in a variety of sexual behaviors including male choosiness, mate guarding, and vibrational signaling in courtship. Genetic evidence of interspecies sexual activity in humans Research into human evolution confirms that, in some cases, interspecies sexual activity may have been responsible for the evolution of new species (speciation). Analysis of animal genes found evidence that, after humans had diverged from other apes, interspecies mating nonetheless occurred regularly enough to change certain genes in the new gene pool. Researchers found that the X chromosomes of humans and chimps may have diverged around 1.2 million years after the other chromosomes. One possible explanation is that modern humans emerged from a hybrid of human and chimp populations. A 2012 study questioned this explanation, concluding that "there is no strong reason to involve complicated factors in explaining the autosomal data". Inbreeding avoidance When close relatives mate, progeny may exhibit the detrimental effects of inbreeding depression. Inbreeding depression is predominantly caused by the homozygous expression of recessive deleterious alleles. Over time, inbreeding depression may lead to the evolution of inbreeding avoidance behaviour. Several examples of animal behaviour that reduce mating of close relatives and inbreeding depression are described next. Reproductively active female naked mole-rats tend to associate with unfamiliar males (usually non-kin), whereas reproductively inactive females do not discriminate. The preference of reproductively active females for unfamiliar males is interpreted as an adaptation for avoiding inbreeding. When mice inbreed with close relatives in their natural habitat, there is a significant detrimental effect on progeny survival. In the house mouse, the major urinary protein (MUP) gene cluster provides a highly polymorphic scent signal of genetic identity that appears to underlie kin recognition and inbreeding avoidance. Thus there are fewer matings between mice sharing MUP haplotypes than would be expected if there were random mating. Meerkat females appear to be able to discriminate the odour of their kin from the odour of their non-kin. Kin recognition is a useful ability that facilitates both cooperation among relatives and the avoidance of inbreeding. When mating does occur between meerkat relatives, it often results in inbreeding depression. Inbreeding depression was evident for a variety of traits: pup mass at emergence from the natal burrow, hind-foot length, growth until independence and juvenile survival. The grey-sided vole (Myodes rufocanus) exhibits male-biased dispersal as a means of avoiding incestuous matings. Among those matings that do involve inbreeding the number of weaned juveniles in litters