Birthing Grounds & Calving | Baja Wildlife Guide

BIRTHING GROUNDS

After arriving in Mexico, gray whales bear their young in offshore lagoons starting in late December. Pregnant females arrive first, proceeded by adult males and females, and followed by juveniles. The whales travel at an average rate of five miles-per-hour and typically arrive at one of three primary lagoons on the west coast of Baja California Sur: Magdalena Bay, San Ignacio, and Laguna Ojo de Libre, formally known as Scammon’s Lagoon. Captain Charles Scammon once hunted gray whales in these areas and published The Marine Mammals of the Northwest Coast of North America in 1874, a classic containing some of the first detailed written accounts of whales.

Early February is peak birthing season, and the majority of gray whales have reached the calving lagoons by mid-February. It is difficult to understand why gray whales choose lagoons for calving. Presumably, this is an ancestral behavior trait. Going south into temperate or subtropical waters, to calve is typical of most baleen whales and is thought to help the newborn whales conserve body heat. The relatively quiet waters also facilitate birth and nursing during the time that the baby is gaining strength for the long migration.

The term “quiet,” however, is misleading. Usually a tidal current of 2 to 3 knots keeps the visibility at less than 5 feet. The interplay of tidal current and wind also produces a surface chop, which seems to make breathing difficult for the infant whale. To compound the difficulties, the uppermost reaches of the tidal channels carry water that has twice the salinity of ocean water—whales must be able to excrete the extra burden of salt. One theory postulates that the additional buoyancy makes it easier for the baby whale to stay at the surface until it gains enough fat to float. However, adult gray whales must adjust to this unneeded ballast by exhaling sizable volumes of air to dive against the current.

Lagoons are subject to change in size and depth through geologic processes, and may be created or destroyed, which may consequently negatively affect the gray whale population. Laguna Ojo de Libre, to the north of San Ignacio Lagoon, is flanked by evaporation ponds to make salt, which is transferred by barge to Cedros Island for loading onto ships. In spite of these barge shipments that occur night and day, whales continue to use the lagoon. The vastness and beauty of the surrounding desert dunes and the winter occupancy by the whales have prompted the Mexican government to proclaim Laguna Ojo de Libre a national park.

CALVING

Females typically mate in years when they are not already bearing a calf, giving birth to one calf every two to three years. During birth, the calf emerges fluke first, weighing between 1,100 and 1,500 pounds and 15 feet in length. Its coloration ranges from dark gray to black, and it may be speckled with white pigmentation. The calf must surface to breathe twice as often as its mother, and can make smooth dives within hours after birth.

Males take none of the responsibility for raising newborns. Females have two teats that are used for suckling, one on either side of their genital slit. The milk that is consumed by the calf consists of 53 percent fat (compared to human milk, which is 2 percent fat). The milk retains water to hydrate the mother and provides the calf with a rich supply of nourishment, as the thick globs of milk are caught in the infant’s baleen. Calves suckle their mothers’ milk until they are between 6 and 8 months old, becoming more independent in the last two to three months of the nursing period after they have journeyed with their mothers back to their northern feeding grounds.

Header Credit: Bill Gent & Diana Russler

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