Quality of seminal fluids varies with type of stimulus at ejaculation

Abstract

The theory of ejaculate economics was mainly built around different sperm competition scenarios but also predicts that investments into ejaculates depend on female fecundity. Previous tests of this prediction focused on invertebrates and lower vertebrate, and on species with high female reproductive potential. It remains unclear whether the prediction also holds for polygynous mammals with low female reproductive potential (due to low litter size and long inter-birth intervals). We used horses (Equus caballus) to experimentally test whether semen characteristics are adjusted to the oestrous cycle of the mare a stallion is exposed to during few moments before ejaculation. We analysed 122 weekly semen samples collected from 16 stallions during exposure to either an oestrous or a dioestrous mare. Semen volume and the rate of motile sperm were higher when stallions were exposed to an oestrous than to a diestrous mare, while total sperm counts and sperm velocity remained unchanged. Sperm collected after exposure to an oestrous mare also showed reduced oxidative degeneration of cell membranes over a period of 48 hours. We conclude that stallions invest more into their seminal fluids when the chance of fertilization is elevated, and that this adjustment of ejaculate quality can happen very quickly.

Stallion semen quality depends on major histocompatibility complex matching to teaser mare

“…Because smell of faeces seems to be an important source of social information (Krueger & Flauger, ), the stallions were first exposed for 15 s to fresh faeces of the teaser mare (presented on a plate on the ground), then kept for about 30 s in front of the respective mare's head, at a distance of about 2 m to prevent physical contact, and then led to the dummy that the stallion could mount and that allowed to collect its semen in an artificial vagina [type “Avenches”; see Jeannerat et al. () for further details; extra‐gonadal sperm reserves were standardized in seven daily semen collections followed by weekly collections every Tuesday starting 4 months before the first experimental semen collection]. The stallions were immediately returned to their box after semen collection.…”

“…An aliquot of semen was cooled to 4°C and analysed for velocity, plasma membrane and acrosome integrity, membrane lipid peroxidation and DNA integrity of sperm at 24 and 48 hr after ejaculation as described in detail in Jeannerat et al. (). Briefly, sperm motility was determined by CASA according to the methods used on fresh ejaculates, and all other sperm parameters were analysed using flow cytometry (Cell Lab Quanta SC MPL, Beckman Couler Inc., Nyon, Switzerland).…”

“…The mares were called in oestrus when they showed typical oestrous behaviour, with a follicle size of >35 mm and a uterus oedema at stage >2 (Ginther, ). They were then given 1,500 IU hCG intravenously (Chorulon ® ; MSD Animal Health GmbH, Luzern, Switzerland) on the evening before the experimental exposure to the stallion in order to induce ovulation the next day for a parallel study on potential effects of oestrus on male behaviour (Jeannerat et al., ) (this parallel study included six further stallions whose MHC types did not allow for the present within‐subject comparison because both mares were MHC similar to these stallions). For each combination of mare and stallion, the mare was exactly once in oestrus and either 2 or 3 times in dioestrus when exposed to the stallion at semen collection, that is, the experiment was factorial with respect to oestrus × MHC sharing (Figure ).…”

“…All statistical analyses were within‐subjects, controlling for the strong differences between stallions that are described in Jeannerat et al. (). We therefore used mixed‐effect models and report only unbounded REML variance component estimates (Kackar‐Harville corrected based on standard least squares in JMP ® 11.2), with MHC similarity of the mare or relatedness r and, in the case of cold‐stored sperm, also time since ejaculation as fixed factors, and with stallion ID and all interaction that include stallion ID as random terms.…”

“…Females attractiveness can vary, for example, in egg number and egg quality in species with high female reproductive potential (including most invertebrates and lower vertebrates), with their level of promiscuity (Pizzari, Cornwallis, Lovlie, Jakobsson, & Birkhead, ), with sexual ornaments (Cornwallis & Birkhead, ; Pizzari et al., ), or with oestrous stage, especially in large mammals with comparatively low female reproductive potential (Jeannerat, Janett, Sieme, Wedekind, & Burger, ). Apart from such generally valid quality traits, some aspects of female attractiveness are predicted to be differently perceived by different males (Wedekind, ).…”