The purpose of this article is to define the types of milk protein, fats, and other ingredients that compose the replacement milks most frequently used in squirrel rehab, discuss their properties and how those properties affect growth and digestion, describe how milk is processed to obtain components that can be remixed into formulas, and help us understand why some replacement milks produce healthier babies than do others. Included is a discussion on developing a dedicated squirrel formula.

I began studying milk components soon after the Esbilac debacle began a few years ago. Before that horrible summer and fall, I was an ignorant user of formula. I held out my money and was given a product in exchange, trusting the manufacturer to give me a well-developed formula in return. I read labels, but didn't really understand them; later, I came to see that the presence or absence of an ingredient influenced the health of a baby, but I had no idea what that meant. I had no reason to be dissatisfied prior to the 2010 change in Esbilac because it grew beautiful babies: thick, fat, big, strong boned, with high energy and great stools. I had tried other formulations and found them poor seconds to the pre 2010 Esbilac. I didn't even dream that one day that formulation would be gone. As shallow as it seems to me when I look back, I also didn't realize how much my rehab success depended on the old Esbilac formula. I had beautiful babies. End of story. Until the formula wasn't there anymore. Suddenly, my babies began failing when they once flourished, and to know why, I realized I had to understand the components of milk in order to understand what was, or was not, in the formulas we were using. I had the idea that if I knew what was wrong I could somehow correct it myself by adding back the missing ingredient or ingredients. That was a naive assessment as I found out, because milk components and their relationships to one another, how they are separated, processed, manufactured, and recomposed into various recipes, are too complex to just add something back. A great formula has to be designed and constructed as tightly as is a fine symphony or grand architecture, with a specific goal in mind, a pinnacle of quality at the top, and an ethical foundation.

I am not a chemist and do not understand milk on a molecular level, nor am I a trained milk formulator, but I have studied the topic of milk and its component parts for three years, gaining an understanding and insight beyond that of most people; I am a layperson and speak in lay language. From each piece of information I have studied, no matter how simple or complex, I have gathered a part of the whole picture of how milk components work, both independently and in unison, to influence digestion and create growth. I have learned how important are the ways milk products are separated from raw milk and processed into individual components that are then recombined to form replacement milks that have desired ratios of proteins, fats, and other nutritional parts. In addition to protein and fat, there are many other ingredients that must be in a milk to produce healthy babies, yet we all too often focus only on those two parts of the whole picture. I have overlaid on my 22 years of experience with baby squirrels the knowledge I have accumulated in these three years to arrive at conclusions of what is right and wrong with the formulas we use, and what milk components should be included in a dedicated squirrel formula.

All of the information I have read and that I discuss in this paper I obtained from internet sources that are easy to find if anyone wants to study this further. Search topics included the chemistry of milk, manufacture and processing of milk, the separation processes of milk ingredients into individual components, the symbiotic relationship of milk proteins, the properties of casein proteins and their effects, properties of whey isolate and whey concentrate, dried milk protein concentrate, what lactose actually is and does beyond the stereotypes and misinformation, raw milk, types of milk, drying processes and how they affect milk products, to name a few. Add your own search topics to the list. I have included at the end of this paper a list of some of the sources from which I gathered information, all of which are professional sources, not commercial sources biased toward selling a product.

I give my permission to copy and share this text, but I ask that the text be copied and dispersed in full, and if added to a website, it becomes more important to display the full article. Context is important to understanding and when a part is lifted out, it can cause confusion and mis-understanding. You can download a copy of this entire paper: What's in that Milk Replacer You Are Using? and Pictorial Comparisons of Babies Given Good and Poor Formulas.

A great formula is more than its ratios of protein and fat. To look only at the numbers and ignore the contents is equivalent to having one-eyed vision. Ratios and contents are two parts of a whole picture; the percentages of protein in a formula are derived from a combination of the protein contents in that formula. Since the contents on which the numbers are based might not be good protein ingredients for some species, or might be in the wrong balance one to the other, the numbers become useless when someone is trying to judge a good formula just by those numbers. The sources for protein are meat, vegetable and milk, with the milk proteins being the most easily digested by babies. Nature developed milk specifically to grow babies, and nature is not wrong.

