Ὁ κάστωρ ἀμφίβιόν ἐστι ζῷον, καὶ μεθ’ ἡμέραν μὲν ἐν τοῖς ποταμοῖς καταδὺς διαιτᾶται, νύκτωρ δὲ ἐπὶ τῆς γῆς ἀλᾶται, οἷς ἂν περιτύχῃ τούτοις τρεφόμενος. οὐκοῦν ἐπίσταται τὴν αἰτίαν δι’ ἣν ἐπ’ αὐτὸν οἱ θηραταὶ σὺν προθυμίᾳ τε καὶ ὁρμῇ τῇ πάσῃ χωροῦσι, καὶ ἐπικύψας καὶ δακὼν ἀπέκοψε τοὺς ἑαυτοῦ ὄρχεις, καὶ προσέρριψεν αὐτοῖς, ὡς ἀνὴρ φρόνιμος λῃσταῖς μὲν περιπεσών, καταθεὶς δὲ ὅσα ἐπήγετο ὑπὲρ τῆς ἑαυτοῦ σωτηρίας, λύτρα δήπου ταῦτα ἀλλαττόμενος. ἐὰν δὲ ᾖ πρότερον ἐκτεμὼν καὶ σωθεὶς εἶτα πάλιν διώκηται, ὃ δὲ ἀναστήσας ἑαυτὸν καὶ ἐπιδείξας ὅτι τῆς αὐτῶν σπουδῆς οὐκ ἔχει τὴν ὑπόθεσιν, τοῦ περαιτέρω καμάτου παρέλυσε τοὺς θηρατάς· ἧττον γάρ τοι τῶν κρεῶν ἐκείνοις φροντίς ἐστι. πολλάκις δὲ καὶ ἔνορχοι ὄντες, ὡς ὅτι πορρωτάτω ἀποσπάσαντες τῷ δρόμῳ, εἶτα ὑποστείλαντες τὸ σπουδαζόμενον μέρος, πάνυ σοφῶς καὶ πανούργως ἐξηπάτησαν, ὡς οὐκ ἔχοντες ἃ κρύψαντες εἶχον.
(Aelian, Nat. Anim. 6.34)
The Beaver is an amphibious creature: by day it lives hidden in rivers, but at night it roams the land, feeding itself with anything that it can find. Now it understands the reason why hunters come after it with such eagerness and impetuosity, and it puts down its head and with its teeth cuts off its testicles and throws them in their path, as a prudent man who, falling into the hands of robbers, sacrifices all that he is carrying, to save his life, and forfeits his possessions by way of ransom. If however it has already saved its life by self-castration and is again pursued, then it stands up and reveals that it offers no ground for their eager pursuit, and releases the hunters from all further exertions, for they esteem its flesh less. Often however Beavers with testicles intact, after escaping as far away as possible, have drawn in the coveted part, and with great skill and ingenuity tricked their pursuers, pretending that they no longer possessed what they were keeping in concealment. (tr. Alwyn Faber Scholfield)