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Mother's Milk - Reflections on Wildlife Species Milks and Formula 





The majority of wild mammals admitted to rehabilitation are pre-weaned juveniles that have been injured, orphaned, or sometimes “kidnapped.” As a result, mammal rehabilitators take on the responsibility of selecting, obtaining, preparing and feeding substitute milk replacer formulas. However, milk replacers will never be as nutritious or as easily digestible as the mother’s milk that has evolved over millions of years.


Species milks


Milk from the mother has evolved to be appropriately balanced for each species to provide nutrition and immunological protection. Biologists and other researchers have collected milk samples of wild mammal mothers to enhance understanding of the species, as well as to provide information for those responsible for feeding wild animals, such as wildlife rehabilitators, sanctuaries, and zoo staff. As one would guess, collecting the milk samples from lactating females or their young is not a small task, especially in a humane manner. It is difficult to gather adequate amounts to be able to conduct a full spectrum chemical analysis, particularly with small species. The limited sample size for some species adds to the difficulty of achieving complete, repeatable, and consistent results. 


Since Dr. D. B Shaul published “The Composition of the Milk of Wild Animals” in 1963, further research has been published on the both the same and additional species. Published studies are available online as well as in a variety of publications, including the NWRA Principles of Wildlife Rehabilitation, 2nd ed. Click here for a list of 90+ published mother's milk studies. The studies describe the percentages of solids and liquids in the species’ milk and may also provide information on the proteins, fats, carbohydrates and kcals. The research may compare components between species, individual animals, different seasons, mothers’ diet, ages of mothers and young, and in some cases, stages of lactation. While people may be very familiar with the milk composition of the wild species that they rehabilitate, they may not be familiar with how the milks differ among species – or the natural history of the species. The chart below displays examples of these milk differences, including variances between some studies.

Mammal Milk Composition Comparison

Renewed focus on nutrition in general


Over the last few decades, people have become more focused on nutrition for themselves, families, and even pets, as they become aware of the dramatic difference it makes on their daily lives and futures. The US Food and Drug Administration has enacted labeling regulations on food product contents and nutritional values, and is enacting even more detailed disclosure requirements. Most consumers embrace this move and have found it instructive. 


There has been similar interest in the foods fed to wild animals in rehabilitation. Unfortunately, manufacturers providing food ingredients or finished products for the “pet industry” operate under far less stringent labeling and disclosure regulations.  It has been difficult for rehabilitators to gain insight into a milk replacer products’ ingredients, sources, quality , and even, manufacturing methods. It is especially concerning since very young animals being fed substitute formula as their SOLE diet are totally dependent on those recipes and products.


Reconsidering Formula Choices


Growth, health, development and, ultimately, SURVIVAL of young mammals are dependent on the milks they are fed – from either their mothers or caregivers. In the absence of mom or a surrogate, caregivers must attempt to create a formula made from commercially available milk replacers.  Over the years, rehabilitators have generally relied upon milk replacer manufacturers, product distributors, biologists and nutritionists, and especially other experienced caregivers for species-specific formula recommendations.  The products and recipes were likely given little thought (or further researched) - because they worked. 


However, once problems began to arise (growth and development problems, gastrointestinal issues, musculoskeletal problems, etc.), rehabilitators began investigating the causes (more here and here).  Initially, many tried to eliminate common factors that could cause such problems, such as feeding practices (amounts, frequency, positions, etc.), parasites, diseases, pain, supplements, medications, etc. They also consulted with other rehabilitators or veterinarians, searching for resources to try to identify causes and options to correct what they hoped was a minor, temporary, and easy resolution to the problem.  Unable to find a solution to the problems, they began to consider whether milk replacer products or formula recipes might be contributing to the problems, and discovered that a milk replacer powder appeared to be the common thread.




Much of the information on this website discusses various aspects of the milk replacer products in terms of nutritional content, energy values (kcals), preparation techniques, etc. Even with that information, people working with wildlife know that understanding the composition of mother’s milk is the primary and logical starting point.  Some rehabilitators may have extensive understanding of the nutrition components (e.g., proteins, fats, carbohydrates, minerals, kcals) for the species under their care.  Those who do not, or just need a refresher, may want to give closer attention to the resources on each of these critical nutritional components available at the links in the previous sentence.


A next step is to consider how a potential product and/or formula recipe compares to the milk composition for the wild species to be fed. Ingredients and nutritional information are provided on the product label, the manufacturer’s website, distributors’ marketing information, and other sources. That information can serve as a starting point in calculating the nutrition in the milk replacer products and the recipes prepared for the wild mammals. Equations are available to calculate the nutrition in various nutrition publications. Another option is to select products and recipes – and then compare that to the milk of the species. 


