To be fair, the shorter average lifespan really only applies to females (possibly only non-breeding females, as some breeders seem to find that their retired does live lives as long as their male counterparts). The cancer issue is fairly insignificant in males, as their reproductive system isn't evolved to operate at 200% the way a female's is.
Female rabbits evolved (in the wild) around being able to mass-produce offspring - right down to having a bifurcated uterus, being "induced ovulators" and being especially receptive towards getting pregnant in the first 72 hours after giving birth. The first two characteristics are relatively unique in the mammal world - a bifurcated uterus allows mom to carry two litters (which can be of different ages and sired by different males) at once; induced ovulation means that they ovulate *after* sex occurs as a direct response to having sex - most non-egg-laying mammals ovulate (aka "go into heat" for animals) on a regular schedule.
Rabbits also have a very short gestation period compared to other mammals with a similar life-span - they're designed for a quick turn-around time between the start of one pregnancy and the start of the next - and they produce somewhat large litters (typically from 4-12 kits).
In the wild, rabbits have a very short life span because they're fragile prey creatures. To succeed (meaning to propagate the species and maintain their overall population), rabbits have to be able to crank out those babies. In addition to all the characteristics mentioned above (which are shared by both domesticated and wild rabbits), wild rabbits' offspring are often out on their own by 4-5 weeks of age so that mom can care for the next litter (compared to 6-8 for the offspring of domesticated rabbits).
Unlike females, the reproductive system of a male rabbit has no major/noteworthy differences from that of males of most other species - it's the female whose biology has evolved to accommodate mass-production of offspring, hence females have display sky-high risks for reproductive cancers compared to males.
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Feel free to skip the rest of this post if you've got no interest in a biology lesson/refresher course - while I LOVE biology, I know this stuff tends to bore most people (kind of like me and Chemistry classes, lol).
For comparison purposes:
In a fertile/"normal" human female of reproductive age who is not currently pregnant, her body prepares itself for pregnancy on a monthly cycle (technically, 25-35 days, varying from woman to woman). Like in ALL mammals, the reproductive system's behavior is regulated by hormones.
In the first half of a woman's cycle, the wall of the uterus thickens and - once an egg is released - the lining is prepared so that it will be receptive to the implantation of a fertilized egg. One egg is released from an ovary (which one it is in any given cycle appears to be random) with rare exception - for example, fraternal twins occur when BOTH ovaries release an egg (identical twins form from a single egg after fertilization occurs). The egg enters the adjacent fallopian tube and waits to be fertilized (it breaks down if not fertilized within a day).
The process that results in an egg being released actually begins at the very start of a woman's cycle (ie right after menstruation ends) - it takes about half the cycle for a woman's body to determine which follicle of many will ultimately be the one to fully develop into an egg and then for that development to occur. If fertilization does not occur, the uterine lining will be shed via menstruation as the final part of the cycle... then her body does the whole thing over again... every freaking month for decades *grumble*.
While that's a (relatively) simplified version of things, a human female's reproductive system is basically "doing stuff" 24/7/365 in order to prepare for the 1-2 days out of each month that she's actually fertile (technically speaking, the fertility window that people looking to prevent or achieve pregnancy are concerned with is longer than this, but that's accounting for the life-span of sperm). Now imagine how busy it would be if it had to do enough "stuff" to be fertile **** near ALL the time...
[Disclaimer: Much of the readily available "scientific" information about rabbits' reproductive systems seems to be publicized through the meat rabbit industry; in particular, through agricultural schools. This makes sense, as those who breed for other purposes are more "hobbyists" than part of "an industry" - you don't go to school to get a degree in breeding show rabbits. This isn't to suggest that non-commercial breeders are less educated/informed about rabbit husbandry - it's just that information specific to non-meat rabbit breeding is disseminated differently and less publicly; also, the agricultural industry has a different focus for their research - responsible non-commercial breeders have no interest in "getting the most" out of their does as they breed for quality, not quantity, so at times, they may follow "conventional wisdom" more than nitty-gritty scientific crap.
What I'm getting at is that some of the links in this post are to sites where the practice of producing/raising meat rabbits is discussed. While there is NO "graphic" information on any pages I've linked to, certain links may never-the-less be considered "offensive" (as a matter of principle) by some pet owners/non-commercial breeders. That said, the links are primarily only there as a pseudo-bibliography anyway, just in case someone wants to check my sources.]
Rabbits don't ovulate on a set schedule, but rather 9-13 hours after sex occurs. Not all female rabbits are always receptive to mating - from
http://www.medirabbit.com/EN/Uro_gen_diseases/Fem_rabbit/Fem_en.htm:
A certain cycle does nevertheless exist. The presence of the estrogen hormone will influence the size and the color of the vulva. Most female rabbits are receptive to a male and prone to mate when their vulva is colored reddish/purple, and will refuse to mate when their vulva is pale and small. This is no clear indication though, as some female rabbits will mate when their vulva is pale and small.
(Another source suggested only about 10% will be receptive to mating and do so successfully when their vulva is pale and small.)
An interesting side note - I just learned that females are actually infertile during a "false pregnancy". (Sources:
UW-Madison animal studies website and
wikivet , among others) Even more interesting, according to an
agricultural university site that I just skimmed, false pregnancy can result from girl-on-girl humping in rabbits (which can be caused by hormones in unspayed females in addition to being a dominance display in any females)
- "Sometime females may stimulate each other to the point of stimulation." (It's phrased a bit awkwardly; basically, it's possible for females to stimulate each other enough to induce ovulation.) [This information is verified by
another site, which is focused on show rabbits - "These false pregnancies may result from infertile mating or from sexual excitement when does 'ride' each other in group housing.] Presumably, the same results might be achievable through a rabbit "humping" one of her toys.
According to the first site that mentioned girl-on-girl action, "It is known that follicles develop and regress in cycles of 15-16 days" (regarding follicles inside the ovaries which have to develop in order to release an egg). However, the second site says "Due to the fact that sexually active does normally have follicles present on the ovary, the time of breeding is dictated, to a large degree, by the number of litters desired per year." Another page from the UW-Madison site says "Estrous cycle: No regular cycle although females are usually receptive to breeding at 4-6 day intervals: Induced ovulator."
I'm guessing the truth is in the middle - that, while follicles are always present, the number of developed follicles (ie ready to release eggs) varies in a cyclical fashion, affecting the desire to mate and odds of successful pregnancy. This means that even though female rabbits can theoretically get pregnant at any time, they're more receptive towards mating and more likely to become pregnant at some times than they are at other times.
Unfortunately, I wasn't able to track down the nitty-gritty scientific details of how many follicles are developing at any given time and only vague information about how the uterine lining is prepared for pregnancy (though it seems to be triggered by ovulation).
The TL
R of all of this is that, while I can't explain the specifics, a female rabbit's reproductive system has to be "ready to go" at a moment's notice rather than having a pre-determined cycle for when eggs are released. One can gather from common sense and what information I *did* find that a female rabbit's reproductive system is pretty much in "overdrive" relative to non-lagomorph mammals (in terms of what it's doing to prepare for the possibility of pregnancy), meaning the rate of cell division/reproduction in the ovaries and uterus is presumably much higher than in most mammals... and cancer cells result from routine cell division gone awry, so their high cancer rates should come as no surprise.
While I didn't cite it specifically, I also found http://www.fao.org/docrep/t1690e/t1690e05.htm to be an interesting read (though it is meat-rabbit oriented, it has some good general information).
Anyway, sorry for the semi-off-topic wall of text! I figured some people might be interested in why spaying is so much more important in female rabbits than in most species and more important than neutering is in male rabbits.
