A case of thanatosis in domestic sheep

Abstract Thanatosis (feigning death, defensive tonic immobility) is a widespread anti-predator behavioural strategy in animals. Animals that have evolved this behaviour react to physical contact with the predator by displaying a persistent tonic immobility that makes them look like dead prey. This phenomenon has often been observed in wild animal species. In domestic animals, defensive tonic immobility is well known in domestic chickens. However, feigning death appears to be unusual in domestic mammals, although similar states of tonic immobility, albeit with different evolutionary significance, have been described in many domestic mammals. Here we report on thanatosis in domestic lambs observed in two separate cases. In both cases, it was apparently a defensive anti-predator strategy, as the tonic immobility began during the capture and brief handling of the lamb due to the regular monitoring of birth weight and marking, while the thanatosis was terminated a few minutes later when the threat ceased (withdrawal of the human, calling and approach of the mother). Both lambs that exhibited thanatosis were male, about one day old and lived in a mixed herd of sheep and goats. Several factors may have contributed to the described occurrence of this phenomenon: the young age of the animals and the relatively low presence of humans in the herd in question. The observations show that evolutionary strategies/adaptations that were crucial in the ancestral environment have not disappeared during domestication and artificial selection and can still come into play when certain conditions and circumstances coincide. Ultimately, this observation can also be applied to the ancestor of the sheep, the mouflon, for which there is no report of this phenomenon to date.