Reproductive cycles of the domestic bitch

Abstract

Domestic dogs are monoestrous, typically non-seasonal, polytocous, spontaneous ovulators and have a spontaneous luteal phase slightly longer (by approx 5 day) than the 64 ± 1 day luteal phases of a 65 ± 1 day pregnancy, a phase followed by an obligate anestrus before the next 2–3 week “heat” (proestrus-estrus). The resulting inter-estrus intervals of 5–12 months are variable among bitches, commonly 6–7 months, and range from highly variable to regular (to perhaps within ± 5–10 day of sequential 7 month cycle, for instance) within bitches, and across studies and do not vary significantly between pregnant and non-pregnant cycles. Hormone levels reported are those observed in this laboratory using previously reported assays and canine gonadotropin standards unless stated otherwise. Endocrine sequences for dog cycles are not unlike those of many other mammals, including selection of ovulatory follicles by increased LH pulsatility, the occurrence of estrus behavior and LH surge during a decline in the estrogen: progestin ratio, a pronounced preovulatory luteinization as in humans and rodents, and luteotrophic roles for both LH and prolactin. Non-pregnant bitches have a spontaneously prolonged luteal phase, often longer and with a more protracted decline in serum progesterone than in pregnancy as there is no uterine luteolytic mechanism. The obligate anestrus of 8–40 weeks is terminated by poorly understood interactions of environment (e.g. pheromones, possibly photoperiod) and a potential endogenous circannual cycle in sensitivities of hypothalamic dopaminergic, serotonergic and/or opioid pathways.

Introduction

Bitches are monoestrous, typically non-seasonal, polytocous, spontaneous ovulators and have a spontaneous luteal phase similar in length to or a bit longer than the 64 ± 1 day luteal phase of the 65 ± 1 days of pregnancy followed by an obligate anestrus before the next 2–3 week “heat” period (Table 1). Inter-estrus intervals of 5–12 months, typically 6–7 months, range from highly variable to regular within bitches, and averages do not vary significantly or consistently between pregnant and non-pregnant cycles. Pubertal estrus occurs variably at 6–14 months in most breeds, with means positively correlated with breed size. The canine cycle is classically divided into 4 phases (Evans and Cole, 1931)—a 5–20 day proestrus, 5–15 day estrus, 50–80 day metestrus (post-estrus portion of luteal phase), and anestrus typically lasting 80–240 days. These phases reflect, respectively, follicular phase rise in estrogen, the initial luteal phase rise in progesterone and decline in estrogen, the remainder of the luteal phase, and the interval between loss of luteal function and onset of next cycle. Timing within the 160–370 day cycle has been variably reported in days post proestrus onset, estrus onset, metestrus onset, preovulatory LH peak or LH surge. The latter is used as day 0 in this review (Fig. 1) unless otherwise noted. Levels of hormones are primarily those observed in the author's laboratory using previously reported assays and canine gonadotropin standards.

This review is based on reports and reviews from this laboratory (Concannon, 1989, Concannon, 2009, Concannon et al., 1975, Concannon et al., 1979, Concannon et al., 1980, Concannon et al., 1997a, Concannon et al., 1997b, Concannon et al., 2001a, Concannon et al., 2001b, Concannon et al., 2006a, Concannon et al., 2009) and from, among others, researchers in Osaka (Hatoya et al., 2003); Tsutsui et al., 2007, Tsutsui et al., 2009 in Tokyo; Hoffmann and Kowalewski and colleagues in Giessen and Zurich (Kowalewski et al., 2007); Verstegen, Onclin and colleagues (Verstegen et al., 1999) in Gainesville; England et al., 2009a, England et al., 2009b in Nottingham; Okkens, Kooistra, Rijnberk and colleagues in Utrecht (de Gier et al., 2008); Gobello and colleagues, Wildt and colleagues (Wildt et al., 1981), researchers at Ghent (Van Cruchten et al., 2004), Fontbonne, Reynaud, and colleagues at Alfort (Reynaud et al., 2006), and Nett, Olson and colleagues at Colorado State (Fernandes et al., 1987), as well as reports in ISCFR symposia Proceedings (see Concannon et al., 1989, Concannon et al., 1993, Concannon et al., 1997a, Concannon et al., 2001a, Concannon et al., 2001b, Concannon et al., 2006b, England et al., 2009a). Important classical texts include Evans and Cole (1931), and Andersen and Simpson (1973).