Adaptive Value

To what degree does roadside foraging increase reproductive fitness?

Grizzly mothers with cubs-of-the-year must invest substantial time and energy in order to successfully rear their young to subadulthood.   The rearing process takes between 2.5 and 3.5 years, and only half of a female's ~10 cubs produced in a lifetime are likely to survive [15].  Thus, any strategy that could increase the less-than-ideal chances of cub survival could have immense adaptive value.

Many have proposed that roadside foraging is a counterstrategy to sexually selected infanticide ((SSI), addressed in greater detail here). Females with offspring will tend to select sub-optimal habitats with decreased adult male presence. However, if roadside foraging is indeed a counterstrategy to SSI, does the added threat of vehicles render it maladaptive?

Infanticide?  Better Hit the Road...

Aggressive adult males pose the threat of infanticide, and tend to avoid roads (e)

Modeling data based on den site locations of mother grizzly bears indicates that mother grizzlies tend to select den sites that are more technically challenging to access (i.e. located on steeper slopes), and in areas that are less resource rich, than do adult male grizzlies [8], one of several examples of such spatiotemporal sex segregation [1,5,7,18]. Essentially, mothers like to go where males wouldn't bother to venture. Though roads carry the threat of automobiles (ref. Table 1 and other human encounters, mothers may increase their reproductive fitness by  sticking close to the asphalt; a first-hand account from a bear-sitting Yellowstone ranger, as well as grizzly road death statistics, indicate that adult males not only fear humans, but they actively avoid roads [5,18].

The question remains as to whether infanticidal threat truly overrides the danger posed by daily proximity to thousands of vehicles. Efforts to mitigate vehicular threat in controlled parks such as Yellowstone, where rangers protect roadside grizzlies by bear-sitting and carefully directing traffic, have certainly succeeded [3]. In such well-managed circumstances, roadside foraging is probably not maladaptive. Most roads are not manned by dedicated bear-sitters, but, then again, most two-lane highways through bear habitat are not constantly swarmed with tourists. The maladaptivity of this behavior really depends on how humans change their own behavior in response to the risk of killing bears.

Roads might shield mothers and cubs from aggressive conspecifics, but cars kill many bears each year (f)