How to tell an animal’s age in a heartbeat

‘The days of our years are threescore years and ten — Psalm 90

For Newton, time was absolute, ‘outside’ all natural events. Relativity theory says, however, that time flows differently depending on how fast one is moving and any gravitational force acting. Then there is ‘physiological time’; species age at differing rates. According to an old rule of thumb, a heart is good for 800 million beats. Small creatures lose body-heat rapidly, so their hearts and lives move faster. Your collie dog, with up to 140 beats a minute, is lucky to celebrate their 12th birthday.