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A female brown bear with three cubs.

The live webcams at Katmai National Park are down right now, and I haven’t been able to successfully conduct any life
field observation as a result, so instead I’m adding a brief post on one of the aspects of sexual selection, mating, and raising offspring in brown bears: Sexually Selected Infanticide (SSI). It’s one of the most interesting, and, from a human emotional point of view, can be somewhat sad and shocking aspects of bear behavior, and one that is sometimes difficult to understand, yet an important one.

The Katmai National Park website addresses the issue of infanticide in their brown bear FAQs, noting that the practice is not completely understood, that there may be many reasons for the practice, and reminding readers that bear behavior does not necessarily conform to human moral and ethical boundaries. Here is an excerpt from their coverage of the topic:

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There have been some studies on this topic, most notably and recently by the Scandinavian Brown Bear Research Project and Telemark University College, Norway. Most of the studies focus on the killing of bear cubs by adult males as sexually selected infanticide, rather than attributing the killings to other reasons. Here I will provide an overview of mating strategies of the brown bears, as well as the strategies and counterstrategies involved in SSI.

Mating System

Brown bears have a polygynandrous (promiscuous) mating system; “in brown bears, reproductive individuals of both sexes mate a variable number of times with a variable number of partners during a given mating season. Females typically mate with three to four males during a breeding season, although females have been observed to mate with up to 20 partners. Males show more variation, and mate with between one and eight females per breeding season, although many males do not obtain any matings.” (source: Steyaert et al 2012)  Females mature sexually between the age of 4 and 8 years of age, while males first mate about a year later on average, when they are large and strong enough to successfully compete with other males for mating rights. Female brown bears are promiscuous, have been observed intiating mating on some occasions, and control mating acts and partners to some extent. “Age and morphological traits, such as body size, weaponry and signs of aggression or willingness to fight, are considered to reflect male quality. The apparent success of larger, older or more aggressive male brown bears might be explained, in part, by female choice for these traits as signs of genetic quality.” (source: Steyaert et al 2012) The mating season lasts from late spring through early summer, approximately 2.5 months. Being serially monogamous, brown bears often remain with the same mate from between several days to a few weeks. 

Fertilization & Oestrus

Female brown bears have delayed implantation after copulation, with the fertilized ova able to remain dormant in the uterus for about five months, prior to implantation in November–December. Because brown bears have delayed implantation, each corpus luteum becomes dormant following ovulation and luteal progesterone secretion stops. This may allow females to re-enter oestrus. It has been suggested that polyoestry (multiple periods of oestrus in the same female) is common in free-living female brown bears, because one study found that 51% of the reproducing females that they monitored engaged in more than one male-female association per breeding season.(source: Steyaert et al 2012) Polyoestrous cycling facilitates the development of different sets of ova, and thereby increases the potential of these to be fertilized by different males. Multiple paternity occurs relatively frequently in brown bears. In Scandinavian populations, in 14.5 and 28% of the litters with 2 and 3 young, respectively, young were sired by different fathers. Brown bear females also show lactational anoestrus. Females that lose offspring by either death or family break-up can enter oestrus already after 2–7 days. (source: Steyaert et al 2012) This relates directly to our discussion regarding sexually selected infanticide.

Cub Birth & Litters

The average brown bear litter has one to four cubs, usually two, with older females giving birth to larger litters, generally, though the size of a litter also depends on factors such as geographic location and food supply. In some cases, there have been records of females adopting stray cubs or even trading cubs when they emerge from hibernation; for example, in 2014 at Katmai National Park, there was an instance of a cub that had been been abandoned by its mother in July, and was seen being taken care of by a second female bear in September, with the “new” cub nursing and nuzzling the female alongside the bear’s biological offspring. (source: Katmai National Park Blog) Males, however, take no part in raising their cubs – parenting is left entirely to the females. Cubs remain with their mother from two to four years, during which time they learn survival techniques, such as obtaining food (hunting, fishing, foraging); how to defend themselves; denning. It is during this time when brown bear cubs are most vulnerable, before they are able to fend for themselves, and before they have established a place in the social hierarchy.

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A female bear teaches her cub offspring.

“Cub mortality in brown bears varies among populations and is typically higher than mortality in other age classes. Annual cub mortality in southwestern Alaska to averages 2.8%, whereas, in Denali National Park in Alaska, annual cub mortality can reach 66%.”(source: Steyaert et al 2012)  Infanticide is considered to be a major cause of death among brown bear cubs, at least in some populations. “Resource competition, exploitation, social pathology and male reproductive strategy may explain infanticide in brown bears. Most cub mortality occurs during the breeding season, and about 80% of conspecific killing is carried out by adult males. Females in poor body condition may abandon their dependent offspring as an adaptive maternal strategy. This allows them to re-enter oestrus during the ongoing breeding season, and produce offspring with potentially higher survival rates.” (source: Steyaert et al 2012)

Sexually Selected Infanticide

“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”

(source: Lisa Warner, TheGoodHuman.com)

Some discussions I’ve found on the topic of SSI:

“During the mating season from May to July, male brown bears have one overriding aim — to impregnate as many females as possible. A hindrance to this is females that are not yet in estrus because of existing cubs. Male brown bears have developed a simple and brutal way of getting round this problem. The offspring of other male bears are unceremoniously killed, meaning that the mother enters estrus again. This instinctive course of action has two advantages. Firstly, the cubs that would have passed on the genes of other fathers are eliminated, and secondly the female is ready to mate and can be impregnated with the male’s genes.” (source: Phys.Org)

“Why Do Male Brown Bears commit SSI? 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. (source: Hrdy, 1979)

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.” (source: Lisa Warner, TheGoodHuman.com)

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. (source: Steyaert et al 2012)

Additional Resources

Two European brown bear (Ursus arctos) cubs climbing pine tree in taiga forest, Martinselkonen, Finland, June

Two European brown bear (Ursus arctos) cubs climbing pine tree in taiga forest, Martinselkonen, Finland, June

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