Siikibam and Piggiesandbuns - this explanation I translated was provided by a Moderator in a German forum:
Female guinea pigs urgently need a male in the group so that they can control their hormonal balance. Because we don't want babies, of course, this should be a neuter. Its male fragrances / pheromones, which are secreted by the caudal gland, are absorbed in the females by tiny indentations on both sides of the nasal septum (vomeronasel organ). This organ transmits neuronal signals to the pituitary gland via the amygdala. These signals regulate the formation of LH (luteinizing hormone). As a result, a female regularly becomes in heat every 14 to 18 days (normal cycle). During sex itself, which also takes place with a castrato, ovulation occurs through mechanical-neural stimuli. The more often copulation takes place, the sooner ovulation will take place. Without copulation / sex, no ovulation. If there is no male MS present and ovulation cannot inevitably take place, then cysts form from the mature egg cells in the ovary, which can reach the size of hen's eggs and / or can also release male sex hormones. The consequences are hair loss, especially on the flanks, masculine and aggressive behavior with dominance behavior, digestive problems (displacement of the internal organs by the cysts) and, unfortunately, tumors on the ovaries and uterus.
Much of the other guidance is the same here but there are some other interesting differences such as; early castration for baby boars is compulsory unless you are a registered breeder; young piggies can only be adopted or rehomed with an older 'educator' piggy present; C&C cages and fleece are only just catching on...I tend to follow the UK guidance where possible.