I came across a note this week that the first rabbit litters of the season are being born right about now.  But that’s only the first of many, right?  Rabbits are notorious for their reproductive habits.  They breed, well, like rabbits – but just what does that mean?  I did some math.

First, some assumptions and averages.  Rabbits are on-demand ovulators, meaning there’s no particular time or season when they’re fertile, and they can literally get pregnant the day after they give birth.  Assuming there are males around, a female rabbit can literally spend her entire adult life pregnant (poor Mama!).  Gestation is about 30 days, and a litter can run anywhere from 1 to a dozen little bunnies.  Female rabbits are sexually mature at 6 months of age, meaning that in a mild climate, a female rabbit can have as many as six litters in the same year she’s born!

767px-Eastern_cottonail_rabbit_close_up_sylvilagus_floridanusSo why are they so prolific?  Generally speaking, habits like this are a step in an evolutionary arms race.  Rabbits have adapted this reproductive efficiency because that’s what has kept them alive as a species.  They are a prey animal for almost every imaginable carnivore, and the vast majority of them wind up on some other animal’s menu.  Anyone who’s read Richard Adams’s “Watership Down” may remember the rabbit folk-hero of mythology and legend, El-Ahrair-Rah, the “Prince with a Thousand Enemies”.  Dogs and cats (both wild and domestic), raptors of all description including crows, snakes, badgers, skunks, weasels – the list is a long one.  Rabbits, whether mature or young, are prey.  They NEED to reproduce quickly, because so many don’t live long.

So what would happen if rabbits were protected, healthy, and free to let nature take its course?  Back to the generalities above, let’s imagine that each litter is a nice average of 6, and that only half of those are female.  We can start with one Mama Rabbit (and of course a male, but we won’t include him in the count).  Mama is part of that first spring litter, born today, and we’ll call her rabbit number 1.  For 6 months, while she’s growing up, the population attributed to Mama holds steady, at one.

800px-New_England_cottontail_rabbit_animal_sylvilagus_transitionalisAt six months of age, Mama becomes pregnant, and has her first litter of 6 babies at month 7.  She immediately becomes pregnant again, and has another litter at month 8, and 9, and so on every month for her entire life until she dies after 7 years, month 84.  In her lifetime, this single Mama will have given birth to 468 rabbits.

But, among those rabbits, half are female.  And starting at month 14, the first batch of three daughters will be giving birth to their own litters – another 18 that month.  The next month, there are TWO litters from Generation 2 that are giving birth, so another 36 are born in month 15, 54 babies in month 16… and so on.  By month 21, Generation 3 is also breeding and adding to the total.

This pyramid rapidly expands, with multiples of 6 occurring over and over, born to ever larger populations of mature females.  By the time Mama dies, on her 7th birthday, the total population will be an astounding 1.34 TRILLION rabbits, all attributable to Mama.  That’s a productive life – and it’s easy to see why backyard breeding operations among protected populations can get completely out of control in a very short time.

1024px-Wild_rabbit_usBack to our initial case though, and the presumption that Mama is in the “first litter of the year”.  If we limit the bunnies to only giving birth during the warm months, say March to August, six months on, six months off, that changes things.  Mama will be mature in August, but she’ll have to wait til next March to start having litters.  Her young will be similarly handicapped – they still have litters every month, but they have to wait until spring to start.  However, the winter months are enough that ALL Mama’s summer litters are mature and are giving birth starting in the following March.

In this scenario, the population is much more manageable – Mama will only be responsible for a paltry 94 MILLION in her lifetime.  If only one daughter from each litter survives to breed, the population drops to just under a quarter million.  So you can see how the actions of a few predators can make a significant dent in what’s possible, keeping the whole system in check.

800px-Rabbit_in_louisianaWhen predation doesn’t occur, things can get out of hand. Tasmania had a population explosion in the early 1800s after colonists arrived in 1788 with caged rabbits, bred for food.  Later, in the 1858, an initial population of 24 European rabbits were released in Australia by an English colonist.  They were cute reminders of home, and provided a bit of hunting sport.  But in a classic example of unintended consequences, the rabbits took over – with few natural predators in this strange land, the rabbits overwhelmed their new home, to the point that their own grazing decimated plant life, causing major erosion problems.  Taking two million individuals in annual rabbit roundups barely made a dent in the population.  They continue to be an environmental stressor to this day.

So two things to take away from this little exercise:
1) Predators are an important part of the ecosystem, and absolutely critical to maintain balance (and in this case prevent a rabbit Apocalypse).
2) Why don’t we humans eat more rabbit?   European and Asian markets know this, and to some extent South American ones do as well, but because of their reproductive efficiency, rabbits are a fairly productive, somewhat renewable resource as a farmed food crop.  For some reason, North Americans don’t eat many (is it the “cute” factor?  Are they too rodent-like?), but one wonders how rabbit meat could contribute to global food security.  I have no idea what the secondary impacts of a rabbit farm are, but I wonder how it compares to beef or poultry.  That’s a research item for another day.

To be clear, we don’t generally have a rabbit population problem.  I’m not advocating killing them on sight – but it does generate an appreciation for the natural balances that keep them in check.  It also makes you think of the power of geometric progression in any protected, healthy, population, even at lower birth rates… Human population curves look awfully similar…

Get Out There

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