Wednesday, August 8, 2007
Briar Jennies and dumb bunnies
Though I set three camera traps for mountain beavers, there was no escaping the brush rabbits. "Briar Jennies" thrive in the coastal scrub.
Kids may believe rabbits lay eggs, but grown-ups deem rabbits to be commonplace, uncharismatic, and often a nuisance.
I've always wanted to believe that knowledge begets appreciation, so my goal today is to convince you that these benign-looking creatures are an evolutionary success.
Let's start with the generalized rabbit body plan (we're talking rabbits and hares here). It's an enduring 50 million-year-old design that first appeared in the middle Eocene when the Lagomorpha split from rodents.
Rabbits and hares (of the family Leporidae) are built for speed. Elongated limbs and feet endow them with a long stride, while their speed comes from the concentration of muscles in the upper legs. They represent two basic models: hares are lanky high-speed runners that out-distance and out-manuever their pursuers. Rabbits are compact sprinters that rely more on cover to elude their predators. Generally speaking, these differences reflect their use of more open versus more closed habitats.
They have a remarkable digestive system that begins with the split upper lip. The "hare-lip" works like two fingers with the jaws, incisors and tongue to select plant parts with magical effect. The single pair of lower incisors cuts vegetation against two pairs of upper incisors, one in front of the other. An effective apparatus for pruning plants.
The two-cycle digestive engine is also uniquely lagomorph. During their wakeful nocturnal phase rabbits frequently churn out numerous bunny beans, which are recycled products of digestion. During the restful daytime hours they eat partially-digested soft feces from the caecum, which is essentially a large appendix or "fermentation vat" attached to the intestine. It sounds yucky, but it works quite well when you have to extract energy from a diet heavy on fiber. The first passage through the gut doesn't quite do the job. That's why rabbits and hares spend their inactive hours dozing, grooming, and eating partially digested feces (refecation).
When it comes to reproduction, evolution has tinkered less with rabbits than hares. The nestbuilding habits of rabbits, and the often naked, blind and helpless condition of their young are more primitive characteristics shared with most rodents. Hares on the other hand, use a dirt depression as a birth site, and the furry and wide-eyed young are hopping about in a few days.
But neither rabbits nor hares are stay-at-home moms. They stoke their young with highly nutritious milk only once a day. With an average litter size of 3 young, and as many as five litters during the 7-month breeding season, the brush rabbit can produce 15 young a year, and an ample food source for predators.
Predators hit the "dumb bunnies" hardest. The complacent and dreamy-looking weanlings are sitting ducks for bobcats, foxes, coyotes, and weasels. Only the quick-witted survive to reproduce.
The brush rabbit seen here looks to be a young of the year. Indeed it was visited by a larger rabbit, possibly its mother.
It spent most of the day lingering in front of this mountain beaver burrow, which it used no doubt for escape. When the weasel visited, it "made itself scarce". Thus, this rabbit seems to have advanced beyond dumb bunnyhood.
If all goes well, it will become a Briar Jenny, and in a few more months will go forth and procreate.
And if it doesn't go well? Then another rabbit will be there to take its place.
Asher, R. J., J. Meng, J. R. Wible, M. C. McKenna, G. W. Rougier, D. Dashzeveg, and M. J. Novacek. 2005. Stem Lagomorpha and the antiquity of Glires. Science 307(5712):1091-1094.
Chapman, J.A. 1974. Sylvilagus bachmani. Mammalian Species No. 34:1-4.
Howell, A.B. 1965. Speed in animals, their specializations for running and leaping. Hafner Publishing Company, New York.
Orr, R.T. 1940. The rabbits of California. Occasional papers of the California Academy of Sciences, XIX. 207 pp.