biology zoology blog crowding swarm benno meyer rochow

Crowding and its effects…

Are generalizations about crowdings possible?

There are still an estimated 5,000 languages spoken in the world (with some sadly only a handful of speakers left) and not being a linguist I may be wrong with my statement that English is probably the language with the most words for a large congregation, assemblage, group, cluster, band, troop, pack, flock, swarm, herd, etc, etc, etc, of organisms. When zoologists talk about “crowding” they commonly mean an unusually large number of individuals occurring together in the same place, in other words, a particularly high population density of specimens in a given area. I avoid crowds because I feel one loses individuality in crowds; one conforms in looks, behaviour, attitude and action. And it is these changes that are of interest to zoologists studying the effects of crowding, Since there are these multiple effects, let‘s examine some of them and insects provide very good examples.

Locusts like Schistocerca can occur in incredibly high numbers during swarming periods and it has long been known that swarming and solitary locusts look different. It starts already with the immature hoppers. If reared under crowded conditions the locust hoppers invariably end up with black patterns on a yellowish background, but reared in isolation they develop very little or no black coloration at all. Amount and quality of the food given to the hoppers had no effect on the coloration of the crowded group. An effect of crowding other than darkening was that sexual maturity, culminating in laying the first batch of eggs, was reached in almost half the duration it took the isolated individuals. Moreover, the offspring of the crowded lot was relatively darker and heavier than that of the isolated parentage.

Very similar observations with regard to body coloration were made on caterpillars of the moths Anticarsia gemmatalis and Prodenia litura: crowded caterpillars turned into darker coloured individuals, but in contrast to the locusts, crowding in the caterpillars was associated with significant decreases in pupal and adult body weights as well as sizes. In the moth Spodoptera littoralis crowding also had some effect on darkening, pupal weight reduction and shortened the larval period, but it lengthened the pupal stage. It was, however, also noticed that darker caterpillars were more active than pale ones. How careful one has to be with generalizations is borne out by observations on crowding in caterpillars of the monarch butterfly, a species famous for its hibernation roosts in the trees of Mexico State’s Valle de Bravo where millions of individuals of it spend their winter months.

In the monarch butterfly caterpillars showed the opposite effect to what had been described in the moth caterpillars (see above) with regard to crowding, for it did not affect coloration and actually resulted in greater rather than smaller body size and in faster development. It seems that feeding together with others stimulated individuals to be more “competitive”, eat more quickly, grow bigger faster and outperform “their neighbours”. Physical interactions between different individuals were, however, not recorded. That in beetle larvae different rules seem to operate from those outlined above for locusts, moths and butterflies are conclusions based on observations in which crowding led to encounters between individuals and a severe reduction in pupated individuals. What crowding does to vertebrates and especially humans is another story yet again that requires a separate essay.

© Dr V.B. Meyer-Rochow and, 2018.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

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