Selection of Laboratory Mice for Improved Reproductive Performance at High Environmental Temperature

When mice are transferred from a 'temperate' to a hot environment their reproductive output is reduced. To test whether this reduction could be countered by selection, three mouse lines, housed permanently as 32�C, were selected for increase in the numbers of young reared to 3 weeks when pairs were allowed to remain together for 12 weeks. In one line (R95) in which all fertile pairs contributed to the next generation, no improvement was observed in 19 generations. In two other lines (W21 and W34) in which only half the pairs in one generation contributed to the next, productivity returned, in about 12 generations, to almost the same levels as those of controls housed at 21�C.