Survey / Flickr / Public Domain Walruses have the longest gestation period of all pinnipeds (a group of mammals that includes seals and sea lions), carrying their young for as long as 15 to 16 months. Seal and sea lion mothers don't get off easy and carry their offspring for about 330 and 350 days, respectively. Walruses also have the lowest reproductive rate of any pinniped.

7 of 12 Whales and Dolphins Jan Daly / Shutterstock Whales and dolphins, also known as cetaceans, are known for their high intelligence, complex societies, and peaceful personalities — so it's no surprise that they also take a lot of care in developing their young. Although all species have different gestation periods, orcas have the longest period among dolphins at about 17 months. Some sperm whales — the largest living predators — have been known to carry their young for up to 19 months.

8 of 12 Elephants john michael evan potter / Shutterstock Elephants have the longest gestation period of all mammals, carrying their young for 18 to 22 months before giving birth. Long developmental periods are common among highly intelligent animals. Since elephants are the largest living and biggest-brained land animal in the world, there's a lot of development for elephants to do in the womb.

9 of 12 Black Alpine Salamanders Thomas Huntke/ Wikimedia Commons / CC BY-SA 3.0 DE Black alpine salamanders are amphibians that live in the Central and Eastern Alps and give birth to live young. Their pregnancies can last from two to three years, depending on the altitude at which the salamanders live. They typically bear two fully developed young. This salamander's life expectancy has been estimated to last from 10 to even 20 years.

10 of 12 Sharks VisionDive / Shutterstock Unlike most fish, sharks are K-selected reproducers — meaning they produce a small number of well-developed young as opposed to a large number of poorly developed young. Pregnancy length can vary significantly by species. The spiny dogfish shark can carry young for about two years, while basking sharks can do so for as long as three years. The frilled shark can wait 3.5 years before giving birth.

11 of 12 Tapirs Nick Fox / Shutterstock A tapir may appear to be a cross between a pig and an anteater, but it's actually most closely related to horses and rhinos and shares a similarly long gestation period. A tapir calf is born after 13 months in the womb. Newborns have special brown- and beige-striped markings that help camouflage them from predators, but the pattern fades after a few months when the young tapir becomes more mobile.