This has been observed with other seemingly less intelligent species as well, for as long as man has kept pets. Anyone who has ever had f.e. dogs or cats at hom

This has been observed with other seemingly less intelligent species as well, for as long as man has kept pets. Anyone who has ever had f.e. dogs or cats at home as part of the family, coupled with one pet in the "litter" dying suddenly or accidentally, knows how notably different the other animals in the home will react to this compared to when one dies from drawn-out illness or old age. It is like that other side of the coin of missing someone, something most of us know is a characteristic that animals share with us. Animals being in mourning during sudden deaths of their kinsmen and companions is something the pet keepers among the population knows very well - there just haven't been much of formal or scientific studies in the topic.