Experiments have shown that animals are able to understand the fact that a set containing zero objects has less objects than a set containing an integer number of objects:
https://science.sciencemag.org/content/334/6063/1664.abstract
https://www.ncbi.nlm.nih.gov/pubmed/27666660
https://www.researchgate.net/publication/325684091_Numerical_ordering_of_zero_in_honey_bees
The most simple animal known to have the ability to understand zero is a honeybee.
See learning_zero.jl for an example of a nueral network much smaller than a honeybee brain that understands zero.