Another obvious aspect of size is growth--that is, variation in size relative to age. Here I present some information that is representative of this phenomenon in bears, featuring data from Yellowstone's grizzly bear population. Another aspect of growth that is particularly relevant to bears pertains to cubs, simply because cubs are so small at parturition (i.e., birth), born in a den, and needing to grow rapidly enough so that when they emerge from the den with their mom roughly 90 days later they can keep up and survive. I start with growth of yearling and older bears and then, below that, go on to examine growth of cubs.
The graphs immediately above show the changes in size (here expressed as weight) for males (left) and females (right) of the Yellowstone grizzly bear population. Weight is on the y-axis and age on the x-axis. Each dot represents the weight of a bear at the corresponding age. The solid black line is fit to these data assuming that weight increases to some sort of maximum (an asymptote) which is characteristic of the population (the number at the termination of the black line). The formula used to estimate this trend is one that is typically used by researchers to describe the growth of animals (the von Bertalanffy equation). This equation includes a parameter called the "growth constant." This constant is an estimate of the characteristic rate of growth for whichever group of animals is being characterized. A high value indicates rapid growth to maximum weight, a low value, slow growth. This growth constant is given for males and females in the small insets immediately above, along with a depiction of how growth rate varies with age, from rapid to slow. The light gray line labeled with "5.8" denotes the average age at which females first give birth to cubs in the Yellowstone ecosystem. Finally, the blue line surrounded by blue shading was fit to the data employing no assumptions about the nature of growth (a method called LOESS). The blue shading encompasses the region within which one can be 99% sure that the estimated line will fall using repeat samples of the same population.
The graph inset in the upper right shows characteristic growth parameters (in bar form) for the Yellowstone population contrasted with the same values estimated for three grizzly bear populations in the arctic regions of Alaska (Western Brooks) and Canada (Tuktoyaktuk and Northern Yukon). Two additional values featured in this graph are, first, the age at which an animal attains roughly 90% of maximum average weight and, second, the average age at which females first reproduce. The contrast between age of first reproduction and attainment of 90% of maximum weight is particularly instructive given that it is difficult for a female to devote energy to producing cubs at the same time that she is accumulating lean body mass. Information on the arctic populations comes from the pioneering work on the topic of bear growth published by Brian Kingsley.
As is the case with virtually all bear populations, the female segment grows to its maximum size much more rapidly than does the male segment, and the maximum is substantially higher for males (230 kg, in Yellowstone) compared to females (125kg, see Differences in bear size). This sex-related difference in growth rate is represented explicitly in the inset graphs. It makes sense that females would grow fast and then stop because of the urgent need to divert resources from growth to reproduction, which is much more demanding on females than on males. On the other hand, males benefit from continued growth because large physical size is an advantage in the violent head-to-head competition that occurs for limited mating opportunities with females (see Life strategy). The other main conclusion derives from comparing growth of Yellowstone's females bears (at a latitude of roughly 44o N) with growth of females bears in the arctic (at latitudes greater than 60oN). Not surprisingly, Yellowstone's females grow to a larger maximum size at a faster rate, which means they attain 90% of maximum weight sooner (at a little over 4 years of age)...which means, in turn, that they can start reproducing sooner. The exception is posed by the Tuktoyaktuk population, which is, indeed, an anomaly among the high-latitude bear populations studied so far. I'm not sure why females were estimated to reproduce at such a young age here; my suspicion is that the answer has to do partly with the methods used for studying and analyzing reproduction in this population.
The above graphs show the growth of cubs during their first year of life. These results derive almost wholly from the work of Charlie Robbins and his students at Washington State University, most notably here, Shawn Farley. (You will see me reference Dr. Robbins' work virtually anywhere there is something to say about nutrition, growth, or physiology). Panels A and B at left above show weight growth, highlighting the period that cubs are in dens--which is roughly the first 90 days of life. Panel B also differentiates the growth of individual cubs that are part of a litter of twins versus a litter of triplets. Panel C translates growth in absolute size into a rate, which is contrasted with the amount of energy that cubs are obtaining from their mother's milk (in Panel D). Finally, in the two graphs to the upper right the fatness of mothers upon entering hibernation is related to (Panel A) the size of cubs after 90 days of growth (at emergence from the den, differentiating male cubs from female cubs) and (Panel B) their birth date (essentially the day in January).
Cubs grow quite rapidly and reach a peak growth rate as late as 7-8 months of age. Their most rapid growth occurs after they leave the den, just prior to the onset of a major decline in intake of energy from nursing. This pattern makes sense given that this period closely coincides with when cubs are obtaining energy not only from milk, but also from sampling food that their mother is finding. Aside from these dominant themes, cubs that are part of litters of two grow faster than cubs that are part of threesomes. This is not surprising given that a cub that is one of triplets is sharing a limited supply of milk with two siblings rather than one, which implicates the trade-offs for a mother of producing triplets versus twins. The mother may increase the odds of replacing herself by having triplets, but only as long as the overall odds engendered by an extra cub offset the reduction in survival of individual cubs--which is a predictable result of being smaller. The other key theme is that cubs born to fatter mothers enjoy several advantages, including having a longer period of growth within the protection of a den, and being able to reach a larger size prior to leaving, regardless of the time advantage. Again, this result is not surprising. Fatter mothers provide their cubs with more milk and are able to endure the rigors of denning for a longer period of time. Interestingly, the benefits of a fatter mother apparently do not differ for male versus female cubs or for cubs that are one of twins versus triplets.
Growth of cubs
The graphs at left show how composition of the mother's milk varies over the course of a cub's first year of life. (Parenthetically, it is often a challenge to know where to place information given that so many topics necessarily intergrade; these graphs could just as well go under Nutrition).
As you can see, composition of the milk changes substantially after the mother leaves the den and transitions from hibernation metabolism to active metabolism--a natural consequence of shifting from subsistence almost wholly on fat stores to subsistence on ingested food (see Physiology). Protein content of the milk increases substantially, as does energy, at least for a while. This shift naturally promotes growth of lean body mass in cubs. Energy more-or-less tracks lipid content of the milk, which reflects the high metabolizable energy concentration of fats.
The graph farthest to the right places the milk produced by bears (the large brown dots) in context of other mammals (the beige dot and bar summaries). Bears produce milk that is amongst the richest in fats and also the least rich in carbohydrates. The dominance of fats in bear milk is a natural consequence of the critical role that body fat (adipose reserves) plays in hibernation, which is when cubs are born and first nursed (for more details on this see Physiology).