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Growth, Development and Reproduction

The content and activities in this topic will work towards building an understanding of how mammals grow and reproduce within the world ocean.

Parental Care

Parental care is any behavior by parents that directly benefits their offspring. These behaviors involve one or more parents staying with their offspring after birth to protect, feed, or train them. From an evolutionary perspective, the advantage of having parental care is that it can increase the chances the offspring will survive to reproduce themselves and pass on the parents’ genes. However there are costs to parental care as well. Parents must use energy to care for offspring which may result in their own decreased survival or loss of additional opportunities to mate with other partners. Despite the costs, parental care has evolved in the animal kingdom several different times in many different species. For example, parental care is thought to have evolved as many as 33 times in ray-finned fish alone. Interestingly, biparental care, or care by both parents, is rare across most groups of animals. Biparental care is most common in species where extensive care is needed for the young. For example, some species of birds commonly have biparental care because both parents must forage for food to feed the offspring.

 

All marine mammal species exhibit some degree of parental care, although male parental care is uncommon. Young marine cetaceans are commonly called calves (Fig. 6.28 A), while young pinnipeds and otters are called pups (Fig. 6.28 B). Juvenile polar bears are called cubs (Fig. 6.28 C). All marine mammal mothers feed their young through lactation. Milk is an energy-rich substance produced by the mother, which allows the offspring to grow quickly and has many immune system benefits. Marine mammals have a relatively long period of parental feeding. For example, sea otter pups feed on milk from their mothers for six to twelve months. In addition to feeding, the mother is also often responsible for instructing the young on how to swim and catch food. Some species have long periods of parental care. Dolphin calves will stay close with their group (called a pod) for several years while elephant seals remain with their mother for less than a year.

<p><strong>Fig. 6.28.</strong> (<strong>A</strong>) Humpback whale (<em>Megaptera novaeangliae</em>) mother and calf, Hawaiian Islands Humpback Whale National Marine Sanctuary, Maui, Hawai‘i</p><br />
<p><strong>Fig. 6.28.</strong>&nbsp;(<strong>B</strong>) Weddell seal (<em>Leptonychotes weddellii</em>) adult and pup, Erebus Bay, Antarctica</p><br />


<p><strong>Fig. 6.28.</strong>&nbsp;(<strong>C</strong>) Polar bear (<em>Ursus maritimus</em>) adult and two cubs</p><br />
<p><strong>Fig. 6.28.</strong>&nbsp;(<strong>D</strong>) Sea otter (<em>Enhydra lutris</em>) mother with nursing pup, Morro Bay, California</p><br />


 

Life History and Reproduction

All marine mammals reproduce sexually. Cetaceans and sirenians undergo the entire process of reproduction in the water (Fig. 6.29). The remaining marine mammal groups generally breed and give birth on land. The gestation period of most marine mammals is approximately one year and is followed by live birth. Time to sexual maturity varies between species but is relatively long compared with other animals, taking several years after birth. Several species of marine mammals are among the longest-lived mammal species. Baleen whales, larger toothed whales, some pinnipeds and sirenians can live as long as humans, while some species of smaller toothed whales have shorter life spans, rarely reaching 20 years old.

 

All marine mammals are born in or very near ocean habitats. They must be well adapted for aquatic life immediately when they are born. Most cetaceans and sirenians can swim immediately after birth (Fig. 6.29). Pinnipeds, otters, and polar bears give birth on land. These marine mammal parents must teach their young how to swim.

<p><strong>Fig. 6.29.</strong>&nbsp;(<strong>A</strong>) Two spinner dolphin (<em>Stenella longirostris</em>) adults with one newborn calf, Kaua‘i, Hawai‘i</p><br />
<p><strong>Fig. 6.29.</strong>&nbsp;(<strong>B</strong>) West Indian manatee (<em>Trichechus manatus</em>) mother nursing a newborn calf, Puerto Rico</p><br />
<p><strong>Fig. 6.29.</strong>&nbsp;(<strong>C</strong>) Humpback whale (<em>Megaptera novaeangliae</em>) mother and calf</p><br />


Representative Image: 
Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.