# The heart of a Pigmy Shrew beats 1300 per minute then why is it so fast?

A pygmy shrew is such a small animal. Even with such fast heart beats their average life span is 1.5 years. So can we consider faster heartbeats to not be good for health?

• Edit: I think for rodents in general, a short life span is preferable, since they are more along the lines of an "r-selected species". en.wikipedia.org/wiki/R/K_selection_theory#r-selection – Ro Siv Aug 25 '15 at 20:25
• I think that if the the heart beats faster then it brokes down very easily. So, a medium heart beat is related with a medium life and etc. – The_Mad_Fish Aug 25 '15 at 20:41

There is a correlation between the heartbeat and the life expectancy as you can see below.

(source: medcan.com)

This relationship is not linear (the y-axis is on a logarithmic scale) and is negative, that is, the faster is the heart beat, the lower is the life expectancy. You'll note the outlier (human) that has a life expectancy that is twice as high as you wold expect from the regression. This shift toward the right is caused by medicine, food quality and hygiene.

Correltation vs causation

Note that correlation doesn't imply causation. Typically, you'll notice from reading the species names on the first graph that species that have a fast heartbeat (and therefore a low life expectancy) are small in size.Allometry is the study of the relationship of body size to shape, anatomy, physiology and behaviour. You might want to have a look at the wiki page.

can we consider faster heartbeat is not good for health?

No! it is wrong to think from from those data that the shrew (that has low life expectancy and fast heartbeat) that if they were to have a slower heartbeat, they would live longer. This is wrong. Shrews have the heartbeat they need to have. If one shrew (due to a mutation or some environmental developmental effect) had a slower heartbeat, it would die very early.

In other words, such inter/species correlation doesn't tell anything about intra-species correlation. To reuse the example of the relationship between mass and life expectancy: Body mass and life expectancy are positively correlated among species (see above) but the mass at which life expectancy is maximized in humans is average. The below graphs shows the relationship between body mass and Relative Risk (RR) of coronary heart disease.

(source: revespcardiol.org)

Health ≠ life expectancy ≠ adaptation

Life expectancy, health and high fitness (biological sense) are three concepts that are eventually related but that are very different. While life expectancy can compare health in inter-species comparison, it makes little sense to compare health of different species. Note also, that natural selection may well favour short lifespan (as suggested by @RioSiv, you might want to have a look at life-history r/K selection theory).

• Where are you getting the figures that the livespan of a dog is greater than 25 years? I wish this was the case, but it is unfortunately more along the lines of 10 to 12 years with larger breeds being shorter. – AMR Aug 26 '15 at 2:16
• Sorry... missed the part about r/K... Deleted the other comment. – AMR Aug 26 '15 at 2:36
• @AMR hum.... good point. Looking at the two figures, there seem indeed to have some weird values for life expectancy. They may consider the world records as averages very much depend on conditions/country. I don't know. The pictures were found on Google Image (there are tons of such graphs with the same crazy values for the dog). I am at home now, I don't have access to the peer-reviewed articles. – Remi.b Aug 26 '15 at 3:35
• Can any one help me with my another question about lips ? – Chloritone_360 Aug 29 '15 at 17:48
• You should NOT use comments to attract attention to other posts. I know nothing about human physiology and can't help you with your lips question. If you want to increase your chances to get an answer you can try to motivate people with a bounty but you'll have to wait 2 days after the question has been asked to be allowed to use a bounty (more info here). – Remi.b Aug 29 '15 at 18:20