Types of Milk Proteins:

Casein - there are four types of casein proteins. Casein proteins are large molecules and stay suspended in the liquid milk until coagulated into curds by the application of heat and acids, or by centrifugation. In the acid method, the curds precipitate out of the liquid milk, taking along most of the fat, which binds to the casein. The fat is later removed from the casein, leaving the pure protein behind. The same coagulation process happens when we consume casein: it mixes with stomach acids and forms a curd in the stomach. Casein proteins can be difficult for small infants such as neonate squirrels to digest, possibly because of this long lasting stomach curd, and could be the cause of bloat in infants who ingest formulas high in Casein. Caseins are less water-soluble than whey proteins, are more difficult and slower digesting than whey proteins, and have lower absorption rates. In cow's milk, casein makes up about 80% of the protein with 20% being whey; in human breast milk, about 40% is casein and 60% is whey. From this example, you can see the balance of casein and whey proteins are different in different species. This example indicates that one milk formula is probably not going to satisfy the needs of several species, such as a formula that advertises it is for use with squirrels, rabbits, and opossums.

Whey - the other types of milk proteins are grouped under the title whey proteins. There are two major whey proteins, but there are many others and they have different functions. During the processing of raw milk when ingredients are separated, the whey proteins stay suspended in the liquid milk while casein and most of the fat are removed. The whey proteins are separated with further processing, after which the separated ingredients can consist of only the whey proteins or the whey proteins and non-protein elements. These proteins are more water-soluble than casein proteins and are fast digesting. Whey proteins possess anti-inflammatory properties and may offer some protection from bacterial and viral infections (lactoferrin, also called colostrum, is one of the whey proteins that provides this protection).

Whey Isolate - the whey proteins have been isolated from the other ingredients in the liquid whey.

Whey Protein Concentrate - contains all of the whey proteins, some fat that didn't coagulate or separate from the casein when it was removed, and vitamins, minerals, enzymes, lactose, lactoferrin and other proteins that help protect against bacterial and viral infections and that promote the growth of healthy intestinal flora.

Dried Milk Protein and Dried Milk Protein Concentrate - the only descriptor I found of dried milk protein as a product was that dried milk protein is obtained from a residue left over from drying coagulated casein after the milk fat has been removed from casein. In all other references, and there are many, dried milk protein is described as Dried Milk Protein Concentrate, which is obtained by the partial removal of non-protein elements from skimmed milk, and contains casein and whey proteins, as well as lactose. The finished dry product contains 40 to 90% protein by weight and the content should include casein and whey in a proportion that reflects the content in the milk from which it was concentrated. The percentage of protein, lactose, and minerals in Dried Milk Protein Concentrate can be adjusted to fit various applications. Some applications might require higher protein content and lower lactose content, while others may require lower protein content and higher lactose content. Generally, the higher the protein content, the lower the lactose and mineral content in Dried Milk Protein Concentrate.

Most Dried Milk Protein Concentrates are foreign made and unregulated, so we don't know what we are actually getting. Additionally, there is no requirement to divulge what animals created the original milks so we have no idea as to what the ratios of casein to whey might be (these natural ratios vary from species to species), under what health conditions these animals live, or what additives and contaminants may be in their environments and diets. The creation of Dried Milk Protein Products has made a problem for the American Dairy farmer because they are cheap to import.

Summation:

Casein and Whey proteins are symbiotic, needing one another, contributing their unique properties to a formula, and working with other nutrients to stimulate growth. As gathered from the sources I read, there is a general consensus that whey proteins are considered superior to casein proteins in growth stimulation, but both types of proteins are necessary. In rank of importance, the composition of proteins in a formula is a large determiner of the food quality, followed by the digestibility of the protein, and availability of the proteins to other growth processes. Casein or whey ratios, as well as lactose and minerals, can be adjusted upwards in a formulation by the inclusion of extra casein, whey isolate, or Whey Protein Concentrate. Dried Milk Protein Concentrate is a combination of these elements and can be obtained in different ratios of ingredients to suit a manufacturer's recipe for a formula.

Milk Fat:

Milk fat is found in the creamy layer that floats on top of raw milk. Cream contains some vitamins and minerals, a small amount of milk protein, and supplies energy and calories. The fatty acids in milk fat assist in the digestion of proteins. Fatty acids are needed to absorb and utilize vitamins and minerals. Milk fats are easily digested. Cream is an oil-in-water emulsion, which will mix readily with water base liquids. When cream is agitated, it solidifies into butter, becoming a water-in-oil emulsion, which does not mix as readily with water base liquids.

