23 August 2005

Bigger is not always better 2: what limits the smallest size in animals?

The effect of egg size on body size

In the first entry in this series, I introduced some general concepts from the literature. If this were a manuscript intended for publication, at this point I would give examples of some of the smallest known animals, but I haven’t finished compiling my list. So, instead I am skipping ahead to a discussion of the limits to minimum size.

Rensch1 discussed one potential factor that may limit how small an animal can get.

...the eggs set a lower limit to body size because their size cannot be reduced ad libitum. This fact seems to be the reason why there is only one ripe egg at a time in the smallest Gastrotricha and Rotatoria, in the minute snail Caecum glabrum, and apparently also in the smallest frog Phyllobates limbatus. There are other such minute types of terrestrial snails, such as Punctum pygmaeum, Pyramidula rupestris, and species of Vallonia and Vertigo, in which only one or two eggs will ripen simultaneously and the smallest cyprinodont, Heterandriaformosa, usually gives birth to only a very few young at short intervals.

Of course, the largest mammals and the largest birds also tend to produce one offspring at a time, but possibly for other reasons.

What Rensch means by egg dimension being a lower limit to body size is that as animals get smaller, the masses (or sizes) of their eggs relative to the adult masses (or sizes) tend to get larger2. An example of this relationship is provided by the change in the ratio of the egg mass to the adult mass in birds as a function of adult mass.

Data were from Table 3-1 in Calder2.

This negative relationship between adult size and relative egg mass arises, because below a certain size eggs would not be viable, perhaps because there wouldn’t be enough room for both the embryo and its food inside the egg. One way to get around this problem would be to have the tiny eggs growing inside the body of a host other than that of the mother. In other words, very small embryos would have to be parasitic and derive some or all of their nutrition from outside the egg.

I have carried out a similar analysis with land snails by substituting linear dimensions for masses3.

Data were from Table 12-1 in Heller4.

Based on this, I tentatively conclude that the rapid increase in relative egg size as a snail gets smaller is one factor that limits the miniaturization of land snails.

1. Rensch, B. 1959. Evolution above the species level. Columbia University Press.
2. W.A. Calder 1996. Size, function, and life history. Dover Publications.
3. The tiniest snail eggs would be difficult to weigh, because they are so small and adult snails are difficult to weigh, because they normally contain large amounts of free water in their mantle cavities. I have taken into account only those species with roughly spherical eggs.
4. Heller, J. (2001) in The Biology of Terrestrial Molluscs, Edited by G.M. Barker. CABI Publishing.

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