From the adored cats in Ancient Egypt, to the more modern rescue-sled-dogs in Alaska; there is no doubt that companion animals are and always have been a fundamental part of our lives. It is also no secret that there seems to exist a very distinct division between cat people and dog people. I personally love both pets (almost) equally, but as a pet owner working in impact assessment, I could not help but wonder what the environmental footprint of owning either a dog or a cat is. So let’s dive into the question: what is the most sustainable pet?
The lifecycle of a pet simplified
To simplify the calculations, the average size and lifespan of the two pets was determined based on available online (average) data. In order to assess their impact, some more assumptions had to be made about each pet (which can be found in the following paragraphs)
Cats
Let’s start with the cat. The average cat weighs 4 kg [1] and lives for 17 years [2]. Per day, an average cat needs 316 grams of wet food or 70 grams of dry food [1]. The impact of the food includes the packaging, the manufacturing and storage of the food itself, and the use of the pet dish the food is eaten out of, including making, washing and disposing.
For their daily business, cats need a litter box, a scoop and litter. For this LCA, it was assumed the litter box to be a simple 60L plastic box weighing about 2kg which is replaced once a year; for the scoop, it was assumed also to be made of plastic and weighing 54 grams, but it gets replaced less, once every 3 years. These values were found by looking at average products online for cat litter box materials and making an estimate.
The most common litter material is bentonite clay, a swelling type of clay which helps to eliminate odors [3]. For a 60L litter box, it was assumed that it would need to be filled to 1/3rd with litter, which equals 20L of litter. The litter comes in a paper bag, the impact of which was included in the LCA.
In this LCA, it was assumed for the litter to be completely replaced once a week, and for it to be collected and thrown out in a regular trash bag and incinerated. The cleaning of the box, including water and soap, was also included in the analysis. In terms of outputs, a cat releases about 46 grams of feces per day [4]; as urine is absorbed in the litter, its impact was omitted from this analysis.
Dogs
The average dog weighs 15 kg [1] and lives for about 13 years [5]. Per day, an average dog needs 1046 grams of wet food or 233 grams of dry food [1]. As for the cat, the impact of the food includes everything from packaging to disposal of pet dishes. For their daily business, it was assumed a 100% collection rate of feces by the owners using an average disposable bag made of compostable plastic (PLA), weighing 1.7 grams.
Dogs on average release about 200 grams of feces [5], one time per day [6]. The impact of the dog bag itself, as well as the impact of the bigger trash bag it is thrown out in, is also included in this LCA. To urinate, dogs go into nature and release about 400ml of urine per day. [5] The impact of the components of the urine was also taken into account and modeled according to the values given by [5]. Finally, transport of materials for e.g. litter and dog bags was not included in the analysis due the uncertainties of the origin of the materials.
There are of course many other variables that could be included in owning a pet, including toys, treats, veterinary care, etc. However, for the purpose of this study these factors were not included in the analysis. Also due to the assumption that this would be relatively similar for both, and the difficulty in retrieving those data.
Impact on Climate change per year of ownership
The impact on climate change of owning a pet per year was calculated based on different diets: 100% wet food diet, 100% dry food diet and a mixed diet of 50% wet and 50% dry food. The difference in impact is quite significant depending on the source of the food, as shown in the graph below.
However, it is evident that this mostly affects dogs rather than cats. Dogs are larger and require a lot more food than cats, therefore this could be expected. According to these results, a dog on a 100% wet food diet has more than twice the impact of a cat on a wet food diet. Interestingly, switching to a 100% dry food diet for dogs can dramatically reduce the impact on climate change, making dogs on a dry food diet the most “sustainable” pet to own. This shows that the impact for dogs largely depends on their diet. For cats, it is not as straightforward.
Impact on Climate change of cats: a breakdown
The impact on climate change for cats also goes down going from a wet to a dry food diet, however that difference is not as dramatic as for dogs. Most of the impact for cats actually comes from the production and disposal stages of the litter material, in this case bentonite clay. Clay made of bentonite used as litter cannot be recycled [7], and does not release energy during incineration, so there are no benefits from the disposal of this material. This dramatically increases the impact on climate change of the end-of-life stage of the litter. The distribution of the impact due to each component is shown on the graph below.
