When rearing poikilotherms at lower temperatures we often find that body size is generally bigger than those organisms reared at hotter temperatures (also known as Bergmann's rule). Why should then an important cell such as a developing sperm cell mature to be smaller in size?
Begmann's rule is most frequently applicable to endotherms. There, and in general, the larger body size conserves heat generated by the body, this is simply very good energy economy, whereas smaller animals need a more active metabolism and thus higher expense of energy per mass, due to the heat dissipation through the body surface, which can be of ecological disadvantage in temperate/colder climates vs. the tropics. And in some ectotherms, this may help conserve heat and thus higher temperature and agility longer after the animal has left a warming environment or after sunset, giving them and advantage at dusk.
As for the sperm, I do not know the answer, but since adaptation always must be viewed as the adaptation of the entire organism in all life stages, it is possible that the small sperm size again has to do with being as economical as possible with the biochemical resources available (cell membranes, proteins, cytoplasma etc.), to make as many sperms as possible and/or have them be faster with the same amount of energy expenditure vs. larger sperm that would be slower with the same amount of energy available to burn for propulsion.
Thanks Sven. I even found some references supporting Bergmann clines in ectotherms. Some say it is adaptive and others perhaps not. The consensus is clearly divided among the literature. In relation to sperm, yes there is some indication of allocational trade-off between the various pathways during development. But again clearly no single mechanism can explain this for now. But I take your answer and thank you for showing interest. Best wishes, Ram
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