can thus be rather fuzzy. Mixed signals may be employed: aggressive mimics often have a specific part of the body sending a deceptive signal, with the rest being hidden or camouflaged. Contrast with defensive mimicry Aggressive mimicry stands in semantic contrast with defensive mimicry, where it is the prey that acts as a mimic, with predators being duped. Defensive mimicry includes the well-known Batesian and Müllerian forms of mimicry, where the mimic shares outward characteristics with an aposematic or harmful model. In Batesian mimicry, the mimic is modeled on a dangerous (usually unpalatable) species, while in Müllerian mimicry both species are harmful, and act as comimics, converging on a common set of signals and sharing the burden of 'educating' their predators. Included in defensive mimicry is the lesser known Mertensian mimicry, where the mimic is more harmful than the model, and Vavilovian mimicry, where weeds come to mimic crops through unintentional artificial selection. In defensive mimicry, the mimic benefits by avoiding a harmful interaction with another organism that would be more likely to take place without the deceptive signals employed. Harmful interactions might involve being eaten, or pulled out of the ground as a weed. In contrast, the aggressive mimic benefits from an interaction that would be less likely to take place without the deception, at the expense of its target. Classification Luring prey In some cases the signal receiver is lured toward the mimic. This involves mimicry of a resource that is often vital to the prey's survival (or more precisely, the survival of its genes) such as nutrition or a mate. If the bait offered is of little value to prey they would not be expected to take such a risk. For example, in all known cases of sexual signal mimicry it is always the male sex that is deceived (in fact, it has been suggested that females of some species have evolved mimicry as a strategy to avoid unwanted matings). In these cases the predator need not move about foraging for prey, but may simply stay still and allow prey to come to it. Some studies suggest that the northern shrike (Lanius excubitor) sings in winter often imitating small passerines that may be preyed upon when lured within reach. There has been one report of a margay using mimicry of the cry of an infant pied tamarin to try to lure an adult tamarin within striking distance.