# Why do pandas have a high probability of giving birth to twins?

According to the BBC documentary "Wild About Pandas", about half of panda birth result in twins. Why do they have such a high probability compared to other mammals? What factor(s) control that fraction in general for mammals?

I am a little surprised that it is not the ratio between the neonatal mass and the adult body mass that enters the formula, but just the neonatal mass. Any ideas on that?

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I don't know that they have a particularly high rate for multiple births up against other mammals when you consider cat litters of up to ten and beyond? – Rory M Mar 9 '12 at 20:17
So you are saying it is rather normal for mammals to have high rate of multiple births, and panda is just an example? If so, why human normally have single birth? – qazwsx Mar 10 '12 at 2:10
See Kevin's great answer :D – Rory M Mar 10 '12 at 15:25

Charnov and Ernest (2006) present data on offspring number per year and neonatal mass for 532 species of mammals. The two are related by the linear regression equation:

ln(offspring/year) = 2.4 - ( 0.3 * ln(neonate mass) )

Giant panda neonates weigh 100-200 g and are weaned at 46 weeks.

So, according to the regression, pandas should have, on average, 2.8 to 2.2 offspring per year (for 100 and 200 g respectively). With a weaning time of 46 weeks, they could have 1.13 (52/46) litters per year. If every litter were exactly twins, that would be 1.13 * 2 = 2.26 offspring per year, which is within the predicted range.

Humans (neonatal mass of 3400 g) are predicted to have 0.96 offspring/year.

Charnov EL and SKM Ernest. 2006. The Offspring‐Size/Clutch‐Size Trade‐Off in Mammals. American Naturalist 167:578-582.

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Thanks for the answer based on allometry. What's a good survey book on animal allometry? I've read Niklas' Plant Allometry for plant allometry. – qazwsx Mar 13 '12 at 2:56
I wonder why the duration of pregnancy (95 - 160 days for panda) doesn't enter the computation? Or is it included in the weaning time? – qazwsx Mar 13 '12 at 3:05
Could you check the correctness of my reasoning in the original post, including the use of duration of pregnancy? – qazwsx Mar 13 '12 at 3:43
Good point. You should probably include gestation in the calculation. There are a lot of other regressions in the paper mentioned above. I'm not sure if neonatal vs. adult mass is included though. – kmm Mar 13 '12 at 5:34

Basically a summary and details-filled version of the above.

Based on

• ln(number of offspring/year) = 2.4 - ( 0.3 * ln(neonate mass) )
(c.f. Charnov EL and SKM Ernest. 2006. The Offspring‐Size/Clutch‐Size Trade‐Off in Mammals. American Naturalist 167:578-582.)
• litter/year = 1 / (pregnancy duration in years + weaning time in years)

it can be deduced that

• number of offspring/litter = exp(2.4 - 0.3 ln(neonatal mass in grams)) / (pregnancy duration in years + weaning time in years)

For pandas, the parameters have values

• neonatal mass in grams = 150 - 200

• pregnancy duration in years = (95 - 160) (days/years) = 0.26 - 0.44

• weaning time in years = 46 weeks/years = 0.88

Therefore, for pandas,

offspring/litter = exp(2.4 - 0.3 ln(200)) / (0.26+0.88) to exp(2.4 - 0.3 ln(200)) / (0.44+0.88) = 1.7 to 2

This means, pandas on average have slightly less than 2 offsprings per litter, which means a high probability of having twins, and sometimes single, but rarely higher multiples. Comparing to human which has about 0.7 offsprings per litter, panda has higher probability of having twins mainly because of much lighter neonatal mass.

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