. ▁Today ▁there ▁are ▁only ▁four ▁gen et ically ▁un m ixed , ▁free ▁ro aming , ▁public ▁b ison ▁her ds ▁and ▁only ▁two ▁that ▁are ▁also ▁free ▁of ▁bru cell osis : ▁the ▁Henry ▁Mountains ▁B ison ▁H erd ▁and ▁the ▁Wind ▁C ave ▁B ison ▁H erd . ▁A ▁founder ▁population ▁of ▁ 1 6 ▁animals ▁from ▁the ▁Wind ▁C ave ▁b ison ▁her d ▁was ▁re - est ab lished ▁in ▁Mont ana ▁in ▁ 2 0 0 5 ▁by ▁the ▁American ▁Pra irie ▁Foundation . ▁The ▁her d ▁now ▁numbers ▁near ▁ 1 0 0 ▁and ▁ro ams ▁a ▁ ▁grass land ▁exp anse ▁on ▁American ▁Pra irie ▁Reserve . ▁ ▁The ▁end ▁of ▁the ▁ran ch ing ▁era ▁and ▁the ▁on set ▁of ▁the ▁natural ▁reg ulation ▁era ▁set ▁into ▁motion ▁a ▁chain ▁of ▁events ▁that ▁have ▁led ▁to ▁the ▁b ison ▁of ▁Y ellow stone ▁Park ▁migr ating ▁to ▁lower ▁elev ations ▁outside ▁the ▁park ▁in ▁search ▁of ▁winter ▁for age . ▁The ▁presence ▁of ▁wild ▁b ison ▁in ▁Mont ana ▁is ▁perce ived ▁as ▁a ▁threat ▁to ▁many ▁cattle ▁ran ch ers , ▁who ▁fear ▁that ▁the ▁small ▁percentage ▁of ▁b ison ▁that ▁carry ▁bru cell osis ▁will ▁inf ect ▁liv est ock ▁and ▁cause ▁c ows ▁to ▁abort ▁their ▁first ▁cal ves . ▁However , ▁there ▁has ▁never ▁been ▁a ▁documented ▁case ▁of ▁bru cell osis ▁being ▁transm itted ▁to ▁cattle ▁from ▁wild ▁b ison . ▁The ▁management ▁controvers y ▁that ▁began ▁in ▁the