Source. Myotis lucifugus. © M. A. Tuttle, Bat Conservation International
One physiological problem (of many) that hibernating mammals face is the preservation of gametes during the winter months. During torpor, most animals do not continue spermatogenesis (the creation of new sperm) because it would require too much energy, but they also need sperm to be immediately available when they awake. This is because in many cases, the earliest born offspring have the best chance of surviving the following winter because they have more time to develop and build energy stores. So, hibernating animals must find a way to preserve the sperm they make during the period leading up to the onset of torpor.
Several bat species, such as the Myotis lucifugus pictured above, have evolved an interesting way to combat this issue: they increase the osmolality (the ratio of solutes to water) in the epididymal tubes where the sperm are stored. This works for three reasons. First, the epididymal tubes are deeper in the body than the testes, so storing the sperm there will reduce the drop in temperature experienced by the sperm. Second, the increase in extracellular solutes draws water out of the sperm cells, making them immobile. Third, the increase of solutes in the extracellular fluid reduces the activation energy of water, affording the cell membranes extra resistance to "cold shock".
Fig 1: Model of events in bat epididymis prior to torpor (adapted from Critchton et al., 1994).
To model how this process might happen, various solutes enter the epididymal tubes, drawing water in with them via osmosis and causing the epididymal tubes to swell. The solutes are then converted into forms that are impermeable to the epididymal epithelium and the sperm plasma membrane, so that water stays in the epididymal tubes and extra water is drawn out of the sperm cells, which shrink and become immobile until the end of hibernation. In some species of bats, the epididymal osmolality can be five times larger than plasma osmolality, and 3-4 times larger than normal epididymal osmolalities during active months!
Fig 2: Electron micrographs of bat spermatozoa at different osmolalities.
A: 100 mmol/kg water, B: 300, C: 500, D: 1000.
The first impression I get from this picture is, wow, bat sperm is kinda scary looking and seems very appropriate for a Halloween post. But back on topic: bat sperm seems to be most active between the osmolalities shown in panels B and C, where D is osmolality during hibernation, and A is when the sperm cells begin to lyse (burst) from the influx of water due to hypertonicitiy (you can see the plasma membrane of the head of the one sperm cell beginning to swell in this photo).
The epididymis of bats normally has a slightly higher osmolality than the blood plasma, and it appears that these bat species have exploited this condition to the extreme during their hibernating months to maintain their vitality upon wakening.
Happy Halloween everyone!
E. G. Crichton, B. T. Hinton, T. L. Pallone and R. H. Hammerstedt (1994). Hyperosmolality and sperm storage in hibernating bats: prolongation of sperm life by dehydration Am J Physiol Regul Integr Comp Physiol, 267, 1363-1370