The birthweight for other bears averages around 0.3% female body weight. The birthweight for pandas averages around 0.1%. So there’s a difference, but when we’re looking at birth weights that are fractions of a percent of the adult I’m not sure how significant that difference is. It may be essentially random variation. After all grizzly bears are almost twice the size on average of black bears. We could speculate endlessly on possible reasons why there is a difference but I’d have to see some evidence that the difference is really significant anyway before I tried.
In addition, Panda gestation is only about 3 months, whereas other bears are longer and Pandas are only fertile for a few times every year, and not until they are 6 or 7 years old. Other bears are quite different. Why?
That has no real validity because one thing we know about mammals is that the largest females tend to produce the largest young. You really need to work with averages or else maximum adult/maximum birth weight or minimum adult size/minimum birth weight.
And given that even these figures suggest that the difference in ratios between a grizzly and spectacled is about the same as between a grizzly and a panda it seems like this is just the normal range of variation within what is a fairly diverse group. Probably no more remarkable than the variation in relative birth rates amongst antelope or new world monkeys.
That’s not to say there wouldn’t be a reason or more likely a number of reasons for these size differences, simply that the panda isn’t a massive abberation, it is simply one extreme on a very wide range. If all other bears had birth weights of 1% and pandas had a weight of 0.1% that would be striking, but when other bears vary as much form each other as they do from pandas it’s really just extreme selection. Or to put it another way if the sun bear became extinct we’d be asking why the spectacled bear was so anomalously large at birth, when in fact no such anomaly exists.
This is really just a variation of the original question. They’re born small and helpless because the gestation period is short. Birth size and gestation period aren’t two independent variables.
Not really. Gestation in bears is a tricky concept because bear embryos don;t develop in linear fashion, Instead they have a prolonged quiescent phase. The animals mate in spring and summer but the embryos only develops within the fallopian tubes and don’t implant. Only if the female has sufficiant body fat in autumn are the fertlised embryos allowed ot implant in the uterus. That ensures that females don’t waste resources producing young that they can’t feed. The fact that the embryo lies dormant for anything up to 5 months makes calculating a ‘true’ getstaion period tricky.
In both pandas and black bears true gestation is about 8 weeks and there can be a delay in implantation of anything up to 5 months in both species.
So pandas aren’t really unusual in this regard.
We don’t know near enough about pandas or other bears to say whether pandas are really different. Sun bears for example cycle several times a year. In contrast pandas appear to be fertile for a few days in spring/early summer just as most other bears . If pandas do turn out to have multiple estrus cycles in a year that is readily explained because they are warm temperate animals. Temperate and arctic bears of course don’t have the luxury of several cycles/year since they need to co-ordinate reproduction with season, but that is widely true of tropical versus temperate animals from amphibians and reptiles through birds and mammals.
We can’t say that pandas are anomalous as far as their estrus cycle goes.
Similarly WRT age at maturity, pandas may mature at 4.5 years or 6.5 years, but we don’t really know. Contrast that with 2 to 9 years for black bears. And one thing we do know for black bears is that “Lack of adequate food resources has been shown to have a direct negative effect on female … age of maturity.
IOW black bears won’t reach maturity inside 9 years on poor quality food. Pandas living on a diet of bamboo reach maturity somewhere between 4 and 7 years. That seems like exactly what we would expect for a black bear living on the same low quality, high volume diet. So once more pandas seem like fairly typical bears in this regard.
the diet of the panda. the panda expends much more energy eating and foraging than other bears. they tend to be on the lower end of the bear weight spectrum.
polar, kodiak, and grizzleys really pack on the pounds with seafood. they are on the high end.
look around you, how many overweight vegans do you see? how many overweight omnivores do you see?
panda babies take more time to grow up and have babies due to the low nutrition from the bamboo diet. at least a black bear will binge on some lovely berries that are higher in carbs than young bamboo sprouts.
the only difference in raising young that the pandas exhibit, is that they leave the birthing den sooner than other bears. they don’t pack on birthing weight (up to 200 lbs for a polar ) and must forage within a week or 2 of birth (appx 9 months for a polar).
any bear can rollover and crush thier young. in a natural birth den (appx3-4 ft high and 5 ft deep for a polar) there isn’t much room for rolling. zoos tend to give them lots of room to manuver.
Blake: Good post. I think it’s clear now that Pandas are easily within the “bear range” of the characteristics discussed in this thread. Any more insistance that they’re not “real bears” is an unlrealistic expactation of typical variation seen in nature.
The key thing is that Giant Pandas are bears beacuse we call them bears: they’re similar enough that it doesn’t stretch the meaning of that term unreasonably, and we haven’t exclude any animals that are more bear-like. If we insisted on calling Red Pandas “bears”, but denied that classification to Giant Pandas, that might be something to gripe about. We can classify animals any way we want, but nature can be messy and these classifications only need to be self consistent to be useful.