Carbohydrates and Lactose:

Carbohydrates are a large category of organic elements that includes lactose, starch and cellulose. Lactose is a carbohydrate present in all natural breast milks. Lactose will be present in a formula that has Whey Protein Concentrate (amount depending on the extent of processing and how much, if any, is added back to the formula as a product in itself), and some amount in skimmed milk, depending on how the skimmed milk was processed. Lactose would not be present in whey isolate or casein, and only a partial amount is in Dried Milk Protein Concentrate. Lactose comprises 40% of the calories in human breast milk, 3.8% in dog breast milk, and 3.0 to 3.7 in squirrel breast milk. As lactose passes through the intestines, it is broken down into two sugars, glucose and galactose. Lactose helps to fight disease and promotes the growth of healthy bacteria in the stomach, as well as improves the absorption of minerals.

Dried Skimmed Milk:

Simply put, fat has been removed from the milk. It should still have casein and whey proteins, lactose, and other nutrients according to the species that produced the milk. A problem with this removal is that fatty acids are a necessary part of the protein chain, as well as to other functions, so, in a formula, fatty acids that have been removed from skimmed milk in some proportion or another must be added back from some source, usually from vegetable oil, cream, or butter. Adding casein to a formula as a separate product will increase the amount of casein relative to the whey proteins that are in the skimmed milk powder, changing the natural values of the milk. The natural protein composition (casein and whey) of skimmed milk is about 34% combined if it is from cow's milk, with a different set of values if it derives from another animal.

Since all of these milk products are first separated from natural milks, manufactured to be single products, then re-blended according to specifications, the amount and strength of a component in replacement milks can be increased or decreased to suit a particular recipe, thereby achieving the protein and fat ratios stated on labels. Other products such as egg yolk, vegetable oil, and non-milk proteins can be added to adjust ratios and nutritional values.

Egg Yolk:

An egg yolk was designed by nature to feed a developing embryo. To that purpose, the yolk contains proteins, including a protein that gives immunities to the embryo and young chick, fatty acids, vitamins, minerals, and carbohydrates.

Vegetable oil:

Other than as a source of fats, I do not know if there are other reasons why vegetable oil is an ingredient in animal milk replacement formulas. If there are other reasons, I could not find them in my research.

Summation:

Once the various milk elements are separated from whole raw milk, they become simply components; they are no longer milk as we knew it, but are now components that can be recombined to suit various recipes and used in many food products. For example, when a formula is advertised as being made from goat's milk, the components may have begun life as raw goat's milk but have been so far removed from their natural state by processing and separating as to be no longer considered goat's milk, just a series of nutritional components that originated with a particular animal that has a particular casein to whey ratio. Most of this kind of advertising appeals to prejudices we already have about the viability of the raw products. We just don't think about the transformations milk products go through during manufacturing and recombination.

There are many more forces at work in the manufacturing, separation, and processing of the components of milk than I have mentioned here. All the steps of manufacture, and how the components of milk are recombined, have an impact on the digestibility and nutrition of a formula. The manufacturing of milk is an interesting and eye opening topic to research.

Ingredients List and Guaranteed Analysis:

An Ingredients List on a product label tells us what products are included in a formulation and may, in some cases, suggest how much of a particular product is in the mix, but, since the products on a list are organized by descending order of weight, with the heaviest ingredient listed at the top, it doesn't necessarily mean an ingredient high on the list is in greater volume than one listed lower, only that it weighs more. We don't know if the product weights were measured in a liquid or dry state, which would affect their weight and, therefore, position in the list.

The Guaranteed Analysis on a product label gives us total percentages of protein and of fat, but does not reveal the specifics within the categories. Only an all-inclusive chemical analysis of a whole product would tell us exactly how much of each protein or fat product exists in a formulation, what carbohydrates and sugars are present and in what volume, how much of the fat content is from oil and how much is from milk fat, what are the vitamins and minerals and their percentage representation, and other factors. An all-inclusive analysis would be more revealing about the nutritional values of a formula. For instance, here is part of a chart that shows a category from a detailed analysis of dog breast milk:

Whey protein and casein (as percent of total composition) in dog breast milk:
Species	Casein	Whey Protein
Dog	5.8	2.1

The ratios of protein and fat are the ratios as they exist in the dry powder as a whole, and the dry powder is a conglomerate of all the products that are in the formula. We don't know how much of the conglomerate powder is protein or fat powder, we only know that within the powder are the listed values. This means that the total amounts of protein and fat listed on a package label are not what will be in a single feeding of the reconstituted powder.

