محققان باغ وحش سن دیگو دو کرکس نر را شناسایی کرده‌اند که بدون جفت‌گیری و مشارکت کرکس نر و در واقع به صورت بکرزایی (تولید مثل بدون لقاح) متولد شده‌اند. این نخستین بار است که بکرزایی در پرندگان گزارش می‌شود و جزو پدیده‌های نادر در بین پرندگان محسوب می‌شود.
بکرزایی نوعی تولید مثل جنسی است که در آن تخمک با اسپرم لقاح پیدا نمی‌کند و بدون حضور مذکر، موجود زنده کامل متولد می‌شود. این پدیده در تعداد زیادی از بی‌مهرگان و مهره‌داران از جمله ماهی‌ها، دوزیستان و خزندگان دیده می‌شود. اکنون شناسایی دو کرکس نر که به صورت بکرزایی متولد شده‌اند، باعث شگفتی محققان شده است، زیرا این نخستین بار است که گزارشی در مورد بکرزایی در پرندگان منتشر می‌شود.
این اتفاق در شرایطی رخ داده که کرکس کالیفرنیایی در خطر انقراض قرار دارد و تنها ۵۰۰ پرنده از این نوع در آمریکا و مکزیک زندگی می‌کنند.
این دو کرکس در سال‌های ۲۰۰۱ و ۲۰۰۹ سر از تخم بیرون آوردهاند و از نظر ژنتیکی فقط دارای ژن ماده هستند و هیچ ژنی از کرکس نر به عنوان پدر به ارث نبرده‌اند. محققان برای رسیدن به این نتیجه تمامی ۴۷۶ کرکس کالیفرنیایی را که در شرایط جف‌گیری متولد شده بودند، مورد بررسی قرار دادند.
گفتنی است هر دو کرکس که به صورت بکرزایی متولد شده بودند، اکنون مرده‌اند. یکی از آنها در دو سالگی و در سال ۲۰۰۳ و دیگری در سال ۲۰۱۷ مرده است. محققان متوجه شدند که هر دو کرکس مادر نیز پیش از این جوجه‌هایی از طریق لقاح به دنیا آوردهاند.
محققان همچنین متوجه شدند که یکی از کرکس‌های مادر که توانسته بود به روش بکرزایی، جوجه‌‌ا‌ی را به دنیا آورد، بعد از این اتفاق دو برابر بیشتر از قبل جوجه به دنیا می‌آورد. یکی از کرکس‌های مادر ۲۳ جوجه کرکس به دنیا آورده است که تعداد زیادی محسوب می‌شود.
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