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Archived Comments for: Fecal estrogen, progestagen and glucocorticoid metabolites during the estrous cycle and pregnancy in the giant anteater (Myrmecophaga tridactyla): evidence for delayed implantation

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  1. Delayed implantation is certainly NOT occurring in the giant anteater

    Franz Schwarzenberger, Vetmeduni Vienna

    24 March 2014

    A.Prof. Dr. Franz Schwarzenberger (Dept. of Biomed. Sci. - Biochemistry; Vetmeduni Vienna; A-1210 Vienna, Austria

    Dr. Christine Osmann (Zoo Veterinarian; Vet. Advisor for the Giant Anteater EEP; Zoo Dortmund, D-44225 Dortmund, Germany)


    The authors Knott et al., 2013 describe the measurement of faecal progesterone, estrogen and cortisol metabolites in faecal samples of the female giant anteater. Results of reproductive hormone measurements confirm those of a previous publication by Patzl et al., 1998. We are co-authors of this study and appreciate the verification of results by the Memphis laboratory using reagents dissimilar to those used by our laboratory.

    However, we harshly criticise the grave errors in the interpretation of results by Knott et al. The authors misinterpret a sharp increase of progesterone metabolite levels by mid gestation as evidence for delayed implantation, or embryonic diapause. The definition for embryonic diapause is ‘developmental arrest at the blastocyst stage’. This phenomenon is occurring in almost 100 mammalian species in seven different mammalian orders. During embryonic diapause the size of a blastocyst is less than 2 mm and can only be visualized by post mortem examinations or by uterine lavage (Renfree & Shaw, 2000).

    There is no morphological evidence presented by Knott et al., or in refereed publications to underline the bold statement of ‘evidence for delayed implantation in the giant anteater’. Instead the authors oversimplified results by presuming the implantation period would occur by mid-gestation only and would coincide with the secondary rise of progesterone metabolites. In fact, this secondary rise most likely is caused by beginning placental steroid hormone production, like it is obvious in several other animal species (Schwarzenberger & Brown, 2013).

    Results of ultrasonography contradict the misappropriate assumption of delayed implantation in the giant anteater. The current methodology of gestation diagnosis for the giant anteater used by experienced veterinarians in European zoos is transabdominal ultrasonography (in line with regular medical training of the animals). Occasionally transrectal ultrasonography is performed under full anaesthesia. Using the transrectal methodology, pregnancy diagnosis is reliable by day 70, when the diameter of the pregnant uterus is about 6 x 6 cm, as compared to the 3 x 4 cm of the non-gravid uterus. The developing foetus is about 2 cm in size, and its heartbeat is clearly visible. By mid gestation (day 90) the foetus is around 8-10 cm in length, and the size of the uterus is approximately 8 x 8 cm. At this stage the foetus and the discoid shaped placenta are reliably visualized by transabdominal ultrasonography. Ultrasonographic images from pregnant anteaters were described in the thesis of N. Schauerte, 2005. A post mortem image of an in situ foetus from a road killed Giant anteater at about mid gestation is shown in the publication of Mess et al., 2012.

    A further severe misinterpretation and oversimplification of results is the statement on age related differences in oestrous cycle length. Knott et al. had results from 3 animals only! The length of oestrous cycles in the two younger females was shorter than those of the older one. However, this result is mere coincidence and is likely due to individual variation. Individual differences of the oestrous cycle length in giant anteaters of up to 10 days have been reported by Schauerte, 2005. Results in this study were supported by faecal steroid analysis, and are in correlation with findings by Patzl et al., 1998.


    Mess, A. M., Favaron, P. O., Pfarrer, C., Osmann, C., Melo, A. P., Rodrigues, R. F., .. & Miglino, M. A. (2012). Placentation in the anteaters Myrmecophaga tridactyla and Tamandua tetradactyla (Eutheria, Xenarthra). Reprod Biol Endocrin, 10, 1-7.

    Patzl, M., Schwarzenberger, F., Osmann, C., Bamberg, E., & Bartmann, W. (1998). Monitoring ovarian cycle and pregnancy in the giant anteater (Myrmecophaga tridactyla) by faecal progestagen and oestrogen analysis. Animal Reproduction Science, 53(1), 209-219.

    Renfree, M.B. & Shaw, G. (2000). Diapause. Annual Review of Physiology 62 353–375.

    Schauerte, N. (2005). Untersuchungen zur Zyklus-und Graviditätsdiagnostik beim Großen Ameisenbären (Myrmecophaga tridactyla) (Doctoral thesis, Dissertation, Universität Giessen).

    Schwarzenberger F., Brown J.L. (2013) Hormone monitoring: An important tool for the breeding management of wildlife species. Wiener Tierarztliche Monatsschrift–Veterinary Medicine Austria, 100, 209-225.




    Competing interests

    We have NO competing interests