Growth differentiation factor-9 (GDF9) and bone morphogenetic protein-15 (BMP15) are co-expressed in oocytes throughout most of folliculogenesis, where they regulate ovulation rate and female fertility in a species-specific manner. Like all transforming growth factor-β superfamily ligands, GDF9 and BMP15 are synthesised as dimeric precursors, where the N-terminal prodomain directs folding and dimerization of the C-terminal mature domain. Mice secrete active GDF9 homodimers, but do not secrete BMP15. Consistent with this, Gdf9-null mice are infertile due to folliculogenesis not proceeding beyond the primary stage, whereas Bmp15-null mice have a very mild phenotype and remain fertile. In contrast, humans secrete inactive GDF9 homodimers, active BMP15 homodimers, and highly active GDF9:BMP15 heterodimers. In sheep, genetic studies have indicated that both GDF9 and BMP15 are important for fertility, with heterozygous carriers of mutations in either gene typically showing greater fecundity, whilst homozygous carriers are infertile; yet biochemical studies indicate that GDF9 and BMP15 homodimers are both inactive. We have now demonstrated that the ovine GDF9:BMP15 heterodimer is highly active, resolving the discrepancy between the genetic and biochemical studies. To gain further insights, we characterised GDF9 and BMP15 synthesis and activity from two additional species where no studies had previously been performed: the domestic cat and the koala. We demonstrated that like mice, cats and koalas secrete active GDF9 homodimers, but do not secrete BMP15. Interestingly, mice, cats and koalas have evolved distinct mechanisms to activate GDF9. Future studies will determine how differences in the secretion and activity of these critical growth factors regulate species-specific fecundity.