Success of reindeer cows depends on calving date

Liv Lønne Dille

When calving early in spring, both reindeer cows and their calves make the most of the short but hectic Arctic summers, and are thus able to start another long winter season with “recharged batteries”.

In autumn 2003, we manipulated the mating dates in a herd of domestic reindeer in Finland. Late mating led to shorter gestation periods and lighter calves at birth. The cows did not compensate late mating by increased late calving. The results indicate that the timing of calving is critical for ensuring the success of female reindeer. The cows’ ability to adjust gestation length and maternal investments can thus be seen as adaptations to the strong seasonal variations of their natural environment in order to secure future reproduction.

Background
Several recent studies indicate that a low percentage of males (and a majority of young males among these) in several deer species can result in late calving and partially also in lower conception rates. Late-born calves are usually lighter in the autumn than calves born at normal calving time. Calf survival prior to weaning is closely linked to calving date and birth weight. In autumn 2003, we manipulated the mating dates in a herd of domestic reindeer with about 100 females to study the calves’ weight gain until weaning, and to see if this had any effect on the females’ condition at the start of the following reproductive cycle.

Results and discussion
In Arctic ecosystems, the short, hectic summers place clear limitations on the development of animals. Within very few months, a reindeer cow must raise a calf that is able to survive its first winter. At the same time, maternal investments must be balanced, so that they do not excessively reduce her fitness prior to the next reproductive cycle. Early heat, and thus, early calving is decisive to ensure that both mother and calf can utilize the protein and energy-rich vegetation early in the growing season.

Female reindeer reduced their gestation periods to approximately 210 days after late mating – compared to about 220 days gestation after mating in the normal breeding season (see Figure 1).

A shorter gestation period led to somewhat lower birth weights (on average, calves were 0.5 kg lighter). The results indicate that this is a compromise between calving date and calf weight at birth, both of which are critical parameters for calf survival. However, the difference in autumn calf weight averaged 5.5 kg between the two groups. Late-calving female reindeer do thus not compensate by increasing maternal investments. In other words, the females rather secured their own future reproductive capacity than investing more in a late-born calf. This is a natural strategy for species with a relatively long life expectancy and many reproductive cycles under harsh natural conditions.

Updated: 14.05.09
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