Lymphoid stem cells in the bone marrow give rise to two primary types of immune cells: natural killer (NK) cells and B lymphocytes. NK cells are part of the innate immune system, providing rapid responses to infected or cancerous cells. B lymphocytes, part of the adaptive immune system, produce antibodies. Platelets and neutrophils, on the other hand, originate from myeloid stem cells, not lymphoid stem cells.
In a deer species that lives in groups, some individuals act as sentinels and are vigilant for predators but there is a trade-off between foraging and vigilance. A researcher collects data on the foraging rates of edge (black) and central (grey) members of the group in two similar forests, one with and one without predators. Which one of the following figures best supports the hypothesis that members at the edge are acting as sentinels in this deer species?