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.
Brown Bear Offspring
The gestational period for brown bears is 6 to 8 weeks, and the females generally give birth to between one and four cubs. The mother cares for the cubs for up to 2.5 years, during which time she will not mate. The males do not contribute to the rearing of offspring.
Why Do Male Brown Bears commit SSI?
Infanticide among animals: A review, classification, and examination of the implications for the reproductive strategies of females gives a suggestion of the potential motivations behind SSI.
Sources of increased fitness from infanticide include:
(1) exploitation of the infant as a resource,
(2) elimination of a competitor for resources,
(3) increased maternal survival or lifetime reproductive success for either mother or father by elimination of an ill-timed, handicapped, or supernumerary infant, and, finally,
(4) increased access for individuals of one sex for reproductive investment by the other sex at the expense of same-sex competitors.
There are however situations where there is no benefit derived from SSI, this can be classified at pathological. In general it is considered that male brown bears commit SSI in an effort to promote their own bloodline.
Female Strategies To Avoid SSI
The cost to the female brown bear is very high. Carrying and rearing cubs takes a toll on her fitness and the loss is significant when the young are killed by males, simply to impregnate her again. As mentioned above females are beginning to develop counter-strategies to avoid SSI whenever possible.
Once such strategy that has been observed in females in an effort to reduce the risk of SSI is multi-male partnering. This multiple mating, or ‘promiscuity’, is considered an attempt to confuse paternity, thereby protecting the cubs from males who think that may have fathered them.
Another tactic is that the females seem to choose to mate with the geographically closest males, which have the highest potential to kill their future cubs. ~ The dilemma of female mate selection in the brown bear, a species with sexually selected infanticide.
Spatio-temporal avoidance is another common, yet high risk strategy, where the females choose to live an elusive lifestyle, staying away from optimal bear resources in order to protect their offspring. Steyaert’s study aimed to see whether this strategy incurred a significant cost in terms of fitness, with the female and her cubs receiving a poor diet as a result of the avoidance of high risk areas.
They analysed forage and faeces to identify the quality of the diet. It was found that changes occurred throughout the season, but the trend showed that females with cubs-of-the-year had lower quality diets than their male counterparts during the mating season, when the SSI risk was highest.
As male brown bears have developed SSI as a reproductive strategy, female brown bears are developing several counter strategies to protect their offspring. The study by Steyaert looked at spatio-temporal avoidance and the impact that it has on the quality of the female brown bear’s diet during the high risk period of time. Analysing the bears’ faeces showed that the quality was generally lower while she was avoiding areas with optimal resources, thereby trading food for safety. Unsurprisingly the reduced nutrition takes a physical toll on her fitness.
It is interesting to look at the evolving practices of the brown bears, and other large mammal species who also commit sexually selective infanticide. What are your thoughts on these animal behaviours?
Copyright © 2002-2013. All rights reserved