How Nature Selects

Thriving cattle develop superior body shape, excellent glandular function and high butterfat

On an island named Umnak in Alaska’s Aleutian Islands, there exists a herd of about 3,700 cattle that have had very little human interaction for 80 years. The exact origins of this herd are unknown, but they were probably part of a cattle operation that shut down decades ago.

The climate on Umnak is similar to the west coast of Scotland. With a limited land base, and limited growth periods for forage, these animals have to be able to get grass “while the getting’s good” in order to have enough backfat to survive the long winter nights.

How did nature select for animals like these to survive in such a harsh environment? While I haven’t personally examined Aleutian cattle — they are wild, after all! — it’s safe to make some educated guesses about those that have stayed alive on their own all these years. The top three traits in cattle like this are wedged body shape, superior glandular function and high butterfat levels.

Shape: Three Wedges

<img src="https://members.acresusa.com/wp-content/uploads/2025/12/8_3-Dec-25-Campbell-Aleutian-cattle-bull-1024x630.jpg" alt="Bull (University of Alaska, Fairbanks, https://hlrm120.community.uaf.edu/wp-content/uploads/sites/332/2013/10/Ebbert_FeralCattle.pdf) — A bull on another Alaskan island, Wosnesenski, is well adapted to the harsh environment." class="wp-image-5310" style="width:1129px;height:auto"/> Bull (University of Alaska, Fairbanks, https://hlrm120.community.uaf.edu/wp-content/uploads/sites/332/2013/10/Ebbert_FeralCattle.pdf) — A bull on another Alaskan island, Wosnesenski, is well adapted to the harsh environment.

An animal has to be deep and wide in the front end to be able to put fat on her back. In a climate as severe as Umnak, a cross section of these animals just behind the front legs needs to look like an “O.” This helps maintain body heat during the cold and wet winters. If Umnak were a tropical island, that cross section might be more of an oval — to help dissipate some of the excess heat.

The cow needs to look like a wedge on three surfaces. The bottom slope from the chest to the back of the ribs needs to be increasing in size. This allows for gut capacity and her ability to “eat enough for three” if her genes are going to get passed along.

The top slope has to be a wedge in the opposite direction. Estrogen stops long bone growth on the front end of the animal first. The heifers and cows producing the most estrogen should look like they are walking downhill when on level ground. Only the most fertile animals conceive; if they have a body that is easy keeping, they can make it through the winter to have their calf the next spring. Keep in mind that she is probably nursing a calf all the while.

The third wedge is looking down from the top. Her rump is always wider than her shoulders. Dr. Michael McDonald found that the best linear measurement to most closely predict fertility is rump length compared to rump width. Another guru of linear measurement, Kenneth Redman, found that old cows have three things in common: they typically have a larger belly than the herd average, they have a wider rump than herd average, and they have more slope from hooks to pins than the herd average. Two of his findings would give us that wedge look — from the side and from the top.

Umnak map

On the male side, testosterone stops long bone growth on the back end first. Among other things, the masculine bulls look a bit like a male buffalo. Keep in mind that early maturity, which is vitally necessary in a harsh environment like Umnak, is going to stop all long bone growth in both sexes, and tall animals will be selected against. Echoing Dr. Michael McDonald again, the best indicator of fertility in the bull is the comparison of the length of the rump to the width of the shoulders. The wider the shoulders, the more masculine the bull.

Horn color patterns also tell us something about fertility. Whenever a horned bull goes through a period of lower fertility, he will leave a tell-tale white ring in the growth of the horn. With the lack of feed in the winter, the older bulls should have horns with a uniform darker/white ring look to them, because the feed was good in the spring and then almost non-existent in the late winter.

It takes 120 days on the best feed for a bull to produce the highest-quality semen he is genetically capable of producing. On Umnak, that is the time of year that cows would get pregnant if they could “eat enough for three.” This is also the time of year when potassium and manganese are highest in the native forages. Potassium, which prevents dystocia, is always highest early in the spring. This is the perfect time to calve from a mineral standpoint. About three months later, manganese levels are highest is the seed heads. Manganese is required for reproduction in both plants and animals. These two minerals, coupled with the ability to put on backfat and 120 days of green-growing feed, get that hardy bull to the most fertile time of the year. To recap, 40-50 days after grass starts growing, the cow has regained enough fat and energy to be able to calve, and then 80-90 days later she and the bull are eating those manganese-laden seedheads.

