Possible explanations for the differences observed between these studies is that tadpoles of B. marinus are inherently less immunocompetent than those of R. catesbeiana and/or they had less time than R. catesbeiana to mount an effective immune response to the globin antigen. Firstly, the cane toad has a very short larval stage of approximately 50-60+days, depending on tadpole density and temperature [28] whereas the larval stage in the bullfrog lasts at least 120 days and up to 2 years depending on environmental conditions [29] . The long larval stage in the bullfrog enabled an initial immunisation using FCA, followed by a boost 1 month later [18] . By contrast, cane toad tadpoles in our study were only large enough to be first injected at stage 26 (approximately day 9), barely 3 weeks before the onset of adult globin synthesis at stages 36-40 (day 30-43) and so a similar boost was not given. Nevertheless we considered that there should be sufficient time for a primary immune response to develop before adult globin appeared, provided cane toad tadpoles recognise the adult globin as a foreign protein. Secondly, due to the large difference between the cane toad and bullfrog developmental time frames, and as age at inoculation was not specified, the bullfrog tadpoles may have been inoculated later in development and been more immunocompetent than in our study. In support of this, studies in Xenopus have demonstrated that immune responses improve with age. The affinity of specific IgY antibodies against dinitrophenol (DNP) in Xenopus larvae is reported to be less than in adults, and in turn much lower than the affinity of mammalian anti-DNP IgG antibodies [30, 31] . The range of antibodies produced to DNP were also less heterogeneous in larval than in adult Xenopus [32] .