Oh Baby

Today I would like to speak with you all about reproduction. Not sex or any of that fun stuff, procreation! Procreation is serious business. All organisms want to make a lot of babies and ensure that as many of those babies survive as possible. But how? Resources and time are limited so you can’t pump infinite babies out indefinitely and expect to be able to provide enough care that they all survive. Choices must be made. And by “choices”, I of course mean reproduction patterns. And by “made”, I mean evolved.

So organisms evolve these different reproduction strategies. At the heart of it, the strategies are based tendencies toward quantity or quality.

By the way, those are squid egg CASES. Meaning each of those holds hundreds of eggs.

 

Not too say that species tending towards larger quantities of offspring will then have lower quality offspring. No, it’s really a question of large quantity reproduction vs. high quality parental care. To get really technical, we say species are r-or K-strategists. “r” as in population growth and “K” as in carrying capacity. For my own memory, I find it useful to think of these as rapid reproduction vs. karing for the kids.

Species in unstable habitats with non-limiting resources (not unlimited resources, but resources where the amount or access to resource is not limiting to population growth) that can rapidly respond to disturbance and can experience rapid population growth are typically r-strategists. Habitat will limit population as opposed to competition within the habitat. Other typical traits of r-strategists include early and frequent reproduction with many offspring that have a high mortality rate. These species also tend to be small and semelparous (adults reproduce once and then die).

Conversely, species with growth limited by competition, in stable environments and that possess a slower growth rate will be K-strategists. K-strategists will live longer, but they will not reproduce as much or as early and offspring have much longer development. However, offspring survival is much higher due to extensive parental care and smaller “litter” sizes. These species also tend to be iteroparous; they’ll reproduce many times over their lifetime.

Of course, nothing in biology is set in stone. There are small starfish that brood over hundreds of eggs and large sharks that leave their young right away (life is unnecessarily hard for shark babies, but that is a tale for another day). So consider r and K reproduction strategies as on a spectrum. A spectrum with some little loops and fangly bits. Because life ain’t simple.

 

Sources

Bohn, K.M. 2011. “Life History Patterns”. Ecology. Whitman College. Walla Walla, WA.

Why Marsupials

Since I have heard through the grapevine that wombats make good pets, my interest in marsupials has greatly increased. Now, I own two dogs and little grassy acreage for a wombat to trundle majestically across, BUT A GIRL CAN DREAM. And who wouldn’t want to dream of one of these fuzzy tanks napping ‘neath the rosebushes.

They really are tanks, though. Although they’ll keep your grass short, they have a habit of walking through obstacles, rather than around.
Wombats probably come third on the list of well-known marsupials after kangaroos and koalas, both much less endearing and well-mannered. But if you ask folks what makes these animals marsupials, the answer will inevitably be somewhere along the lines of “they have pouches”. Unless you’re a smartass biologist that likes to talk peoples’ eyes and ears off with a detailed explanation of marsupial reproduction. Lucky for you, I am just such a smartass! I’ll try not to talk off any sensory organs, though.
Marsupials are defined as an order primarily by their unique reproductive tracts. Though they also have more incisors and an opposable toe on the hind feet, the reproductive tract is much more recognizable. While “having a pouch” is a grand simplification of things, it does summarize the situation pretty well. Although if that’s all you know, trying to figure out why the pouch is there gets tricky. I know, I know, “they have a pouch to carry their young in”. But think about it, why do marsupials get a pouch and humans don’t? Wouldn’t our ancestors have enjoyed a built in baby carrier? Ask a few moms if they’d prefer shelling out for a fancy stroller or baby sling to a tummy pouch and see what they say. Actually don’t, that would sound creepy. The answer is in the baby marsupial’s development; they need to chill in a pouch for several months because when born, they still qualify as embryos. Gestation (time spent developing in the womb) is very short, down to 9 days in some, so the newborn eyes, ears and hind limbs will not be fully developed. They can eat, breath and self-maintain, but these naked little suckers will need to remain pouch embryos for a while to finish development. Oh, and did I mention? They are TINY. Since “fractions of an ounce” is vague, imagine pink little kangaroos and wombats the size and shape of jelly beans and bumblebees. TINY. Zoos that own marsupials will often not even know a female is pregnant because they can’t tell until the baby is big enough to peek out of the pouch. Some marsupials, like wallabies, will overlap pregnancies and until big sister is kicked out of the pouch, the fertilized and developing ball of cells that is little brother will actually be put on hold (embryonic diapause). Yep, the mother can hit “pause” and “play” on her pregnancy. Busy human mothers be jealous.
Another particularly interesting trait of marsupial reproduction are the internal organs themselves. While eutherians (non-marsupial mammals, you are a eutherian) have one uterus and vagina connected to a pair of ovaries, marsupials possess paired uteri and vaginas with one ovary leading to each uterus/vagina set. BUT WAIT, there’s more. Young are actually born through a third, middle vagina that (in most marsupials) is temporary. Yet they have only one cloaca from which the embryos exit the mother. It’s a mystery to me, but I will update if I find out more.

Source
-. 2013. Kosciuszko National Park. 27 Jan 2013. NSW National Parks and Wildlife Services.
Cogger, Harold G., Joseph Forshaw, Edwin Gould, George McKay and Richard G. Zweifel. 2002. “Marsupials”. Encyclopedia of Animals. Barnes and Noble Books, New York.
Jackson, Tom. 2010. “Common Wombat”. Illustrated Encyclopedia of Animals of the World. Metro Books, New York.
Marinacci, Peter. 2013. “Wombat Behavior”. Wombania’s Wombat Information Center. 27 Jan 2013

Note: Apologies if the citation for the wombat photo is odd, there wasn’t a lot of info on the photo itself so my citation is mostly on the slideshow it was in on the NSW National Parks and Wildlife Services website.

Another Note: I do not condone keeping wild animals for pets, and considering how wombats like to go all Kool-Aid Man on walls and fences, I see the trouble of owning one. Also, those claws! However, generations of selective breeding can produce animals (dogs, cats, domestic foxes) that live well around humans so I just hope to all that is bright and beautiful that SOMEONE will get on that with wombats.