Stockmen know about animal breeding. Livestock programs include selection for traits like birth and weaning weights based on a bull’s performance, crossbreeding to avoid inbreeding and gain heterosis, and choosing the breed, crosses and sires best suited for your environment and operation.
Research at state universities, USDA and breed associations provide modern science on genetics and breeding, and common sense and experience knowing what works in your operation are all used in livestock production. It might be of interest to WLJ readers that livestock genetics concepts have been applied to wolves, as described below.
Wildlife can affect your operation with loss of animals and increased costs. For example, wolves kill livestock and cause stress that can affect reproduction, behavior and body condition (see WLJ July 13, 2026, page 3). Mexican wolves in Arizona and New Mexico are an endangered subspecies and genetically different than northern wolves in the other states and Canada, although these differences are not very large or scientifically definite. Mexican wolves’ body sizes are on average smaller than northern wolves, but they regularly kill cattle and elk.
The Mexican wolf population started with a small number of captive animals and experienced inbreeding. A captive breeding program using pedigrees and genetic data, like a livestock program, was designed to minimize this. The captive population grew and wolves were released into the wild in Arizona and New Mexico. Northern wolves were introduced into Colorado in 2023 and 2025, and could disperse southward into the range of the Mexican wolf (see the WLJ references below).
Possible interbreeding of northern and Mexican wolves has been discussed by biologists with two considerations. First, interbreeding could result in heterosis and increased fitness of the Mexican wolf population. Second, interbreeding could change the genetics of the Mexican wolf subspecies and potentially disrupt adaptation to the environment in southern Arizona and New Mexico. These considerations are similar to those for crossbreeding livestock in which heterosis can be beneficial and choosing breeds with complementary traits is important.
What will be the effect of northern wolves interbreeding with Mexican wolves? I previously provided numerical data for this question by calculating possible genetic changes to Mexican wolves for a coat-color gene (WLJ, November 27, 2024). I recently expanded the analysis to include 39 SNP (single nucleotide polymorphism) genes like the ones used for cattle genetics.
With the migration of one northern wolf into the Mexican wolf population per generation, new genes are introduced, genetic variation (heterozygosity) increases and gene frequencies change by about 1% in 10 generations and 6% in 100 generations. The black coat-color gene increases substantially with selection in two generations (see the References).
Interbreeding of northern wolves could also increase the body size of Mexican wolves. Research in Yellowstone Park found that larger wolves had better success at killing large prey (elk) than smaller wolves. Northern wolves weigh 70-120 pounds (middle value 95 lbs.) and Mexican wolves weigh 50-80 lbs. (middle value 79 lbs.), so northern wolves are on average about 16 lbs. larger than Mexican wolves.
The average genetic effect on body weight (heritability) of dogs is 0.51, and dogs are closely related to wolves, so this heritability is a reasonable approximation for wolves.
Application of a standard breeding equation (response to selection = heritability x selection differential) to body weight and considering a hypothetical breeding population of 1/2 northern wolves and 1/2 Mexican wolves results in an increase of about 4 lbs. in offspring (see the References for the calculations).
These are hypothetical calculations, but the point is that northern wolves interbreeding with Mexican wolves can increase body size, and hence their effectiveness at killing large animals like cattle and horses.
Wolves are a designated federal endangered species in all of the Lower 48 states except Montana, Idaho, Wyoming and eastern Washington and Oregon, where they are managed by the states. I think an important scientific aspect is that the Mexican wolf is a subspecies, and this is an indefinite biological category (see the References).
There is not a scientific consensus about wolf subspecies, so the Mexican wolf endangered species status is not based on the best science. This is a very controversial topic with some biologists insisting on the validity of subspecies and others recognizing that subspecies are indefinite.
Whether interbreeding of northern and Mexican wolves is good by increasing fitness and body size, or bad by changing the genetics of Mexican wolves, is a question considering the management objective of having a viable Mexican wolf population. But this wildlife objective doesn’t focus on ranchers’ management objectives of raising livestock and reducing the negative impacts of wolves.
Maybe the more important question is: Do we need more fit and bigger Mexican wolves that are better at killing livestock? Some readers of the WLJ probably think the answer is no.
The Endangered Species Act (ESA) is the law that enabled the spread of wolves in the western states. Perhaps the Multiple-Use Sustained-Yield Act of 1960 should be expanded beyond U.S. Forest Service lands to be equally as important as the ESA so farmers and ranchers, who know a lot about breeding and fitness, can have more input into wildlife management. — Matt Cronin, WLJ columnist
(Matt Cronin is a biologist with Northwest Biology and Forestry Company LLC in Bozeman, MT, and a teaching professor at Montana State University. He may be contacted at croninm@aol.com.)
