The sperm heads are thus considered rather fragile as there is no capacity for the formation of stable crosslinked nuclear protein-DNA complexes. ![]() It is also worth noting that, as with marsupials (with the exception of some planigales), and, unlike eutherian mammals, the amino acid sequence of protamine 1 of monotreme spermatozoa does not include cysteine residues ( Retief et al., 1993). Monotreme yolky oocytes are fertilized within the female’s oviduct where they meet the spermatozoa ( Grutzner et al., 2008). The microanatomy of the epididymis has been likened to that of birds and reptiles ( Bedford and Rifkin, 1979 Carrick and Hughes, 1982). The spermatozoa are filiform and bear more resemblance to avian and reptilian spermatozoa than to spermatozoa of other mammals during sperm maturation in the epididymis, the spermatozoa become organized into bundles of approximately 100 cells, which swim together in synchrony. The monotremes are testicond mammals, which means that the males’ testes are situated inside the body cavity, rather than in a scrotum. For detailed information on comparative mammalian reproductive biology the reader should consult the encyclopaedic book by Asdell entitled “Patterns in mammalian reproduction” ( Asdell, 1965) and a subsequent revision of the same book, which presents further detailed information ( Hayssen, 1993). In fact, some aspects of their biology seemed highly improbable to earlier generations of scientists, and indeed, when the first description of a platypus was sent to the Royal Society in London in the early 19th century ( Home, 1802), it was widely regarded as a hoax!Įchidnas and the platypus are both seasonal breeders, and it is worth noting here that seasonality is an important reproductive adaptation for many, if not most, mammals. Detailed studies of platypus and echidna reproductive biology have only been conducted over the last few decades (see, e.g., Grant and Temple-Smith, 1998 Morrow and Nicol, 2009) and given that these species are secretive and until recently ( Wallage et al., 2015) have not been easy to breed in captivity, knowledge about their general biology is still rather rudimentary ( Johnston and Keeley, 2015). Genome research has suggested that monotremes diverged from the other mammalian lineage (therian mammals which later further diverged, forming the marsupials and the placental mammals) about 166 million years ago. These species are geographically restricted and occupy different habitats echidnas are terrestrial mammals and are found only in Australia and New Guinea while platypus are aquatic, living in the river systems of eastern Australia and Tasmania. The four extant species of echidna within the order Tachyglossidae, together with the duck-billed platypus ( Ornithorhynchus anatinus) are the egg laying mammals, and show many reproductive characteristics reminiscent of birds and reptiles. William Vincent Holt, Stephen Johnston, in Encyclopedia of Reproduction (Second Edition), 2018 Monotremes However, these teeth are replaced by horny ridged plates that are used by adults to crush food. The teeth are multirooted and have a thin layer of enamel, which shows only traces of prismatic structure but has some incremental markings and some atypical features ( Lester et al., 1987). Adult platypuses lack teeth but juveniles possess a number of vestigial teeth the dental formula is given as I 0 5 C 1 1 P 2 2 M 3 3 = 34 ( Pasitschniak-Arts and Marinelli, 1998). Platypuses are nocturnal and aquatic they are opportunistic predators of bottom-living arthropods together with mollusks, annelids, and fish eggs ( Pasitschniak-Arts and Marinelli, 1998). After loss of the egg tooth, echidnas are toothless. ![]() The egg tooth is shed a day or two after hatching. ![]() Monotremes are oviparous, and hatchlings, like those of reptiles, are equipped with an egg tooth, which has a basis of dentine and is used to pierce the eggshell ( Green, 1930 Hughes and Hall, 1998). The Monotremata consists of two families: the Tachyglossidae (two genera of echidnas or spiny anteaters) and the Ornithorhynchidae, which contains only one species, the platypus. Barry Berkovitz, Peter Shellis, in The Teeth of Mammalian Vertebrates, 2018 Monotremes
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