When companies list Calories, or Kcals, on a label or in some other location, (Calories or Kcals are the total amount of caloric energy in a product), that listing does not tell us how much of the calories derive from what component in the formulation, just the total amount of calories (there may be some instances where these numbers are revealed). Some of the calories could be coming from a pure sugar product in the formula. The calories in pure sugar products are empty calories, providing no growth factors.

Summation:

To judge how viable is a particular formula, we have to not only look at the ingredients list and the guaranteed analysis, but we have to look at how babies grow on a formula: stools, fur growth, skin condition, bone mass, hydration, growth rate, energy levels, appetite, and body fat. We can then compare these results one formula with another to draw an opinion about what might or might not be present in a formula that is affecting growth, good or bad, and what formula is the best choice.

Formulations Most Frequently Used as a Milk Replacer for Squirrels:

Old pre 2010 Esbilac 33/40 (no longer available):

1. Vegetable oil
2. Casein
3. Whey Protein Concentrate
4. Dried skim milk
5. Butterfat
6. Egg yolk

2010 Version of Esbilac 33/40 (being replaced with the 2013 version)

1. Vegetable oil
2. Dried skim milk
3. Casein
4. Cream

Whey Esbilac 2013 version 33/40:

1. Vegetable oil
2. Casein
3. Whey Protein Concentrate
4. Cream
5. Dried skim milk

(probiotics were added to this new version)

#1 Zoologic 33/40:

This list of Zoologic ingredients is what is still on the Pet Ag website (2013), but has been replaced by the next version in this list. I do not know when this switch took place. Website product listings are not usually up to date with package labeling.

1. Vegetable oil
2. Casein
3. Dried Skimmed milk
4. Egg yolk

#2 Zoologic 33/40:

This list is taken from the label on a container of Zoologic (2013).

1. Vegetable oil
2. Casein
3. Dried milk protein concentrate
4. Dried skimmed milk

Fox Valley 32/40 (2013):

1. Vegetable oil
2. Casein
3. Dried milk protein
4. Dried skimmed milk
5. Dried corn syrup solids

Formula Comparisons:

Old Esbilac (no longer available):

The combination of ingredients in the old, pre 2010 version of Esbilac that was discontinued has a long track record of success in providing proper nutrition to infant squirrels. There was a combination of added casein and added whey protein as Whey Protein Concentrate, which includes lactose and other non-protein nutrients. Fats are provided in both the vegetable oil and butterfat. Egg yolk in this old version contributed more high quality proteins with protective factors, as well as nutrients such as fatty acids and minerals. The ingredients in this version combined to produce an easily digested formula with high growth factors. The presence of butterfat may have caused the dry product to form clumps in the can, and difficulty reconstituting the product with water.

2010 Version of Esbilac (being replaced with the 2013 whey version):

In the 2010 version of Esbilac, which replaced the old Esbilac, whey concentrate was eliminated entirely as an added ingredient. Extra casein was added, but no extra whey was added to balance that. The only whey is in the skimmed milk and if the skimmed milk has a ratio of that of cow's milk, 80% casein and 20% whey, the added casein overwhelmed the whey, probably causing some, if not all, of the digestive problems reported. Gone along with the whey concentrate were the lactose and other non-protein nutrients that are in whey concentrate. A nutritionist at Pet Ag, during an email correspondence with her a couple of years ago, told me that the lactose content in this version of Esbilac is 0.5%, an insignificant number. Nonfat skimmed bovine milk should have an average lactose percent of 5.2, so it seems from this that the lactose content in the skimmed milk was further processed to lower the percentage. Dog breast milk has, at mid stage lactation, 3.8% lactose and mother squirrel milk is 3.0 to 3.7% (see chart later in this paper for the analysis of mother squirrel milk). The amount of lactose in this version is too low for a dog and too low for a squirrel. The egg yolk was eliminated, with all of its nutritional values. Butterfat was replaced with cream, which may be a neutral exchange, or could have some other influence. This product mixes more readily with water than did the old Esbilac.

