Animals establish security of food, shelter, social contact, and mating because proximate mechanism, if they do not seek these necessities they will not survive. All vertebrates share similarities in body structure; they all have a skeleton, a nervous system, a circulatory system, a digestive system and excretory system. Similar to humans, non-human animals also have a sensory system. The sensory system is responsible for the basic five senses from touch to tasting. Most of the physiological and biochemical responses found in animals are found in humans. Neurophysiologists have not found any fundamental difference between the structure and function of neurons and synapse between humans and other animals. Recent studies using positron emission tomography (PET) and magnetic resonance imaging (MRI) has provided evidence proving that chemical changes that occur with emotions are similar between humans and non-human animals. In a study comparing guinea pigs and humans, it was determined that the distress experienced by offspring separation in a guinea pig and a human going through depression activates the same region of the brain. The opiate receptor was also examined, allowing observation of the pleasure stimuli. In the procedure both a human and a rat had their receptors blocked with a certain drug. Once receptors were blocked, both the rat and the human were exposed to pleasurable food, but both were disinclined to eat the food. Engagements of sexual activities during non-breeding seasons have been observed in the animal kingdom. Dolphins and Japanese macaques are two of the many species that engage in sexual activities that do not lead to fertilization. Great varieties of non-copulatory mounting are expressed in several species. Male lions engage in mounting with other male lions, especially when in search for another pride. The varieties of mounting include mounting without erections, mounting with erection but no penetration, and mounting from the side. Expressions of affection are displayed in the animal kingdom as well. Affectionate behaviors do not include penetration or genital rubbing, but are still seen as a manner of sexual behavior. An affectionate activity can be as simple as licking. Male lions are known for head rubbing, bats engage in licking, and mountain sheep rub horns and faces with each other. Animals have also engaged in kissing, touching of noses, mouths and muzzles have been witnessed in African elephants, walruses, and mountain zebras. Primates also engage in kissing that is incredibly similar to human display of kissing. Chimpanzees have full mouth-to-mouth contact, and bonobos kiss with their mouth open and mutual tongue stimulation.