Sex Allocation in Hermaphrodites

Abstract

Organisms in which individuals can reproduce both as males and females are called hermaphrodites. Sex allocation in hermaphrodites involves the division of reproductive resources between the male and female function, and presents an interesting contrast to species that alter sex allocation by adjusting offspring sex ratio. Theoretical and empirical research in the past four decades have largely attempted to explain when hermaphroditism is favoured, and how sex allocation in hermaphrodites is controlled. Furthermore, the application of endocrinological methods has elucidated some of the molecular processes that underlie hermaphroditic sex allocation, especially in sequential hermaphrodites. Despite significant advances in our understanding of hermaphrodites, some fundamental predictions such as the adaptiveness of changes in sex allocation have not been tested, and some basic questions on hermaphroditism have not been answered.

Key concepts:

  • In hermaphroditic organisms, individuals are capable of reproducing as both males and females.

  • Simultaneous hermaphrodites have male and female reproductive organs that are functional at the same time, and are both involved in a mating event.

  • Simultaneous hermaphroditism is favoured, when an increase in male and female reproduction yields diminishing fitness returns.

  • Factors such as limited mobility, low density or self‐fertilization can result in diminishing fitness returns for males, and favour simultaneous hermaphroditism. However, local resource competition and physiological limitations could saturate fitness through female function.

  • Simultaneous hermaphrodites can adjust sex allocation according to the social environment (e.g. mating group size), and the amount of resources available for reproduction (e.g. body size).

  • Sequential hermaphrodites, or sex changers, reproduce as one sex for a part of their lifetime, and then switch to reproducing as the opposite sex.

  • Sequential hermaphroditism is favoured in mating systems where individuals have consistently greater reproductive success as one sex earlier and as the other sex later in life.

  • Sequential hermaphrodites can adjust the optimal timing of sex change according to relative size or social status in a mating group.

  • There are also mixed hermaphroditic systems where individuals can repeatedly switch between sexes, or simultaneous hermaphrodites coexist with pure sexes.

  • Sex change in fishes involves complex behavioural and morphological changes that are regulated by an endocrine gland system conserved across all vertebrates.

Keywords: sex allocation; hermaphroditism; reproduction; life history

Figure 1.

Male and female fitness as a function of male allocation (fitness gain curves). In each graph, the blue curve (M) and the red curve (F) give male and female fitness gain curves, respectively. The black arrow denotes the predicted optimal sex allocation that maximizes the total fitness of a hermaphrodite through male and female functions (H). (a) The male fitness curve is strongly saturating compared to the female fitness gain curve. A female‐biased allocation is predicted. (b) Male fitness curve saturates less strongly than (a). As a result, sex allocation is less biased towards the female function. Reproduced with permission from Baeza .

Figure 2.

The size advantage hypothesis that explains when sequential hermaphroditism is favoured. In each graph, the blue curve (M) and the red curve (F) give size‐specific fitness for males and females, respectively. The black arrow denotes the predicted size of sex change. (a) Male fitness increases more rapidly with size than female fitness. Female‐to‐male (protogynous) sex change is predicted. (b) Female fitness increases more rapidly with size than male fitness. Male‐to‐female (protandrous) sex change is predicted. Reprinted from Munday et al., (), with permission from Elsevier.

Figure 3.

A sketch of the hypothalamic‐pituitary‐gonadal axis. The hypothalamus produces GnRH, which induces the production of gonadotropins in the pituitary gland. Gonadotropins stimulate steroidogenesis in gonads, and consequently, the restructuring of gonads. Gonadal steroids may further interact with the hypothalamus and also trigger behavioural change (dashed lines) along with another hypothalamic product, AVT (see text for details). Reproduced with permission from Godwin et al..

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Warner RR and Munoz RC (2008) Needed: a dynamic approach to understand sex change. Animal Behaviour 75: e11–e14.

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Kazancioğlu, Erem(Dec 2009) Sex Allocation in Hermaphrodites. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0021911]