Sex differences in the onset of seasonal reproductive quiescence in hamsters

Abstract

Day length is the primary cue used by many mammals to restrict reproduction to favourable spring and summer months, but it is unknown for any mammal whether the seasonal loss of fertility begins at the same time and occurs at the same rate in females and males; nor it established whether the termination of mating behaviour in males and females coincides with the loss of fertility. We speculated that females, owing to their greater energetic investment in reproduction, are the limiting sex in terminating offspring production in short days (SDs). Oestrous cycles and production of young were monitored in Syrian hamsters (Mesocricetus auratus) transferred from long days (LDs) to SDs. Females were mated to LD males after three to eight weeks of SD treatment; in a parallel experiment, males housed in SDs were mated to LD females. After five and eight weeks in SDs, at least twice as many males as females were fertile. Both males and females continued to copulate for several weeks after becoming infertile. The onset of seasonal infertility occurs earlier in females than males and the decline in fertility precedes the seasonal loss of mating behaviour in both sexes.

Figure 1

Fertility of SD Syrian hamsters mated to LD partners. (a) Female fertility: filled circles, per cent of females that mated during short day treatment. Filled squares, per cent of females that produced litters. (b) Male fertility: closed circles, per cent of males that mated in SD. Closed squares, per cent of males that sired offspring. LD controls are shown as 0 weeks in SD. Significance of difference between SD treatment and LD controls is indicated as ^**p<0.01, ^***p<0.001.

Figure 2

Relation between siring at week 8 or 10 after transfer to SD and estimated testis volume. Males that did not sire litters had significantly smaller testes at the end of week 10 (^**p<0.005). No male with ETV<1500 (n=11) sired young, and every male with ETV>1500 (n=5) sired offspring.

Figure 3

Fertility decline in males and females. At weeks 5–8, both mating and fertility were significantly lower in females than males (^*: each p<0.05, FET).

Figure 4

Relation between mating, fertility, and cyclicity in females. The white portion of each bar represents hamsters that did not mate, the light grey portion those that mated but did not litter, and the dark grey portion those that mated and littered. Open circles denote individuals who had regular 4-day cycles; filled circles females who missed one or more cycles. Cyclic and acyclic females appear in each category.