Data from: Aggressive behaviours track transitions in seasonal phenotypes of female Siberian hamsters

Seasonally breeding animals exhibit profound physiological and behavioural
responses to changes in ambient day length (photoperiod), including
changes in reproductive function and territorial aggression. Species where
aggression persists when gonads are regressed and circulating levels of
gonadal hormones are low, such as Siberian hamsters (Phodopus sungorus)
and song sparrows (Melospiza melodia), challenge the well-established
framework that gonadal hormones are important mediators of aggression. A
solution to this apparent paradox is that a season-specific increase in
sensitivity to hormones in brain areas associated with aggression offsets
low levels of gonadal hormones during periods of reproductive quiescence.
To test this hypothesis, we manipulated photoperiod to induce natural
fluctuations in seasonal phenotype across multiple stages of the annual
reproductive cycle in female Siberian hamsters that display increased
aggression during short-day reproductive quiescence, suggesting that
behaviour persists independent of gonadal steroids. Females were housed in
long “summer” days or short “winter” days for 10, 24 or 30 weeks to
capture gonadal regression, transition back to a reproductively functional
state and full gonadal recrudescence, respectively. Long-day animals
maintained reproductive functionality and displayed low aggression across
all time points. By week 10, short-day reproductively responsive females
underwent gonadal regression and displayed increased aggression;
non-responsive animals showed no such changes. At week 24, animals were in
a transitional period and displayed an intermediate phenotype with respect
to reproduction and aggression. By week 30, short-day females were fully
recrudesced and returned to long-day-like levels of aggression. Consistent
with our hypothesis, gonadally regressed females displayed decreases in
17β-oestradiol (oestradiol) levels, but site-specific increases in the
abundance of brain oestrogen receptor-alpha (ERα) in regions associated
with aggression, but not reproduction. Increased site-specific ERα may
function as a compensatory mechanism to allow increased responsiveness to
oestradiol in regulating aggression in lieu of high circulating
concentrations of hormones. Collectively, these results broaden our
understanding of how breeding phenology maps onto social behaviour and the
mechanisms that have evolved to coordinate behaviours that occur in
non-breeding contexts.