02 August 2005

Bigger is not always better 1: miniaturization in animals

Some generalizations

This is the first entry in what I hope will be a series of posts on the subject of miniaturization in animals, specifically in land snails.

I am writing these partly to organize my own thinking, so the content may get technical at times. Also, if this were a paper intended for publication (it actually is, but not yet), I would start out by giving examples of the tiniest animals. But, the nice thing about writing for the Internet is that I can start anywhere I want and post what I’ve written in any order I want. I will revise and reorganize as I go along.

The subject of miniaturization brings 2 broad questions to my mind:
1. What are the evolutionary processes that favor and result in miniaturization of animals?
2. What are the consequences of miniaturization?
Tied in with the last question are additional ones. What are the limits to miniaturization? Or, in other words, what is the smallest animal that has evolved and can theoretically evolve? Also, can one species become very small and still retain the morphological and physiological characteristics of larger species?

Rensch1, early in the 1950s reviewed the then available evidence on the genetics of body size and concluded that “body size is brought about by single genes in some cases and by several genes in others”. This is probably true, but then, he continued with the following statement:

Nevertheless, body size must invariably be considered as a single character in processes of selection, and if this character is favored, a large number of morphological, anatomical, histological, physiological, and developmental relations will be changed in the process.
I have to do some more thinking to understand what he exactly meant by this and to decide whether I agree with him or not.

Regarding the anatomical changes that accompany miniaturization, Rensch1, also offered the following generalization: “In approaching the lower size limit of phylogeny, the organisms usually reduce special structures and special organs and only the indispensable mechanisms are maintained.”

Hanken & Wake2 further developed this idea. From their review, I have extracted following 3 possible evolutionary consequences of miniaturization in animals:

1. Underdevelopment or loss of organs.
2. Variation in the presence/absence of an organ.
3. Morphological novelties.

There are numerous examples of underdevelopment or loss of organs and related variation in miniature land snails. I will present examples and continue this discussion in a future post.

1. Rensch, B. 1959. Evolution above the species level. Columbia University Press.
2. Hanken, J. & Wake, D.B. 1993. Miniaturization of Body Size: Organismal Consequences and Evolutionary Significance. Annual Review of Ecology and Systematics, 24:501-519.


Tim Pearce said...

Here are some instant reactions to your post, not yet very well thought out.
My first thought is that there must be a lower size limit, because cells have a lower size limit, don't they? However, cells themselves vary widely in size, for example didn't they recently find a 1mm diameter bacterium (that is a prokaryote, by the way)?
Then again, the number of cells in an organism is variable, for example, in the phylum Mesozoa, the beasts (which are parasitic in cephalopods) contain only about 50 cells. Certainly single celled organisms exist. So at this level it would seem that reduced cell number would not preclude life, although a reduced cell number might drastically change an organism relative to other species in its lineage.
Regarding cell size, I believe that the putative bacteria found in a Martian meteorite several years ago were about 1/20 the size of the smallest known bacteria on Earth. That size difference sparked debate on whether the Martian items were indeed life, but it remains possible.
In another example of cell size, a researcher in Ontario was interested to learn more about the land snail Punctum minutissimum because it is only 1mm diameter, and he was studying larger land snails that had single cells in their brains that were 1 mm diameter! Since it is possible to locate the homologous brain cells in different species of land snails, he was curious to see the homologous cell in the brain of P. minutissimum. Unfortunately, he hasn't found time to look.
Juveniles raise another issue. Since a juvenile is an individual, then the size of a newly born or newly hatched individual could be taken as one measure of the size of that species. The juveniles of P. minutissimum are definitely less than 0.5 mm, so I find it completely believable that adults of some Diplommatinidae could be 0.7 mm long, e.g., Diplommatina (Moussonia) typica (see fig. 1254 on p. 484 in Wenz 1938).

Wenz, W. 1938. Gastropoda, Teil 1: Allgemeiner Teil und Prosobranchia. Pp 1-948, Volume 6, in Schindewolf, O.H., Handbuch der Palaozoologie. Gebruder Borntraeger, Berlin.

h0g said...

Interesting topic. Look forward to reading more.

Cheers - Pete.

deniz bevan said...

okay, here we go with an extremely lay person's comment, kind of along the lines that Tim already suggested - when talking about the size of animals, the "invisible to the naked eye" microscopic ones don't count, right? I mean, does the exploration of miniaturization relate only to animals that may evolve into smaller versions of themselves? And related to that, would the domestication of animals or even the growing human interaction with ecosystems over the years have a measurable impact? I'm thinking mainly of the smaller/cuter/cuddlier aspects of animals, mainly mammals, surviving inordinately due to human affection for those types (preservation of tigers and pandas vs. certain walrus species maybe?). Then again, cat and dog breeders seem to act almost entirely against this idea, as they seem to keep breeding these elongated, hairless and generally unattractive varieties...


The microscopic ones would count if they were miniatures of bigger ones.

Many large mammals & large flightless birds went extinct at about the same time humans were first migrating to their territories, for example, Australia & North America. Their demise is attributed to either overhunting by humans or to climate changes. I believe the recent evidence is putting the blame more on humans.