2013 Whey Version of Esbilac:

The first aspect of this formula that jumps out is the re-inclusion of Whey Protein Concentrate and the second aspect is that the ingredients list resembles old Esbilac, with variations found in a different list location for skimmed milk, cream instead of butterfat, and probiotics have been added. This version should have a higher content of lactose because it is a part of Whey Protein Concentrate. The additional whey proteins in the concentrate balance the added casein proteins. Higher whey content relative to the casein, plus higher lactose, would stimulate growth and should reduce or eliminate digestive issues that were evident in the 2010 version. Since products are listed by weight, and if the weights of skim milk in two volumes are equal, then the total amount of skim milk included here is either less or the skimmed milk has gone through some reduction or drying process that changed the weight. Seven probiotics have been added to this version. What effect on digestion the probiotics will have is not yet known. Only time and use will expose if the probiotics have a beneficial, negative, or neutral influence on a formulation that is otherwise an excellent combination of growth ingredients.

Early reports are that babies are growing on it as well as they did on old Esbilac, and have excellent stools. As of this writing, there has been one report that I know about of poor stools in neonates, but this could have been caused by some other health factor and not the formula.

If you are using Esbilac, once the supplies of the 2010 version have disappeared from shelves, warehouses, and refrigerators, this is the formulation you will be using. If you are using a combination of another formula and Esbilac, this version of Esbilac will improve the nutritional content of that mix.

Zoologic #1:

There are no milk fats in this product. The fats are supplied by vegetable oil and a little fat in the egg yolk. The skimmed milk, if it is bovine milk, would have approximately an 80% casein and 20% whey content, unless that content has been adjusted, so the addition of casein makes this product casein heavy and whey light because it doesn't contain additional whey to balance the casein content.

Zoologic #2:

The ingredients in Zoologic #2 are the same ingredients listed, and in the same order of weight, as those of FV 32/40, except for the inclusion of corn syrup solids in Fox Valley, and a difference in the way Dried Milk Protein is listed in each formulation: Dried Milk Protein Concentrate is listed in Zoologic. Dried Milk Protein Concentrate should include both casein and whey proteins, lactose and other non-protein elements in some ratio as ordered by the formula manufacturer. More casein has been added to this formula in addition to what is in the skimmed milk and the Dried Milk Protein Concentrate. The added casein is not balanced by additional whey, making this product casein heavy and whey light. There should be more lactose in this formulation than the last version of Zoologic due to its presence in Milk Protein Concentrate and in skimmed milk, and might contribute to better digestion of the product. There are no milk fats in this formula. All the fat is in the vegetable oil and we don't know what oils they are.

Fox Valley 32/40:

The listing of Dried Milk Protein is confusing because this product is always referred to as Dried Milk Protein Concentrate in the professional literature that I have read, which includes the two protein categories plus some lactose and other non-protein elements. If the listing is accurate and it is a protein only product, it would mean this product is extra heavy in casein proteins; there would be only a small representation of whey, lactose, and non-protein elements in the skimmed milk, depending on how it was processed. This formula would be very whey and lactose light. Caseins are less water-soluble than whey proteins, more difficult to digest, digest more slowly than whey proteins, and have lower absorption rates. It could be very difficult for neonates and pinkies to digest this formula, particularly because they are fed at frequent intervals. More casein is poured into the stomach at every feeding. Eventually the entire digestive system is overwhelmed with undigested casein just sitting there. A rehabilitator in Florida had a necropsy done on a bloated infant who had been fed 32/40. The results were undigested formula in the stomach and intestines that still resembled the powder. The high presence of casein and low presence of whey and lactose would explain the bloating and poor growth of pinkies. Squirrels who are three weeks or older may show less negative effects of this casein heavy formulation. Two reasons for that are because their digestive systems are more mature and able to handle the amount of casein, and feeding intervals are usually spaced wider apart for older babies, giving their digestive systems additional time to process the casein before they are given more.

Lactose, with all of its beneficial components is under-represented, contributing to low growth weights and development. Not present in FV 32/40 at all are milk fats: no cream, no butterfat. Vegetable oil is the only contributor of fat. Corn Syrup Solids do not add any positive nutrients to this formula. They are simple sugars and have no complex nutritional components or interactions with other nutrients. They do not go through any digestive processes to become sugars. They add empty calories. The inclusion of corn syrup solids would boost the kcal listing, a listing that discloses the total amount of caloric energy in a product, but that would be a somewhat incorrect indicator of the nutritional value of the formula.