A lot of us think, “I don’t want that great big bull — he’ll tear up my cows come calving time!” Mother Nature has a way of handling this, though. Dr. McDonald also found that at one year of age, each inch the shoulders are wider than the length of the rump results in two-and-a-half fewer days of gestation. With that in mind, bull #1, with a plus-three-inch shoulder, would throw approximately a fifteen-pound lighter calf at birth. That’s seven and a half days less gestation at a time when the calf is gaining two pounds a day in utero. Bull #2, with even shoulder and rump measurements, would play out a bit differently. After he picked himself up out of the dust, where bull #1 put him, he might find a heifer that #1 had not yet serviced. But come next spring, when the calf goes full term, it’s too large to come out, and bull #2 and that heifer’s genes do not get passed along.

Glandular Function

We now have this ideal shape for a bull and a cow in mind. What else do we need? We need glandular function and butterfat. These two are “joined-at-the-hip,” so to speak. You almost can’t have one without the other. Let’s first talk a bit about glandular function.

Dr. Jan Bonsma taught us that glandular function controls everything in the cow. Without glandular function, the cow would not produce the estrogen that gives her the wedge shape. Without glandular function, the bull would not have those massive shoulders with all that testosterone. And the bull needs all that testosterone and those massive shoulders for two main reasons: first, he needs to make sure his genes are passed along, and being king-of-the-hill makes the competition think twice about challenging him; and second, being that fertile, he can service a lot of cows, with the added bonus that the quality of his semen helps maintain their pregnancies.

Digestion efficiency, and the superior epigenetics that come from that, both play a part in the animal’s ability to survive in a closed environment like Umnak. Dr. Don Faulkner of the University of Arizona made the following statement at a conference I attended in the spring of 2014: “We are getting very good at predicting what a group of open cows, pairs, yearlings, bred heifers, etc., will consume, depending on whether fed grain, ensilage, alfalfa, grass hay, etc. What we can't tell you is what individuals will consume in those groups. Some animals consume half as much as other animals in those groups.” If the group average is 3 percent of bodyweight, then some animals eat 2 percent of bodyweight while others consume 4 percent. Similarly, Anibal Pordomingo of Argentina has found that the average beef cow in America digests 55 percent of what she ingests. Yet there were some cows who digest 70 percent of what they ingest.

With that information in mind, I speculate that the Aleutian cattle that have survived are the ones eating 2-2.5 percent of bodyweight per day and digesting 65-70 percent of what they ingest.

Glandular function also keeps the animals free of internal and external parasites. Healthy sebaceous fluid is a natural fly repellant. The bulls bothered by the fewest flies during breeding season get more females bred, everything else being equal. Bugs are Mother Nature’s cleaning crew. In our row crop fields, they are trying to do their nature-made job, which is eating what is not fit for man nor beast. Then we come along with chemical warfare agents and … but I digress!

The healthy rumen is not a house for worms. The good biology working in there does not leave a good environment for worms to show up, let alone survive. Once a cow is impregnated, she stops producing estrogen and starts producing progesterone to maintain the corpus luteum and the pregnancy. If the cow is too fat (not a problem in the harsh environment of Umnak) her own fat gobbles up the progesterone (and a lot of other toxic things), and she sheds the corpus luteum and cycles again.

Butterfat

How does butterfat come into play in this Aleutian Island story? It would be virtually impossible for a calf to stay alive through the winter unless he/she can put some fat on the back.

Umnak island (David W. Shaw, Adobe) — The brief summer on Umnak Island provides a grazing period for the island’s wild cattle.

Without a fully developed rumen (the development of the rumen takes approximately ten months at mother’s side), the main way for the calf to get that fat layer is through the butterfat in the milk. Too much milk is a problem because the energy required to produce it pulls the cow’s body condition down too much.

A few things indicate that a cow is producing a lot of butterfat. A bald udder is the main one — while there is of course going to be more hair on the udder of a cow on Umnak Island than on Fiji. The second thing to look for is vertical folds in the hide. The closer they are, and the farther back over the rib cage they go, the looser the hide is; this is also an indicator of more butterfat.

Surviving the Winter

With all of this in mind, let’s throw some of nature’s variability into the equation. As I stated above, there are approximately 3,700 animals on Umnak. Some die each year due to old age, inability to survive the periods of low or no feed, or sickness (low glandular function). Some calves die because of the cow’s poor phenotype. Some cows have a calf too early (not enough feed for her to breed back on time), and some have a calf too late (lack of fertility in the cow resulted in a calf too far behind to survive the winter).

Let’s say there’s a year with 25 percent more feed than normal. The winter die-off will be less than normal, and now we have 4,000 head on the island. Next year is a normal year for forage growth, and we have more mouths to feed. Nature is really going to sharpen her selection knife that coming winter. In some winters on Umnak, 25 percent of the animals die. The next spring we wind up with 3,000 head, but they are Aleutian Island cattle 2.0.

Only the very best shapes, best glandular function and high-butterfat “package” animals survive a winter like that. And farmers and ranchers who seek out and select for these three traits will likewise build superior herds.