Humans ovulate once a month during a normal hormonal cycle, regardless of how much action (or not) we get.  Llamas, however, have a much more efficient system. Did you know that male llamas can induce ovulation in females by copulating with them? This means that females llamas only ovulate when they’ve actually had sex! While this might mean that they don’t have to deal with the annoyance of monthly cycles, it also means hormonal birth control would not work on them. 

Sketch and fact contributed by Victoria Farrar

Source: Bravo, P. W., Stabenfeldt, G. H., Lasley, B. L., & Fowler, M. E. (1991). The effect of ovarian follicular size on pituitary and ovarian responses to copulation in domesticated South American camelids. Biology of Reproduction, 45 (4), 553–559. [PubMed]