Photographs of Babies Given Poor Formulas Compared to Babies Fed Nutritious Formulas

A Milk Replacer Just for Squirrels:

The power manufacturers of replacements milks have over rehabilitators and the health of the babies in our care, and the constant conflicts within our community surrounding which formula is best (it is ironic that we are debating and criticizing over products that are insufficient to begin with), will only end when we have a dedicated replacement milk and the contract that once it has been tested, adjusted, and fine tuned, will not be changed. A rehabilitator should be included as part of product development by partnering with the formulator as an advisor, and a selected group of rehabilitators with many years of experience and credibility should test the product before it is released for public use.

It is quite possible to create a replacement milk just for squirrels. What is lacking is the will to do so. The milk should be designed around the values of mother squirrel milk that we currently have (hopefully more thorough studies will be forthcoming at some point), using quality milk ingredients in the correct relationships one to the other that have already been proven to grow beautiful babies. To get a dedicated squirrel milk, we have to first admit that the products we use today are insufficient, drop our allegiance to one manufacturer or another, and demand a dedicated squirrel milk, not just one or two of us, but a concerted effort by all. A company that has the equipment, raw ingredients already on the shelf, employees, formulators, a purchasing market in place, and last but certainly not least, the ethical concern and openness of spirit to include rehabilitators in the development process, would be able to produce this formula with a nominal financial investment, and at a quick pace. What is lacking is the will to do so.

Until we have a formula that has been developed just for squirrels, we will continue to be at the mercy of companies who produce inadequate formulas, or produce a formula that works well for a period of time, but is changed every few years, changes that don't necessarily improve the formulation, as we have seen happen. We will continue to watch helplessly as babies suffer on inadequate formulas, and we will suffer right along with them, until we have a milk replacer created just for squirrels, created with the input of knowledgeable and experienced rehabilitators.

Composition of Gray Squirrel Milk
Species	Total Solids	Water	Protein	Fat	Carb/ Lactose	Ash	Calcium	Phos	Source
Gray Squirrel	25.4	74.6	9.0	12.1	3.0	1.3	.36	.45	Nixon and Harper, 1972
Gray Squirrel	39.6	60.4	7.4	24.7	3.7	1.0	--	--	Jenness and Sloan, 1970
Gray Squirrel	27.6	72.4	9.2	12.6	3.4	1.4	--	--	Ben Shaul, 1962
(NOTE: Ash represents a combined mineral content. It is called Ash because the minerals are burned, then the ash resulting from the burn is measured to arrive at the total percentage of minerals.)

In Memory of Nonda Surratt,
who taught us to not take things at face value,
but to look deeper for true meaning.

My deepest thanks and appreciation go to Pam Spragins, who has been my listener, commentator, editor, occasionally my fellow researcher, and created this web page. I also want to thank Christy Reeves, who caused me to write this paper by asking me to answer someone's question about the causes of bloat in a pinkie squirrel.

Reading List:

Milk Chemistry - An Introduction
European Whey Products Assoc. - Lecturer's Handbook on Whey and Whey Products
Effect of Bovine Lactoferrin Addition to Milk in Yogurt Manufacturing
Whey vs. Casein Protein
Comparative Lactation - Cats and Dogs
Milk Composition - Proteins
Milk Protein Concentrates
American Dairy Products Institute - Whey Protein Concentrate
Wikipedia - Whey concentrate
U.S. Dairy Export Council - Nonfat Dry Milk & Skimmed Milk Powder
Wikipedia - Casein
True Protein versus Crude Protein
MedicineNet - Stool Color and Texture Changes - (article about the influence bile and bilirubin have on stool color)
Gut Hypertrophy in Response to the Ratios of Casein and Whey Protein in Milk Formulas in Artificially Reared Rat Pups. (National Center for Biotechnology Information, U.S. National Library of Medicine)
Nixon, Charles M.; Harper, W. J. Composition of Gray Squirrel Milk. Ohio Journal of Science: Volume 72, Issue 1 (January, 1972) - You can read the full Nixon and Harper study at this link.