Bats: Saving weight
The bats have many of the modifications developed by birds in order to save body weight. The bones in the tail are thinned to mere straws to support the flying membrane or have been lost altogether. Though they have not lost their teeth, their heads are short and often snub nosed and so avoid being nose heavy in the air. They had one problem that birds did not face. Their mammalian ancestors had perfected the technique of nourishing their young internally by means of a placenta. Evolutionary developments can seldom be reversed so bats have not been able to revert to egg laying with the associated benefit of weight saving that occurs in birds. The female bat must therefore fly with the heavy load of her developing foetus within her. In consequence, bats usually have one young born per breeding season. This, in turn, means that if the population is to be maintained, the females must compensate by having long reproductive lives, and bats are for their size, surprisingly long-lived creatures, with a life expectancy of up to twenty years.