Saturday, 14 March 2015

The reproductive resource gap

It's a commonplace observation that reproduction requires resources.

Reproduction often creates a transient, local resource depletion that decreases the chance of future reproduction in the short term. This "reproductive resource gap" is the topic of this article.

I've long known about this "resource gap". It seems to apply to many organic and cultural systems. Recently, I have been thinking about how general this principle is. It clearly doesn't apply to all creatures. For example a termite queen doesn't have a measurable reproductive gap - her reproductive system is a pretty continuous assembly line. However, for many creatures - from bacteria to elephants, there is a "reproductive gap" - where the mother has to accumulate resources after splitting and before splitting again.

Splitting typically produces offspring which are smaller than the mother. One hypothesis is that size is responsible for part of the effect - that bigger things are more likely to split than smaller things.

However, we know that - in many cases, the reverse relationship holds - i.e.: smaller things are more likely to split than bigger things are.

This is true for most organisms in the biosphere: there's a strong negative correlation between an organism's adult size and their reproductive rate. Smaller critters reproduce faster.

It's also true of very large objects. For planets and stars, smaller objects are more likely to break up than larger ones are - because the larger ones are held together more effectively by gravity.

Also, if erosion or corrosion are involved in the splitting - then these forces apply to the surface of objects - so again, small objects would be more strongly affected - because they have relatively larger surfaces.

Another perspective on the issue comes from considering a simple, common case. Some of the most common particle interactions in the universe consist of photons hitting dust particles. The photons form a a clear family tree with a few high energy photons near the root and enormous numbers of low-energy photons at the tips of the branches and a clearly-defined set of branching points - when the photons hit the dust particles and split.

In this system, the distance (or time) from one branching point to the next doesn't increase with proximity to the root. If anything, there's a tendency for collisions to be quickly followed by more collisions (if you just hit some dust there may be more things to hit nearby). This is another case where splitting tends to lead to more splitting - rather than to less splitting.

There are many other cases where splitting leads to more splitting. In a landslide, rocks that lave been stable for a long period of time might suddenly split many times in quick succession. There are many cases where splitting generates jagged edges and jagged edges result in more splitting. Beach pebbles are an example of this. Another similar case involves splitting reducing structural integrity. An egg is the stereotypical example, but similar considerations apply to many structures with membranes or skins. Breaching the outer wall leads to splitting and rapid disintegration.

At this stage a brief recap. A reproductive resource gap seems to be a pretty common feature of organic and cultural evolution - often the mother seems to need time to recharge. This applies to K-Selected creatures - and also to many r-selected ones - such as bacteria. However in many simple physical systems, there's often no reproductive resource gap. Instead, we find the opposite: splitting is likely to be followed by more splitting.

At this stage it might be tempting to conclude that the reproductive resource gap is an adaptation for managing limited resources - and that the reason simple systems don't have a resource gap is that they are degenerative systems which can't accumulate adaptations.

However, I've skipped over presenting some significant data. There are, in fact, simple physical systems that do exhibit a reproductive resource gap. One example involves raindrops in a condensation cloud. Smaller droplets have larger surface area to volume ratios - and so are more likely to be held together by surface tension. Another system involves falling ink droplets in water. The easiest way to explain this is with a video.

The video pretty clearly illustrates droplet reproduction - and a family tree of droplets.

However, there's a characteristic delay between one droplet reproduction event and the next. It is as though the falling droplets need to build up some kinetic energy before they can reproduce again.

In my opinion, these examples demonstrate that the reproductive gap is more than just an adaptation. There's a simple physics of needing to accumulate resources after splitting and before splitting again.

This is, I think, an interesting result in a poorly-studied area. Future investigations into the topic could look into how widespread this "reproductive resource gap" is in simple physical systems and what the interactions are between the simple physics of needing to accumulate resources before splitting and adaptations for resource management in organisms.

1 comment:

  1. Fascinating topic worthy of further investigation. Physics plays a significant role in stellar evolution as well as organic chemistry, so the above is plausible. Anyone pursuing such an enquiry may benefit from exploring E.O. Wilson's observations of ant colonies as a superorganism, where average workers are deprived of reproduction to specialize in food collection & defense, exerting pressure on the queen to endlessly feed and reproduce. Equally important to consider is the physical composition of insects (exoskeletons, different vascular systems) as well as insects' location within the trophic pyramid--all fundamentally relating to your assertion that the ease or cost of reproduction is grounded in physics.
    Looking forward to a future post which fleshes out this relationship to memetic reproduction!