What is the ruling on watching sexual acts between animals in documentary films?

There is no objection to watching the animals you mentioned, for the default rule regarding things is permissibility unless there is evidence to the contrary, and no such evidence exists here. However, a person should avoid this type of matter because it compromises one's dignity without any benefit. This permissibility is conditioned on the absence of temptation (fitna), otherwise it would be forbidden, as Ibn 'Aqil transmitted in Al-Insaf: "It is forbidden to look with lust, or to engage in tribadism, or to look at an animal one desires and avoids, and similarly, to be in seclusion."