Others may prefer to use nutrition/milk formula calculators designed to do that analysis. One such wildlife formula calculator is specifically designed for calculating formula recipes for wild mammals and is available for free downloading. While that nutrition calculator includes milk composition analyses for some common North American mammal species (and several optional studies), it will be adding more. It also allows the user to add milk composition analyses for other species. 


After choosing the species on the Wildlife Formula Calculator, the user then selects formula ingredients (e.g., various brands of milk powders, proteins, fats, water) and then chooses the desired amounts and proportions. The calculator then shows how that recipe compares to the milk composition analysis for that species. The user may further customize other aspects of the recipe, such as using product weights rather than volume, or adding other ingredients. This allows the user unlimited options to consider how the formula products can combine to create a formula comparable to the species milks. While the Wildlife Formula Calculator provides the option of using information printed on the nutrition label, another option is to use the results from independent lab tests of milk powder components, since there have been some unexpected variances from guaranteed minimums. Instructions on how to use the calculator can be found here and on the 'Read Me First' tab of the calculator.


Beyond the formula math 


It is certainly useful to consider how the formula compares to the composition analysis for the  species’ milk.  However, that can simply be a numbers exercise if one stops there. For example, the calculator does not assess the quality, digestibility or utilization of the formula. So even if the nutrient profile compares closely to the species milk, the animal will not obtain the full benefits if it is unable to fully digest the formula. Digestibility is influenced by many factors, such as the basic nature of the ingredients, manufacturing process used, recipe, and preparation (e.g., rehydration, reconstitution). The product’s quality and freshness are also important, influenced by the date it was manufactured and how it has been stored. The animal’s age, health, and feeding practices are other key considerations. Thus, while it is important to understand the nutritional components, it is also essential to monitor the effectiveness of the formula based on growth, development, health, quality of the stool and urine, and more. 



References and further reading (not intended as an exhaustive list)


Capuco, A.V. and R.M. Akers. 2009.  The origin and evolution of lactation. The Journal of Biology. V. 8 (4).


Fox, Pat F. 2009. Milk: an overview. Milk Proteins: from Expression to Food. Abby Thompson, Mike Boland, and Harjinder Singh, ed. Elsevier Inc. Boston, MA.


Green, Brian, William Krause and Keith Newgrain. Milk composition in the North American opossum (Didelphis virginiana) Comparative Biochemistry and Physiology. V. 113, pp. 619-623.


Hinde, Katie. 2013. Mega Milk Composition Analysis. SPLASH! Milk Science Update. November. 


Kunz, al. 1994. Changes in milk composition during lactation of three species of insectivorous bats.  Journal of Comparative Biology. 165, pp. 543-551.


Moore, Adele and Sally Josten. 2002. NWRA Principles of Wildlife Rehabilitation: the essential guide for novice and experienced rehabilitators, 2nd ed. National Wildlife Rehabilitators Association. St. Cloud, MN.


Nixon, Charles and J.W. Harper. 1972. Composition of Gray Squirrel Milk. Ohio Journal of Science. V. 72, pp 3-6.


Oftedal, O.T. et al. 1993. Nutrition and growth of suckling black bears (Ursus americanus) during their mothers’ winter fast. British Journal of Nutrition. V. 70, pp. 59-79.


Oftedal, O.T. and S. J. Iverson. 1995 Comparative analysis of nonhuman milks. A. Phylogenetic variation in the gross composition of milks,” in Handbook of Milk Composition, ed. Jensen RG. Academic Press, pp.749-788.


Oftedal, O.T. 2000. Use of maternal reserves as a lactation strategy in large mammals. Proceedings of the Nutrition Society. V 59, pp. 99-106.


Oftedal, O.T. 2012. The evolution of milk secretions and its ancient origins. Animal Ecology. V. 6, pp. 355-68.


Oftedal, O.T. 2013. The evolution of the nutrient composition of mammalian milk. Journal of Animal Ecology. V. 82, pp. 1254-1264.


Riek A. 2008 Relationship between milk energy intake and growth rate in suckling mammalian young at peak lactation: an updated meta‐analysis. Journal of Zoology. 274. pp.160-170.


Shaul, Ben. 1963. The composition of the milk of wild animals. International Zoo Year. V. 4, pp. 333-342.


Skibiel A.L., et al. 2013. The evolution of the nutrient composition of mammalian milks. Journal of Animal Ecology.

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