Responsa for Bekhorot 47:11

הנכנס לתוך עדרו בלילה וראה כעשר כחמש עשרה מבכירות ושאינן מבכירות יולדות למחר השכים ומצא זכרים תלויות בשאין מבכירות ונקבות תלויות במבכירות אינו חושש שמא בנה של זו בא לו אצל זו:

And so R'Simeon B'Gamaliel used to say: If one goes among his herd at night and sees about ten or fifteen animals, both those which had not borne previously and those which had previously given birth, and, the next day, he rises early and finds the males<span class="x" onmousemove="('comment',' Who were born now, clinging to and being given suck by animals which had already been exempted from the law of the firstling. And the females born now, he found clinging to and being given suck by animals that had now given birth for the first time. In these cases, the priest receives nothing, for as we presume that each offspring is near its own mother, the law of the firstling is not here applicable, as the males come from animals already exempted and the females are not subject to the law of the firstling.');">⁶</span>

Teshuvot Maharam