Complex sentence: It is noted that those species with such swellings have a higher average of males per group than those without; over twice as many, and of the "Old World" primate species which have multi-male social systems, 71% show exaggerated swellings.
Split into simpler sentences: It is noted that those species with such swellings have twice as many males per group than those without. Additonally, of the groups "Old World" primate species which have multi-male social systems, 71% show exaggerated swellings.