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One million rabbits

If you have ever sat around a campfire with a bunch of mates, you will inevitably have heard countless yarns and tall tales - it’s actually one of the things I love about going bush. At one such recent event, a statement made with complete conviction was: “One pregnant rabbit can generate one million rabbits in a year!”

Now this was met with the usual concerned enquiries from the ‘campfire jury’ as to the likelihood of the speaker having recently suffered a possible concussion. However, the speaker held firm and added: “That is, if none die from natural causes or predation.” This generated a similar reception but it did start me thinking.

Putting aside the impossible parameter that no rabbits died from natural causes or predation, a Google search led me to discover that the gestation period for a rabbit is around 30 days and that the female rabbit (doe) can fall pregnant again almost immediately. Depending on the breed, a doe has a litter of between six and 12 young (kittens) and some of these can reach sexual maturity at between four to six months of age. I also discovered that although it is not ideal, rabbits are able to inbreed. So I started to do a few quick calculations...

Let’s say a pregnant doe (obviously accompanied by a male buck) has a litter of 10 kittens. If we estimate the average number of kittens at 10 to make things a bit easier to calculate, the herd would be 12 in total.

Assuming things went well, one month after that, the herd would be 22 all from the same original pair of rabbits. But after six months, I found that things started to become a little complicated. This is because the initial kittens would have reached breeding age in month five and half of those (the five does, based on 50:50 ratio of does to bucks) would have now had a litter of their own. This means the number jump up by 50 from month five to month six. This new branch of the family tree continues to pop out 10 kittens a month, but for each month after that, a new lot of the offspring from the original pair of rabbits start having their 10 kitten litters.

It was at this precise moment that I needed a cup of hot, sweet tea and went and had a lie down. Once my headache subsided, I did what I should have done right from the start and contacted my brother, affectionately known as ‘Donk’, who studied mathematics at university and leaves me for dead in this field. The conversation went like this:

“Donk, it’s me. How’re you goin’?”

“Yeah, good, thanks. And you?” came the reply.

“Yeah, all good,” I said. “I’ve got a question for you.”

Pause. “Hmm… yes?” This point in the conversation marked a distinct change in the tone of my brother’s voice as he came to the realisation that this was going to be yet another example of what I considered to be essential and fascinating research, but what he considered to be a gross imposition on his time and an exaggerated stretch of his patience.

“It’s about rabbits and their breeding cycle,” I said.

This led to a longer pause followed by a barely audible sigh, but unperturbed, I pressed on and outlined the details of the maths involved. This was followed by a pause so lengthy that I wondered if he was still on the line, but eventually he muttered: “Leave it with me.” This meant I had won. I had been able to hook his interest and I knew that once he started thinking about how to solve this problem, it would bug him until he somehow worked it out. I can be a cruel sibling, I know.

After a week or so, I received an email with a spreadsheet attachment titled ‘Ben and his rabbits’, which almost became the title of this article. In the accompanying spreadsheet, you can see that everything up to July 2016 progresses as you would expect (the total rabbit numbers are on the top of each column). But in September 2016, the numbers jump up by 50, which is a total increase in numbers of 60, as you have still another 10 kittens from the original pair of rabbits in there as well (the adult rabbit numbers are colored yellow).

After one year, February 2017 (not January 2017, because the original statement said a “pregnant doe giving birth to a litter” and this happened in February according to our sheet), the number has reached 1182. So, that is a far cry from a bold million as predicted.

However, if we continue to follow the maths, after 18 months in August 2017, the rabbit numbers are just shy of 13,000, which is a frightening thought, but it doesn’t stop there. The final interesting fact is that in the 12-month period following this (from August 2017 to August 2018), the rabbit numbers go from 12,842 to a staggering 1,282,562 which is an increase of more than 1.25 million rabbits. This is only two and a half years after our theoretical breeding program began.

As stated, it is sound in theory only, and of course rabbit numbers could not increase at this rate in the wild. However, it does definitively prove that the more rabbits you have, the more rabbits you are going to have to tolerate. Indeed, after three years of theoretical breeding, the numbers are just simply staggering! The least we can do as responsible hunters is to do our bit to stop them taking over the world. .

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