A study conducted in a boreal forest in Dalarna and Gavleborg counties, Sweden, by Steyaert et al has analysed the behaviour of female brown bears during mating season. It has been found that they have adopted counter strategies to protect their offspring from sexually selective infanticide (SSI), which is thought to be an adapted mating strategy for males. Some of these strategies are shown to have a cost on the health of the female and her young.
What Is Sexually Selective Infanticide (SSI)?
Infanticide is common among mammals and can be a male reproductive strategy provided that three requirements are fulfilled, that is
(i) males only kill offspring that they have not fathered,
(ii) victimized mothers enter oestrus shortly after offspring loss and
(iii) the perpetrating male has a high probability of fathering the victimized mother’s next offspring
SSI is commonly seen in polygamous mammal species, with sexual dimorphism (where the size between genders is significant). It is more prevalent with animals that have an extended period of maternal care, with a period of lactational anoestrus (a phase of sexual inactivity between periods of ‘heat’).
SSI is discussed in the following excerpts from Mating Strategies in Relation to Sexually Selected Infanticide in a Non-Social Carnivore: the Brown Bear ~ Eva Bellemain.
This phenomenon should not benefit females, so one would expect females to evolve mating counter strategies in order to protect their infants from infanticidal males.
Moreover, all infanticide cases occurred during the mating season. We expected that primarily immigrant males were infanticidal, as in social species. However, we found that resident adult males commonly committed infanticide. Perhaps they recognize females they have mated with previously.
Steyaert’s study evaluated an area of around 1300 km2, with a bear density of approximately 30 individuals per 1000 km2. They hypothesised that female bears with cubs-of-the-year were put in a position where they had to trade food for safety. With SSI being such a major cause of offspring mortality the females are required to follow strategies to protect their cubs, and this is particularly difficult when resident males are most likely to be a danger to her young.
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