The consequences of large body size

The four panels to the left, two top and two bottom, show the relationship between some key life parameters (the vertical or y-axis) and body mass, expressed in grams (the horizontal or x-axis). Each dot represents a parameter summarized for a given species, all of which are terrestrial mammals--ranging from the smallest (i.e., shrews) to the largest (i.e., elephants). The various bear species are denoted by a larger symbol colored brown. The central relationship between each life parameter and body mass is represented by a solid black line. Note that adult body mass is consistently log-transformed, which tends to compress values, especially at the high end of the range.

The four panels above and Panel D below emphasize relationships with body mass from which the bears do not deviate substantially--in other words where they conform to what you might expect given their size. Graphically, this shows up as dots representing bears clustering near the central trend line. This holds for the number of years between litters (Panel A), litter size (number of cubs per litter, Panel B), age at which females first reproduce (Panel D below), longevity (Panel C), and the number of offspring produced in a lifetime (Panel D above).

 

That said, because of their large size, bears first reproduce late in life, have long intervals between births, and have small litters, all of which is offset by the fact that they tend to live for a long time. This dependence of life-time reproductive success on longevity highlights how critical the annual survival of females is to maintenance of populations. In other, a high rate of female survival from one year to the next is critical to conserving virtually all bear populations.

Three of the four panels at left highlight relationships where some (if not all) bear species deviate substantially from what might be expected given their size. Panel B shows the numbers of days before newborns open their eyes, which is an indicator of how helpless these neonates are. Such helplessness is more common among smaller-bodied species; bears comprise the majority of large-bodied species giving birth to such altricial young. This, of course, because birth happens in a den where helpless newborns are relatively secure. In line with this, bears give birth to the smallest neonates--relative to the size of the mother--of just about any species with a placenta (in other words, barring marsupials). Again, this goes hand-in-hand with relative helplessness, and the opportunities afforded by denning (but more on all this under Life strategy).

 

Finally, Panel C at left requires some explanation. This shows length of gestation relative to body mass. All but one of the bear species represented here have gestation lengths that seem to fall in line with the dominant trend. However, this is deceptive. Three of six bear species are known to delay the implantation of fertilized eggs, presumably as a hedge against times of food shortage (again, see Life strategy for more information). The upshot is that, even though among brown bears fertilization typically occurs in June and birth the following January, actual gestation is only 90 or so days. So duration of sustenance from the placenta is actually quite short for bears, and belies the ostensible results in Panel C, which do not account for delayed implantation.