Cattle American Simmental Association Hay, E.H., and A. Roberts. 2023. Genomic analysis of heterosis in an Angus X Hereford cattle population. Animals. 13:191. https://doi.org/10.3390/ani13020191 MacNeil, M.D. 2009. Research contributions from 75 years of breeding Line 1 Hereford cattle at Miles City, Montana. Journal of Animal Science. 87:2489–2501. doi:10.2527/jas.2009-1909. Greenlee ranchers raise concerns over Mexican gray wolves | Local News | eacourier.com Wolf subspecies A wolf is a wolf is a wolf | International Wolf Center Khidas, K. 2023. Morphological relationships among populations support a single taxonomic unit for the North American Gray Wolf. Journal of Mammalogy, 104(3):562–577. https://doi.org/10.1093/jmammal/gyad012 Cronin, M.A., A. Cánovas, A. Islas-Trejo, D.L. Bannasch, A.M. Oberbauer, and J.F. Medrano. 2015. Wolf Subspecies: Reply to Weckworth et al. and Fredrickson et al. Journal of Heredity 106:417-419. Fredrickson, R., P.W. Hedrick, R.K. Wayne, B.M. vonHoldt, and M. Phillip. 2015. Mexican wolves are a valid subspecies and an appropriate conservation target. Journal of Heredity. 106:415-416. https://doi.org/10.1093/jhered/esv02. Weckworth, B., N. Dawson, S. Talbot, and J. Cook. 2015. Genetic distinctiveness of Alexander Archipelago wolves (Canis lupus ligoni): reply to Cronin et al. (2015). Journal of Heredity 106:412-414. doi: 10.1093/jhered/esv02 WLJ articles on wolves April 2026 Resource Science: Reforming the science used with the ESA | Western Livestock Journal 23 Feb 2026 Resource Science: Science and management of wolves | Western Livestock Journal 25 August 2025 Resource Science: Wolves and science | Western Livestock Journal 24 April 2025 Resource Science: What is a dire wolf? | Western Livestock Journal 27 November 2024 Resource Science: Black and gray wolves and the ESA | Western Livestock Journal April 2024 Resource Science: Black and gray wolves | Resource Science | wlj.net August 2023 Resource Science: Possible black wolf sighting in Arizona | Resource Science | wlj.net 3 April 2023 Resource Science: Predicting the numbers of wolves in Colorado | Resource Science | wlj.net June 2023 Resource Science: Wolf ESA populations are too complicated | Resource Science | wlj.net March 2023 Resource Science – Wolves in Colorado: The need for management | Resource Science | wlj.net November 2023 Resource Science: Wolves in CO and Mexican wolves | Resource Science | wlj.net May 2023 Resource Science: Draft EIS for introducing wolves to CO | Resource Science | wlj.net March 2022 Resource Science: Wolves—When north meets south | Top Headlines | wlj.net 22 July 2022 Resource Science: Enviros propose wolf plan | Resource Science | wlj.net 29 August 2022 Resource Science: CO wolf environmental impact statement | Resource Science | wlj.net April 2021 Resource Science: An assessment of wolf numbers, predation in CO | Top Headlines | wlj.net 24 July 2020. Wolves in Colorado. Western Livestock Journal. 8 April 2020. Resource Science: Livestock losses to wolves | Western Livestock Journal. 19 Feb 2019 Resource Science: Wolf species, subspecies, populations, and the ESA | Western Livestock Journal 17 December 2019 Resource Science: Wolves and Colorado | Western Livestock Journal Wolf biology and Genetics Mexican wolf body weights etc. Conserving the Mexican Wolf | U.S. Fish & Wildlife Service Northern (Montana) wolf weights etc. Wolf Identification.pdf MacNulty, D.R., DW. Smith, L. D. Mech and L. E. Eberly Body Size and Predatory Performance in Wolves: Is Bigger Better? 2009. Journal of Animal Ecology. 78, (3):532-539 https://www.jstor.org/stable/27696397 Plassais J, Rimbault M, Williams FJ, Davis BW, Schoenebeck JJ, Ostrander EA. 2017. Analysis of large versus small dogs reveals three genes on the canine X chromosome associated with body weight, muscling and back fat thickness. 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Molecular and evolutionary history of melanism in North American gray wolves. Science 323:1339-1343. Breck, S.W., B.M. Kluever, M. Panasci, J. Oakleaf, T. Johnson, W.B. Ballard, L. Howery, and D.L. Bergman. 2011. Domestic calf mortality and producer detection rates in the Mexican wolf recovery area: Implications for livestock management and carnivore compensation schemes. Biological Conservation 144:930–936. Carey, J. 2023. Wolf observation report by Catron County Wildlife Investigator. Catron County, New Mexico Information Investigation Report, Case Number IR-052. Colorado Parks and Wildlife. 