Personality and gonadal development as sources of individual variation in response to GnRH challenge in female great tits

Abstract

Seasonal timing of reproduction is a key life-history trait, but we know little about the mechanisms underlying individual variation in female endocrine profiles associated with reproduction. In birds, 17b-oestradiol is a key reproductive hormone that links brain neuroendocrine mechanisms, involved in information processing and decision-making, to downstream mechanisms in the liver, where egg-yolk is produced. Here, we test, using a simulated induction of the reproductive system through a Gonadotropin-Releasing Hormone (GnRH) challenge, whether the ovary of pre-breeding female great tits responds to brain stimulation by increasing oestradiol. We also assess how this response is modified by individual-specific traits like age, ovarian follicle size, and personality, using females from lines artificially selected for divergent levels of exploratory behaviour. We show that a GnRH injection leads to a rapid increase in circulating concentrations of oestradiol, but responses varied among individuals. Females with more developed ovarian follicles showed stronger responses and females from lines selected for fast exploratory behaviour showed stronger increases compared to females from the slow line, indicating a heritable component. This study shows that the response of the ovary to reproductive stimulation from the brain greatly varies among individuals and that this variation can be attributed to several commonly measured individual traits, which sheds light on the mechanisms shaping heritable endocrine phenotypes.

Olfactory detection of trace amounts of plant volatiles is correlated with testosterone in a passerine bird

“…We found no correlation between estradiol and time spent in the infested arm in female 470 blue tits. Estradiol does respond to GnRH within 30 min in female great tits (Parus major) and 471 dark-eyed juncos (Junco hyemalis) (Caro et al, 2019;Needham et al, 2019), suggesting we 472 would see similar results to testosterone in male blue tits, if perception of a reproductively-relevant 473 stimulus like HIPVs can indeed induce a surge in sex-steroid hormones. Yet female birds control timing of reproduction more closely than males (Caro et al, 2009).…”

“…Plasma testosterone was measured using a commercially available enzyme immunoassay kit on a single plate (Enzo Life Sciences, ADI-900-065). Testosterone was 278 extracted from 35 μL of plasma using solid phase extraction with C18 columns (100 mg C18 279 material, Sep-Pak Vac 1cc, Waters, Milford, MA, USA), dried under nitrogen gas at 40 o C, and 280 reconstituted overnight with 250 μL assay buffer (Caro et al, 2019). Concentration was adjusted 281 for samples that did not have 35 μL of plasma available (n = 9 of 22).…”

“…We validated the assay in blue tits following the methods of (Caro et al, 2019). Briefly, 290 pooled plasma samples were spiked with exogenous testosterone and serially diluted.…”

“…No sample was found to be below the detection 311 limit. The assay procedure has previously been validated for blue and great tits (Caro et al, 2019). 312…”

Induction of reproductive activity and egg production by gonadotropin‐releasing hormone in non‐laying hens

“…Non‐laying hens are a concern in egg production farms. Furthermore, the ovary's response to reproductive stimuli from the brain differs significantly between individuals, shedding light on the processes inducing heritable endocrine phenotypes (Caro et al., 2019). It was important to find a solution to this issue, which is caused by these females' lack of ovarian activity.…”