Článek se zabývá studiem poměru pohlaví narozených mláďat u hrocha obojživelného (Hippopotamus amphibius) a hrošíka liberijského (Choeropsis liberien­sis) chovaných v zoologických zahradách Evropy (hroch) a celého světa (hrošík). U obou druhů jsme zjistili vychýlení poměru pohlaví od vyrovnaného poměru 1 : 1, přičemž u hrocha je to ve prospěch synů a u hrošíka ve prospěch dcer. U každého z obou zkoumaných druhů ovlivňují poměr pohlaví jiné faktory a zřejmě také podmiňuje jiný mechanismus.

Použitá a citovaná literatura:
Bercovitch, F. B., Loomis, C. P. & Rieches, R. G. 2009: Age-specific changes in reproductive effort and terminal investment in female Nile lechwe. Journal of Mammalogy 90, 40-46.
Boisserie, J.-R., Fisher, R. E., Lihoreau, F. & Weston, E. M. 2011: Evolving between land and water: key questions on the emergence and history of the Hippopotamidae (Hippopotamoidea, Cetancodonta, Cetartiodactyla). Biological Reviews 86, 601-625.
Brown, G. R. & Silk, J. B. 2002: Reconsidering the null hypothesis: Is maternal rank associated with birth sex ratios in primate groups? Proceedings of the National Academy of Sciences of the United States of America 99, 11252-11255.
Cameron, E. Z. & Linklater, W. L. 2007: Extreme sex ratio variation in relation to change in condition around conception. Biology Letters 3, 395-397.
Clark, A. B. 1978: Sex ratio and local resource competition in a prosimian primate. Science 201, 163-165.
Clutton-Brock, T. H. & Iason, G. R. 1986: Sex-ratio variation in mammals. Quarterly Review of Biology 61, 339-374.
Edwards, A. M. & Cameron, E. Z. 2014: Forgotten fathers: paternal influences on mammalian sex allocation. Trends in Ecology & Evolution 29, 158-164.
Eltringham, S. K. 1999: The hippos. Academic Press, London.
Estevez, J. A., Landete-Castillejos, T., García, A. J., Gaspar-López, E., Ceacero, F. & Gallego, L. 2011: ENSO affects sex ratio progeny in captive Iberian red deer despite a steady feeding regime. Acta Theriologica 56, 323-328.
Fisher, R. A. 1930: The genetical theory of natural selection. Clarendon, Oxford.
Grant, V. J. & Chamley, L. W. 2010: Can mammalian mothers influence the sex of their offspring peri-conceptually? Reproduction 140, 425-433.
Hewison, A. J. M. & Gaillard, J. M. 1999: Successful sons or advantages daughters? The Trivers-Willard model and sex-biased maternal investment in ungulates. Trends in Ecology and Evolution 14, 229-234.
Hewison, A. J. M., Gaillard, J. M., Kjellander, P., Toigo, C., Liberg, O. & Delorme, D. 2005: Big mothers invest more in daughters - reversed sex allocation in a weakly polygynous mammal. Ecology Letters 8, 430-437.
Karstad, E. L. & Hudson, R. J. 1986: Social organisation and communication of riverine hippopotami in southwestern Kenya. Mammalia 50, 153-163.
Laws, R. M. & Clough, G. 1966: Observation of reproduction in the hippopotamus (Hippopotamus amphibius). Symposia of the Zoological Society of London 15, 117-140.
Leimar, O. 1996: Life-history analysis of the Trivers and Willard sex-ratio problem. Behavioral Ecology 7, 316-325.
Linklater, W. L. 2007: Translocation reverses birth sex ratio bias depending on its timing during gestation: evidence for the action of two sex-allocation mechanisms. Reproduction Fertility and Development 19, 831-839.
Marshall, P. J. & Sayer, J. A. 1976: Population ecology and response to cropping of a hippopotamus population in eastern Zambia. Journal of Applied Ecology 13, 391-404.
Orliac, M., Boisserie, J. R., Maclatchy, L. & Lihoreau, F. 2010: Early Miocene hippopotamids (Cetartiodactyla) constrain the phylogenetic and spatiotemporal settings of hippopotamid origin. Proceedings of the National Academy of Sciences of the United States of America 107, 11871-11876.
Pluháček, J. 2013: European studbook for common hippopotamus Hippopotamus amphibius Linné, 1758, 7th edn. Ostrava Zoo, Ostrava.
Pluháček, J. & Steck, B. L. 2015: Different sex allocations in two related species: the case of the extant hippopotamus. Ethology 121, 462-471.
Roth, H. H., Hoppe-Dominik, B., Mühleneberg, M., Steinhauer-Burkart, B. & Fischer, F. 2004: Distribution and status of hippopotamids in the Ivory Coast. African Zoology 39, 211-224.
Saltz, D. & Kotler, B. P. 2003: Maternal age is a predominant determinant of progeny sex ratio variation in ungulates: a reply to Hewison et al. Oikos 101, 646-648.
Saragusty, J., Hermes, R., Hofer, H., Bouts, T., Göritz, F. & Hildebrandt, T. B. 2012: Male pygmy hippopotamus influence offspring sex ratio. Nature Communications 3.
Sheldon, B. C. & West, S. A. 2004: Maternal dominance, maternal condition, and offspring sex ratio in ungulate mammals. American Naturalist 163, 40-54.
Silk, J. B., Willoughby, E. & Brown, G. R. 2005: Maternal rank and local resource competition do not predict birth sex ratios in wild baboons. Proceedings of the Royal Society B-Biological Sciences 272, 859-864.
Smuts, G. L. & Whyte, I. C. 1981: Relationships between reproduction and environment in the hippopotamus in the Kruger National Park. Koedoe 24, 169-185.
Steck, B. 2013: Pygmy hippopotamus Choeropsis liberiensis (Morton, 1844) international studbook for the year 2012 19th edn. Basel Zoo, Basel.
Trivers, R. L. & Willard, D. E. 1973: Natural selection of parental ability to vary the sex ratio. Science 179, 90-92.
Zschokke, S. 2002: Distrored sex ratio at birth in the captive pygmy hippopotamus, Hexaprotodon liberiensis. Journal of Mammalogy 83, 674-681.

We studied the birth sex ratio in captive Common Hippopotamus (Hippopotamus amphibius) and Pygmy Hippopotamus (Choeropsis liberien­sis). In both species the birth sex ratio differed from 1 : 1. In Common Hippos more sons than daughters were born, whereas the opposite was found in Pygmy Hippos. Various factors affecting the birth sex ratio were analysed and discussed.