2023. Colorado wolf restoration and management plan. Colorado Parks and Wildlife Department, 6060 Broadway, Denver, CO. Cooke, R.F., L.R. Mehrkam, R.S. Marques, K.D. Lippolis, and D.W. Bohnert. 2017. Effects of a simulated wolf encounter on brain and blood biomarkers of stress-related psychological disorders in beef cows with or without previous exposure to wolves. J. Anim. Sci. 2017.95:1154–1163. doi:10.2527/jas2016.1250 Cronin, M.A. 2023. Possible black wolf sighting in Arizona. Western Livestock Journal 101(38):2. August 21, 2023. Resource Science: Possible black wolf sighting in Arizona | Resource Science | wlj.net Cubaynes, S., E.E. Brandell, D.R. Stahler, D.W. Smith, E.S. Almberg, S. Schindler, R.K. Wayne, A.P. Dobson, B.M. vonHoldt, D.R. MacNulty, P.C. Cross, P.J. Hudson, and T. Coulson. 2022. Disease outbreaks select for mate choice and coat color in wolves. Science 378:300-303. Fitak, R.R., S.E. Rinkevich, and M. Culver. 2018. Genome-wide analysis of SNPs is consistent with no domestic dog ancestry in the endangered Mexican wolf (Canis lupus baileyi). J. Hered. 109:372-383. Fredrickson, R., P. Siminski, M. Woolf, and P.W. Hedrick. 2007. Genetic rescue and inbreeding depression in Mexican wolves. Proc. R. Soc. B. 274:2365–2371. doi:10.1098/rspb.2007.0785 Hedrick, P.W., D.W. Smith, and D.R. Stahler. 2016. 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Detailed methods and results of my genetic analysis I calculated allele frequencies (p and q) and Hardy-Weinberg expected heterozygosity (He = 2pq) for 38 autosome and X chromosome single nucleotide polymorphism (SNP) loci and the K-locus (the CBD103 gene encoding the beta-defensin protein on chromosome 16) considering an island model of one-way migration with one migrant per generation from a source population to the Mexican wolf population with: Equation 1: qt = qm + (q0 − qm)(1 − m)t (Hartl and Clark 1989:307- 308) where the migration rate m = 0.0031 represents one migrant into the current population of 319 Mexican wolves, qm is the allele frequency of the population providing migrants, and q0 is the initial allele frequency and qtis the allele frequency after t generations in the population receiving migrants. I calculated allele frequencies for the pre-migration generation (G0) and for 1, 2, 10, and 100 generations (G1, G2, G10, and G100)of migration. I assumed the SNP and K loci are selectively neutral for these analyses. For the X chromosome SNP locus, I assumed allele frequencies in males and females were at equilibrium in the pre-migration Mexican wolf population (i.e., q0 = qmales = qfemales). I calculated allele frequencies for G1, G2, G10, and G100 for females with equation 1, for males qmales = (qfemales in generation Gt-1, Hartl and Clark 1989:44), and for the total population of females and males with: Equation 2: q = 0.6666 x qfemales + 0.3333 x qmales(Falconer 1989:16). I used allele frequencies for 1 polymorphic SNP locus on each of the 39 Canis chromosomes with the Mexican wolf population (n = 8) as the recipient of migrants and the Wyoming wolf population (n = 24) as the source of migrants that is geographically closest to the Mexican wolf population for which there are SNP data (Cronin et al. 2015a). I used the K allele frequency for wolves in Yellowstone National Park (qm = 0.2530, Hedrick et al. 2016) as the source of migrants to the Mexican wolf population for the K-locus. The K-locus controls coat color, with the dominant K allele resulting in black coat color in KK homozygotes and Kk heterozygotes. I used the K allele frequency q0 = 0.0000 for the Mexican wolf population because there are no black wolves in the extant Mexican population (Oakleaf et al. 2023). I calculated the change of allele frequency (∆q) for the allele that increased in frequency in the Mexican wolf population following migration from G0 to G1, G2, G10, and G100 for all loci. I also considered assortative mating and selection at the K-locus after migration of 1 wolf with a KK genotype from the Yellowstone population to the Mexican population with the methods of Hedrick et al. (2016). Negative assortative mating of gray and black wolves (proportion of negative assortative mating A = 0.430) and a selective advantage of heterozygotes (selection coefficients of genotypes KK 0.987, Kk 1.00, kk 0.221) were demonstrated in wolves in Yellowstone National Park (Hedrick et al. 2016, see also Cubaynes et al. 2022). Results The