Can relaxed time constraints on sperm production eliminate protandry in an ectotherm?

Abstract

In most temperate-zone reptiles, as in many other ectothermic taxa, males emerge from periods of inactivity (e.g. hibernation) before females, a pattern referred to as protandry. In the large body of theory depicting its evolution it is assumed that as selection pressures on females moderate female emergence time, net sexual selection on males shifts male emergence time accordingly. This is because early-emerging males might (i) benefit from advantages in behavioural interactions (e.g. obtain more matings, better territories, or a higher social status), or (ii) produce and mature more spermatozoa before the mating season. These putative advantages of early emergence occur simultaneously in most temperate-zone reptile species, because sperm production and copulations occur soon after emergence from winter inactivity. Thus, it is difficult to distinguish between the two hypotheses. To do so would require a system in which behavioural interactions occur at one time of year and sperm production at another. The southern snow skink (Niveoscincus microlepidotum) provides such an example; adult males fight with each other immediately after emerging from hibernation, but examination of the testes and epididymes of males throughout the year show that males do not produce spermatozoa until around 3 months later. Thus, the ‘behavioural interactions’ hypothesis predicts protandry in snow skinks, whereas the ‘sperm production’ hypothesis predicts synchronous spring emergence in males and females. Our field data, collected over 5 years, show that emergence dates in spring are the same for male and female skinks. Hence, when the selection pressure for rapid sperm production is relaxed, we no longer see the protandric emergence pattern characteristic of most temperate-zone reptiles.