Getting Old(er): How Long is a Lifespan?

Dogs are amazing for many reasons. One reason is their range in size within one species – from less than 1 kg right up to nearly 100kg – a one-hundred-fold increase.

Another reason is their lifespan. Smaller dogs – on average – live more than twice as long as giant breeds despite near-identical physiology, diet and environmental conditions.

For cats, lifespan is on a much more linear trajectory. Members of the Felidae family range from 15 to 30 years old. Larger cats like lions live longer than smaller cats like your own moggie domestic cat.

Why do big dogs die young? And how do small dogs – and cats – manage to avoid this early death trap?

Image by free-photos [CC0 1.0]

This article is the first in an occasional series on getting older – in pets, animals in general and, of course humans. Obviously, one critical factor that determines a lifespan is lifestyle, and one factor in premature death is discussed in another article HERE. Lifespan is also determined by hormonal and/or genetic factors and, in dogs and cats, by breed-specific vulnerabilities to disease and other environmental factors.

In this article, we explore the biological correlates of ‘normal’ aging in animals. In other words, we ask the question – what determines a natural, healthy lifespan in different species of animals?

It starts with actuarial senescence

In ecological studies of wild animals, the term – actuarial senescence – is often used to describe increasing adult mortality rates with age. Actuarial senescence operates, either through internal degenerative changes that kill the animal regardless of its environment, or through making the animal more vulnerable to hazards in its environment, for example the presence of predators, including other carnivorous animals and, of course, humans – probably the most significant global influence on unnatural death in wild animals.

Cross-species studies on a wide range of animal species have identified several factors that correlate with actuarial senescence including –

1. The gestation period of the animal.
2. The age at which the animal reaches sexual maturity.
3. The adult body mass of the animal.

Across-species comparisons
When comparing one species with another species across the animal kingdom, a larger body mass is a good predictor of a longer lifespan.

Image by laurentmarx [CC0 1.0]
Within-species comparisons
In complete contrast, comparing animals of the same species with each other (within-species comparison) shows the reverse – smaller body size in correlated with longer lifespan and larger body mass is correlated with a shorter lifespan, for example in mice and in dogs.

For a wild animal, being small means being vulnerable so there is a selective advantage in being big, and in growing big as quickly as possible. However, the cost of growing fast is a shorter life span.

Image by Alexas_Fotos [CC0 1.0]
Birds are an exception to this rule as many species of bird grow fast and live very long lives.

Dogs in particular are hugely valuable as models for research into ageing because of their unprecedented range in size within one species, from less than 1 kg right up to nearly 100kg, a one-hundred-fold increase.

The lifespan of dogs also varies significantly, with smaller dogs living more than twice as long as giant breeds, despite near-identical physiology, diet and environmental conditions.

Image by Wallula [CC0 1.0]

Why is it that big dogs die younger than small dogs?

There are 3 possible reasons –

1. Big dogs have a shorter lifespan because of an earlier onset of senescence.
2. Big dogs have a shorter lifespan because they are more vulnerable to hazards in their environments.
3. Big dogs have a shorter lifespan because, biologically, they have an increased rate of ageing.

One study on variability of aging in dogs analysed the data collected over 20 years from the deaths of 74,556 dogs, representing 74 different dog breeds.

The results showed that –

1. For every 2kg increase in body weight, there was a one month reduction in lifespan.
2. Senescence onset was earlier in larger dogs, and once it started they aged more rapidly.

Image by ASSY [CC0 1.0]
How does this influence aging?
Large breeds have to grow rapidly and for an extended period of time to reach their adult weights. This is associated with higher levels of the insulin/insulin-like growth factor-1 (IGF-1), required to stimulate skeletal and muscle development. However, increased IGF-1 is also linked to cancer, diabetes and obesity.

Take-home message

The conclusion that can be drawn from this is that larger dogs die younger because an earlier onset of senescence triggers an accelerated rate of ageing (actuarial senescence) compared to smaller dogs.

© copyright Robert Falconer-Taylor, 2018
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