mean positive ∆q for 38 autosome loci from G0 is 0.0007 in G1, 0.0013 in G2, 0.0065 in G10, and 0.0571 in G100. The X chromosome locus ∆q from G0 is 0.0003 in G1, 0.0010 in G2, 0.0076in G10, and 0.0630 in G100 including both sexes. There are similar ∆q in males and females analyzed separately. The K allele frequency increased from 0.0000 in G0to 0.0008 in G1, 0.0016 in G2, 0.0077 in G10, and 0.0675 in G100. Sixteen of 38 autosomal loci and the K-locus have a new allele introduced by migration into the Mexican wolf population. Expected heterozygosity increased at 27 autosomal SNP loci, the X chromosome locus, and the K-locus, and decreased at 11 autosomal SNP loci from G0 to G100 in the Mexican wolf population following migration. Mean He at the 38 autosome SNP loci increased from 0.2192 in G0 to 0.2198 in G1, 0.2204 in G2, 0.2248 in G10, and 0.2658 in G100. He at the X chromosome locus (in females only, males are hemizygous for the X chromosome) increased from 0.2188 in G0 to 0.2194 in G1, 0.2207 in G2, 0.2304 in G10, and 0.3055 in G100. He at the K-locus assuming no assortative mating and selection increased from 0.0000 in G0 to 0.0016 in G1, 0.0031 in G2, 0.0153 in G10, and 0.1259 in G100. For the K-locus with one KK genotype migrant in one generation followed by assortative mating and selection, the dominant K allele frequency increased in the Mexican population from 0.0000 in G0 to 0.0069 in G1 and to 0.0088 in G2, and He increased from 0.0000 in G0 to 0.0137 in G1 and 0.0174 in G2. The increase in the K allele frequency and He with assortative mating and selection in two generations is > five times the increases in two generations assuming no assortative mating and selection. The K allele frequency of 0.0088 in G2is close to 0.01 which will increase to an equilibrium q = 0.242 in ten generations (Hedrick et al. 2016). Migration into the Mexican wolf population could result in increased numbers of alleles and heterozygosity and enhance fitness with heterosis and introduction of new alleles (e.g., the K allele). Genome Wide Association Studies reporting SNP genotypes associated with phenotypic traits in dogs (C. familiaris, e.g., Plassais et al. 2019) could also be used to assess fitness in wolves. The magnitude of changes of allele frequencies and heterozygosity and whether migration results in genetic rescue or negative effects on the Mexican wolf gene pool will depend on the level of migration and fitness of new genotypes in Mexican wolf habitat. Migration will also reduce genetic differentiation of the Mexican wolf and northern wolf populations which is relevant to subspecies taxonomy (Cronin et al. 2015b). Calculation of body weight following hypothetical interbreeding of northern and Mexican wolves. Mexican wolves typically weigh 50 – 80 pounds, Midpoint 79 lbs. Northern (Montana) wolves weigh 70-120 pounds. Midpoint 95 lbs. Average of midpoints Northern wolves 16 lbs. > Mexican wolves. Consider “genetic swamping” of the Mexican wolf population with northern wolves as half the breeding population. Population before breeding all Mexican wolves: 79 lbs. Breeding population select ½ northern and ½ Mexican wolf breeders, so breeders’ average weight = (95+79)/2 average = 87 lbs. Selection differential S = mean weight of selected breeders – mean weight of the population S = 87 – 79 = 8 lbs. Response to selection = heritability x selection differential = R = h2 x S R = 0.51 x 8 = 4.08 lbs. offspring References Cronin, M.A., et al. 2015a. Single nucleotide polymorphism variation of wolves in Southeast Alaska and comparison with wolves, dogs, and coyotes in North America. J Heredity 106:26-36. Cronin, M.A., et al. 2015b. Wolf Subspecies: Reply to Weckworth et al. and Fredrickson et al. J Heredity 106:417-419. Falconer, D.S. 1989. Introduction to quantitative genetics. Third edition. Wiley, New York, USA. Hartl, D.L., and A. G. Clark. 1989. Principles of population genetics. 2nd ed. Sinauer, Sunderland, MA. Hedrick, P.W., et al. 2016. Negative-assortative mating for color in wolves. Evolution 70:757-766. Oakleaf, J. K., et al. 2023. Gray Wolf (Canis lupus). Pages 303-344 in J-L. E. Catron and J. K. Frey editors. Wild carnivores of New Mexico. New Mexico University Press, Albuquerque, USA. Plassais, J., et al. 2019. Whole genome sequencing of canids reveals genomic regions under selection and variants influencing morphology. Nature Communications 10:1489.
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