Hormones and Reproductive Cycles in Anuran Amphibians

Anuran amphibians display a variety of reproductive modes including, in one genus, a non-placental viviparous species. In frogs and toads, reproduction is hormonally controlled and, in most species, environmentally mediated. The reproductive cycle includes several components: oogenesis and spermatogenesis, including cell maturation and vitellogenesis; courtship and mating; spermiation and oviposition; and the timing of these events in terms of seasonality or its absence. These topics are briefly summarized. In many species, breeding is synchronized within the population, whereas in others it is not. Some species of AQ1 anuran have predictable breeding season, whereas others are more flexible and some breed opportunistically in response to unpredictable humidity availability. Temperature is the principal environmental cue used to time and tune the pre-breeding stages of the reproductive cycle, allowing anurans to adapt their physiology in advance of predictable environmental changes. Endocrine and neuroendocrine plasticity allows non-thermal cues (photoperiod, food availability) to modulate timing to enable individuals to reproduce. Circumstantial evidence indicates that GnRH from the hypothalamus exerts its regulatory functions on the pituitary gonadotropins via blood circulation. The gonadotropins are responsible for the seasonal and/or annual developmental changes in gametogenesis and in the production of sex steroids that drive reproductive organ function, culminating in reproduction. There are many areas of uncertainly, and future integrated studies are required.