Like-size dog vs donkey: a contrast in pace of life

Imagine that you live on a small farm where you keep Minidonk, the smallest donkey you know, together with Maximutt, the biggest dog you know.

Your two pets are about the same size, and they play together in the paddock behind your house.

Which pet costs you more to feed, and why?

Your first thought may be 'the dog of course, because the donkey can graze for herself', but the question is much deeper than that. So deep that if you figure out its biological meaning you may never see your pets - or any other animals - in the same way again.

Maximutt burns up energy much faster than Minidonk does. This is not because he is a carnivore but because canids have a fast pace of life in a physiological sense, whereas equids have a slow pace of life in the same sense.

Even if you lived in a waterless area, and you had to buy all the straw needed by Minidonk, feeding your donkey would still cost you much less than feeding Maximutt.

Behold the body size of the smallest-bodied breeds of the donkey: https://soumo.eu/cute-donkeys-10/ and https://www.guinnessworldrecords.com/world-records/shortest-donkey and https://www.dailymail.co.uk/video/news/video-1609300/Video-Tiny-Tim-Mini-Donkey-enjoys-playing-pillows.html and https://www.youtube.com/watch?v=MIQKlHSD89E and https://www.alamy.com/old-man-riding-a-donkey-in-a-mountain-village-image158358068.html and https://travel.mongabay.com/china/images/china_104-8142.html.

Now behold the body size of the largest-bodied breeds of the domestic dog: https://es.123rf.com/photo_654533_giant-dog-and-woman.html and https://imgur.com/gallery/OhYPphE and https://www.distractify.com/p/wolfdog-kill-shelter-rescue and https://www.dreamstime.com/beautiful-woman-big-fluffy-dog-nature-caucasian-guard-sheepdog-image212481454.

Obviously your pets, one derived from large ancestors and the other derived from small ancestors, have come to converge in body size through selective breeding in domestication. But how close has this brought them in pace of life - the rate at which the animal uses resources physiologically, as reflected by its metabolism, growth, reproduction, and senescence?

The answer is: much less than you might assume.

This thought-experiment is worth doing because body mass is one of the most important descriptors of any organism. Other factors being equal, the smaller the body the faster its pace of life per unit mass of the body. However, Maximutt is an exorbitant pet and like-size Minidonk an economical one because even mammals similar in body size can differ in pace of life as part of their ecological niches.

In the case of the dog, wild ancestors weighing perhaps about 15 kg have been stretched via selective breeding into modern breeds weighing more than 50 kg. In the case of the donkey there has been scant selective breeding for minimal body size, but wild ancestors weighing perhaps about 250 kg have nonetheless been compressed enough to produce individuals weighing as little as 90 kg.

Given that more of the weight of the body is gut contents in donkey than in dog, we should make a discount to correct for the difference in gut-fill. This brings the maximum body mass in the canine species to perhaps 60 kg, comparable with perhaps 75 kg as the minimum body mass of the equine species.

Bone probably also contributes more to body mass in the donkey than in the dog: equine jaws are particularly solid and the bones of hoofed feet are likely to be denser than those of pawing feet. How could miniatures of the donkey carry adult humans if their feet were not made of really strong bones?

So, after discounting both gut-fill and non-metabolising matter in the form of teeth and bone, we probably have the same mass of flesh in Minidonk, the smallest individual of the donkey, as Maximutt, the largest individual of the dog.

Now, why is it that the flesh of Minidonk has a far slower pace of life than the flesh of Maximutt?

Well, African wild asses (ancestors of the donkey) are adapted to stony semi-deserts - poor environments beyond the mainstream of life. They have evolved to cope with a poverty of resources by slowing down their consumption of food and water, and thus even their breathing (though oxygen is not in short supply). And this economical way of life remains in the donkey because selective breeding concentrated on making the animals docile and obedient, and at the same time as cheap as possible to keep.

So it is in the nature of equines - and particularly the donkey - to metabolise, grow, and reproduce more slowly than do canines. The selective breeding of particularly large or small individuals has affected the species-specific physiological processes relatively little because these processes are genetically 'hard-wired' in the wild ancestors with their respective niches.

The body temperature of Minidonk is only about 36.6 degrees Celsius, whereas that of Maximutt is about 38.7 degrees Celsius. This difference of two degrees - which remains even in miniatures of the donkey and giants of the dog - makes a great difference to the rate at which the cells use energy and oxygen, consume food, produce wastes, and wear out.

Partly because his metabolism is so much more rapid than that of Minidonk, you can expect Maximutt to become senile by ten years old (https://en.wikipedia.org/wiki/Aging_in_dogs). Minidonk may become senile only after 30 years. This three-fold difference is a biological clue to the cost of life from day to day.

Though you do not intend to breed your pets, consider the divergent gestation periods. If Minidonk were to conceive, she would give birth after about one year. By contrast, any mate of Maximutt would gestate for only about two months regardless of how large-bodied she is (https://www.researchgate.net/publication/276212655_Factors_affecting_pregnancy_length_and_phases_of_parturition_in_Martina_Franca_jenny and https://pubmed.ncbi.nlm.nih.gov/8122353/).

Despite the donkey gestating so much longer, its litter sizes are about tenfold less. The donkey bears only one at a time, whereas large-bodied breeds of dog bear on average about ten. The maximum, recorded for a Neapolitan mastiff, is 24 newborns (https://www.akc.org/expert-advice/dog-breeding/average-litter-size/#:~:text=A%20normal%20litter%20size%20can,according%20to%20AKC%20registration%20data), compared with a maximum of only two in the donkey. This difference in fecundity is consistent with the donkey being long-lived whereas the dog is short-lived.

So return now to watching your two pets, resting in the paddock, and this time imagine that the oxygen they breathe is being combusted rather than metabolised. Can you see that Minidonk would look like more like a glow, and Maximutt brighter, more like a blaze? The fire of life only smolders, as it were, in the donkey because frugality is its niche in life, just as it was for the ancestral wild asses.

Because money is a proxy for resources, particularly energy, imagine the different paces of life of your two pets as the main determinant of how rapidly you need to pay to maintain their lives. Do you now see why Maximutt is likely to be more expensive than Minidonk?

Julkaistu syyskuu 25, 2021 08:51 AP. käyttäjältä milewski