Yesterday a new scientific study was published, documenting that even seals train dry (doing terrestrial apneas) to improve their dive response (specifically they tell the purpose is developing of bradycardia). They begin as young as in the age of one month only:
Terrestrial apnoeas and the development of cardiac...[J Comp Physiol . 2008] - PubMed Result
Terrestrial apnoeas and the development of cardiac...[J Comp Physiol . 2008] - PubMed Result