Nice Buck Slim9300. I'd guess that it is a genetic trait that is expressed to a greater extent as the animal ages. Much like Male Pattern Baldness and/or complete change of hair color from dark to silver, the older the human, the more impressive the change is compared to normal youth appearance. There definitely seems to be some level of genetic expression throughout the range of BT deer.
I've always felt that the average Sitka Blacktail double throat patch was often much more substantial/impressive than those seen in BT deer significantly south of that region. From memory - they are essentially the same animal genetically. The few genetic traits that differentiate them from herds farther south likely resulted from their (small) population's isolation following the last major glaciation in N. America. A Founder Effect type small population of deer migrated north from SW WA/NW OR to populate areas north. Thousand of years of interbreeding resulted in a situation of reduced genetic diversity in that population, which likely strengthened of traits like the double throat patch, and reduced body size in size, (which is often seen in populations experiencing limited food resources), but similarly, created genetic problems not seen in deer populations south of there. Certainly, there doesn't seem to be a genetic advantage for a deer to have a double throat patch (unless the breeding females find it more impressive than animals without the trait......often seen in birds etc.
https://en.wikipedia.org/wiki/Founder_effecthttps://www.researchgate.net/profile/Jim-Heffelfinger/publication/24216147_Species-wide_phylogeography_of_North_American_mule_deer_Odocoileus_hemionus_Cryptic_glacial_refugia_and_postglacial_recolonization/links/5a146f7945851500521301ea/Species-wide-phylogeography-of-North-American-mule-deer-Odocoileus-hemionus-Cryptic-glacial-refugia-and-postglacial-recolonization.pdfFrom the abstract of that paper:
"Patterns of genetic diversity within the black-tailed deer
lineage suggest a single refugium along the Pacific Northwest coast, and refute the hypothesis
that black-tailed deer persisted in one or more northern refugia. Our data suggest that
black-tailed deer recolonized areas in accordance with the pattern of glacial retreat, with
initial recolonization northward along a coastal route and secondary recolonization inland."
example: 70% of Sitka BTD have condition where neither testes descend:
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.544.2888&rep=rep1&type=pdfI didn't read this so it may completely refute my theory.....
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/cordilleran-ice-sheet(not my trophy) Taken from Google images of Sitka BT deer
