calving interval

In whales, giving birth is called calving, and the calving interval is the time a female takes between one calf and the next. Researchers measure it by following known individuals over many years, which for North Atlantic right whales means matching photographs of each animal against a long-running catalog of identified whales.

The interval is set by biology first. NOAA Fisheries says female right whales become sexually mature at about age 10 and give birth to a single calf after a pregnancy lasting more than a year, then spend much of their energy nursing it. It considers about three years a normal, healthy interval for the species.

Because a female must rebuild her condition before she can carry another calf, a lengthening interval is an early warning. NOAA reports that right whale females have recently been calving every 7 to 10 years on average, and links the longer gaps to the stress of entanglement in fishing gear and to difficulty finding enough food. A shorter interval is read the other way, as a sign that breeding females are in better health.