- Gorilla females tend to have one man, hence there is no sperm competition between different males' sperm
- Orangutan females tend to choose their man, so, once again, there is no sperm competition between different males' sperm
- Bonobos are matriarchal and have concealed ovulation, hence they can also choose the father of their children
- Male chimpanzees pick their female partners, but female partners are not necessarily faithful, hence a race sometimes takes place within female chimps to determine which sperm will impregnate them.
From the paper:
In conclusion, the major findings of this study are: (i) That the contrasting patterns of DAZ and CDY variability in chimpanzees (P. troglodytes) and bonobos (P. paniscus), initially suggested by comparative FISH , are similarly reflected by real-time qPCR data. (ii) Although chimpanzee and bonobo share promiscuous mating behaviors, it is only in chimpanzees that male dominance is sufficiently developed to influence sperm competition , . This results in high selective pressure on male fertility genes. Bonobos on the other hand, are characterized by a matriarch-dominated societal structure [63,64, reviewed in 65] which, coupled to concealed ovulation , permits female mate choice, thus rendering sperm competition redundant. (iii) That monoandrous mating in gorillas (G. gorilla) – and female mate choice in orangutans (P. pygmaeus and P. abelii) , ,  similarly accounts for the dearth of intraspecific Y-chromosomal variation in the ampliconic fertility genes DAZ and CDY among Y-chromosomal lineages in these species.PLoS ONE 6(12): e29311. doi:10.1371/journal.pone.0029311
Y-Chromosome Variation in Hominids: Intraspecific Variation Is Limited to the Polygamous Chimpanzee
Gabriele Greve et al.
We have previously demonstrated that the Y-specific ampliconic fertility genes DAZ (deleted in azoospermia) and CDY (chromodomain protein Y) varied with respect to copy number and position among chimpanzees (Pan troglodytes). In comparison, seven Y-chromosomal lineages of the bonobo (Pan paniscus), the chimpanzee's closest living relative, showed no variation. We extend our earlier comparative investigation to include an analysis of the intraspecific variation of these genes in gorillas (Gorilla gorilla) and orangutans (Pongo pygmaeus), and examine the resulting patterns in the light of the species' markedly different social and mating behaviors.
Fluorescence in situ hybridization analysis (FISH) of DAZ and CDY in 12 Y-chromosomal lineages of western lowland gorilla (G. gorilla gorilla) and a single lineage of the eastern lowland gorilla (G. beringei graueri) showed no variation among lineages. Similar findings were noted for the 10 Y-chromosomal lineages examined in the Bornean orangutan (Pongo pygmaeus), and 11 Y-chromosomal lineages of the Sumatran orangutan (P. abelii). We validated the contrasting DAZ and CDY patterns using quantitative real-time polymerase chain reaction (qPCR) in chimpanzee and bonobo.
High intraspecific variation in copy number and position of the DAZ and CDY genes is seen only in the chimpanzee. We hypothesize that this is best explained by sperm competition that results in the variant DAZ and CDY haplotypes detected in this species. In contrast, bonobos, gorillas and orangutans—species that are not subject to sperm competition—showed no intraspecific variation in DAZ and CDY suggesting that monoandry in gorillas, and preferential female mate choice in bonobos and orangutans, probably permitted the fixation of a single Y variant in each taxon. These data support the notion that the evolutionary history of a primate Y chromosome is not simply encrypted in its DNA sequences, but is also shaped by the social and behavioral circumstances under which the specific species has evolved.