Comparison of cats and dogs over a lifetime
In life cycle assessment the life time of a product, or in this case a pet, can really make a difference in the impact caused. While we would love nothing more than for our pets to live as long as us, the sad truth is that the average life of a pet is much, much shorter than ours. Nonetheless, cats can live up to 4 years longer than dogs, according to the average lifespan. The overall impact of cats and dogs over a lifetime, based on their diets, is shown in the graph below.
Despite the difference in lifespan, a dog on a wet food diet still has the highest impact on climate change, although the gap with the impact of cats on a wet food diet is smaller than on the per year comparison. It is logical that the shorter the lifespan, the lower the impact.
Impact on Ecotoxicity per year
Climate change is not the only indicator used in life cycle assessment, in fact there are many more indicators that can be looked at to evaluate the impact of a product, or in this case a pet. One of the indicators which food has no impact over is Ecotoxicity, which is the impact on freshwater organisms due to the release of toxic chemicals and substances released to the environment.
If we look at the impact on this indicator for example on the graph below, one could derive that dogs are a lot more sustainable than cats. Most of the impact on Ecotoxicity for cats comes from the production and incineration of litter, which dogs do not need.
So, what is the most sustainable pet?
The answer depends on which parameter you take; there is no straightforward answer that is undisputedly true. This analysis was made using very generic information and assumptions. But it does give some good insights on where to make more sustainable choices, when it comes to our furry babies.
Firstly, it is evident that a wet-food based diet has an enormous impact on climate change. Switching, or mixing with dry foods for either pet can have a significant contribution in lowering the environmental impact.
Secondly, litter materials such as clay bentonite have a significant impact on the environment. Switching to more sustainable options can help cat-owners reduce their impact and be more environmentally friendly.
Furthermore, this is an analysis per pet. And whilst dogs on a wet food diet do have the largest impact, cat owners often tend to have more than one cat. Which would increase the impact of their total pets per household.
So the conclusion is, fine, maybe I am a dog person after all.
References
[1] European Pet Food Industry Federation (FEDIAF) (2018); C&D Foods; FACCO, Chambre Syndicale des Fabricants d’Aliments pour Chiens, Chats, Oiseaux et autres Animaux Familiers (the French Pet Food Association for Dogs, Cats, Birds and Other Domestic Pets); Mars PetCare Europe; Nestlé Purina PetCare Europe; Saturn Petcare GmbH, and Quantis. Product environmental footprint category rules (PEFCRs): prepared pet food for cats and dogs, final version; European Commission: Brussels, Belgium.
https://ec.europa.eu/environment/eussd/smgp/pdf/PEFCR_PetFood_FinalPEFCRs_2018-05-09.pdf
[2] Lovejoy, J. (2023). How long do cats live? here’s what to expect. Retrieved from https://www.petmd.com/cat/care/how-long-do-cats-live
[3] Mitchell, C. (2022). What Is Cat Litter Made Of? Helping You Make the Right Choice Retrieved from https://worldanimalfoundation.org/cats/what-is-cat-litter-made-of/
[4] Michael, B. (n.d.). How much does cat poop weigh? Retrieved from https://catstourguide.com/how-much-does-cat-poop-weigh/
[5] Yavor, K. M., Lehmann, A., & Finkbeiner, M. (2020). Environmental impacts of a pet dog: an LCA case study. Sustainability, 12(8), 3394.
[6] Mitchell, S. C. (2022). How Often Should Dogs Poop? Retrieved from https://www.petmd.com/dog/general-health/how-often-should-dogs-poop
[7] Maticic, A. (2022). Retrieved from https://litter-boxes.com/is-clay-cat-litter-compostable/#:~:text=Clay%20cat%20litter%20should%20never,if%20put%20down%20the%20toilet
