See also:

Caribou population
 
Caribou calf survival
 
Caribou calving habitat use
 
Off-site: Porcupine Caribou Herd

Arctic Borderlands Ecological Knowledge Co-op

What is happening?

• This graph shows the general pattern of annual birth rates for the Porcupine Caribou Herd. In early June of each year, a sample of 40 to 100 female caribou are located by tracking down the signal from the radio collar they are wearing. The sighting of each of these females is then used to record signs of pregnancy or recent calving.
• The birth rate estimate in 2005 was the lowest on record for this herd. During the last six years the birth rate has been slightly above the long term average of 81%.

Why is it happening?

• Having and feeding a calf requires a lot of energy. After giving birth and nursing a calf, a cow is in poor shape and this makes it difficult for her to get pregnant in the fall. Cows that do not give birth in the spring or lose their calves soon after birth, are in good shape during the fall rut and have a very good chance of getting pregnant. So, a summer when lots of calves are born and survive results in many cows in poor condition with a low probability of getting pregnant in the fall. This means that fewer calves will be born the next summer. Then, because lots of cows did not give birth, there are more cows able to get pregnant during the fall and so more calves are born the next summer.
• Researchers have not documented abortions in the Porcupine Caribou herd, unlike some other caribou populations. In 1990, however, a number of cows in poor shape during the rut suffered early loss of their embryos. This situation is uncommon, however. Thus, low birth rates are most related to the conditions in the previous summer and fall.

Why is it important?

• The birth rate gives us valuable information. It lets us know how healthy the herd is and, along with other indicators, it helps us to predict how the population is changing.

Technical Notes

• Radio-collared caribou cows are checked for signs that they have recently given birth (e.g. calf at heel), lost a calf (e.g. showing enlarged udders and hard antlers) or not pregnant (e.g. no hard antlers and udders not distended).