Reproductive Inactivity Research Articles

Lifetime trajectories of male mating effort under reproductive conflict in a cooperatively breeding mammal.

The costs of reproductive conflict can shape the evolution of life-histories in animal societies. These costs may change as individuals age and grow, and with within-group competition. Social costs of reproductive conflict have been invoked to explain why females might gain from delaying maturity or ceasing reproduction midway through life, but not in males. Here, we analyse more than 20 years of data to understand how individual male banded mongooses adjust their reproductive activity in response to the costs of reproductive conflict. In banded mongoose groups, multiple female breeders enter oestrus synchronously that are each guarded by a single male that aggressively wards-off rivals. The heaviest males in the group gained the greatest share of paternity. Those lighter males that are reproductively active paid disproportionate survival costs, and by engaging in reproductive activity early had lower lifetime reproductive success. Our results suggest that reproductive inactivity early in life is adaptive, as males recoup any lost fitness by first growing before engaging in less costly and more profitable reproductive activity later in life. These results suggest that resource holding potential of males and the intensity of reproductive conflict interact to shape lifetime schedules of reproductive behaviour.

On the reproductive strategies post-colony foundation: major termite pest species with distinct ecological habits differ in their oviposition dynamics.

Termite colony foundation precedes the incipient stage, when the first oviposition cycle takes place, followed by months of reproductive inactivity. The royal couple is supposed to cease oviposition during this period, investing energy to care for the first brood. When a suitable number of alloparents differentiate, egg-laying resumes. Here we followed oviposition dynamics, embryo development and queen/king body changes in laboratory colonies of the major pest species Coptotermes gestroi (Rhinotermitidae) and Cryptotermes brevis (Kalotermitidae) during 9 months. We show that they differ in these oviposition dynamics, as C. gestroi queens displayed an uninterrupted oviposition whereas C. brevis laid a cohort of eggs and ceased oviposition during a 3-month period (lag phase). C. gestroi oviposition dynamic was remarkable and suggests that occurrence of progeny was not a limiting factor, thus queens and kings were able to concomitantly invest energy in reproduction and parental care. These findings contrast those reported for rhinotermitids from temperate areas, and we discuss the likely reasons for such a condition, including endogenous rhythms, avoidance of a high mortality rate of the first progeny and adaptation to the weather conditions of the Neotropical region. Oviposition dynamic in C. brevis resembled those of several termite species, in which the royal couple cease reproduction to care for the first brood. Rearing conditions did not influence oviposition dynamics (egg-laying cycle followed by a lag phase), thus our results on the oviposition of C. gestroi and C. brevis correspond to different reproductive strategies post-foundation adopted by these pest species.

Comparative Characterization of the Cellular Composition of the Black Scorpionfish (Scorpaena porcus L.) Hematopoietic Organs during the Spawning Season and the Period of Reproductive Inactivity

Hematopoiesis in teleosts has a number of characteristics that are not fully understood. In the present work, the cellular composition of the hematopoietic organs (head kidney and spleen) of the black scorpionfish during the spawning season and the period of reproductive inactivity was studied using light microscopy. The morphology and the percentage of blood cells were described. The head kidney was shown to be the main hematopoietic organ of the black scorpionfish: immature blood cells of all hematopoietic lines at the different stages of differentiation were observed there. They were divided into 3 clusters depending on the average cell diameter. Lymphocytes, thrombocytes and colony-forming cells, the precursors for all types of blood cells, were observed within the cluster of small cells. The intermediate-size cluster comprised blast forms (erythroblasts and blasts of white blood cells). The large-size cluster consisted of maturing granulocytes, monocytes, macrophages, plasma cells, and mature erythrocytes. The spleen mainly contained mature erythrocytes, showing signs of senescence, and erythrocyte ghosts. Therefore, it was concluded that the spleen of the black scorpionfish performs the function of depositing and utilizing erythrocytes. The study also demonstrated the seasonal dynamics of hematopoiesis. The increase in the number of erythroblasts was recorded in the head kidney of spawning individuals. Erythroblasts were also found in the spleen, in spite of their total absence in the reproductively inactive fish. Consequently, the spleen of the black scorpionfish is an organ of secondary erythropoiesis, which functions when the hematopoietic capacity of the kidneys is insufficient

Reproductive morphophysiology of the male scorpion mud turtle (Kinosternon scorpioides Linnaeus, 1766) in captivity

Kinosternon scorpioides, popularly known as scorpion mud turtle (jurará in Brazil), is a fresh water species. There is little information about its reproduction and the present study aims to morphologically characterize the reproductive organs of male K.scorpioides bred in captivity in two seasons of the year. The reproductive tracts of adult animals under went macroscopic ultrastructural analysis of the lumen, as well as scanning electron and transmission microscopy. Macroscopically, the male genital organs consist of a pair of testicles, epididymis, the vas deferens and a penis. Testicles, epididymis and deferents ducts were characterized by reproductive activity during the rainy season and reproductive inactivity in the dry period. The morphometry regarding the tubular and luminal diameter and epithelial height of the testicles, epididymis and deferents ducts showed changes along the studied periods. The rainy season presented higher averages than the dry period. The penis did not show any changes during both periods. It was concluded that K. scorpioides exhibits reproductive seasonality.

Low birthrates and high levels of female reproductive inactivity may characterize the reproductive biology of wild Peruvian red uakaris (Cacajao calvus ucayalii).

Birthrates are key parameters for population models and hunting sustainability analyses frequently used in conservation, but for many rare species, these data do not exist. We examine the reproductive organs of endangered red uakari monkeys to calculate birthrates in the wild. We collected reproductive organs from wild uakari monkeys hunted for subsistence by indigenous hunters and examined them for embryos or fetuses. We extrapolated birth dates to test for breeding seasonality and calculated birthrates. Breeding was seasonal, and birthrates were low relative to other neotropical primates. We recorded unexpectedly high numbers of reproductively inactive females compared to other neotropical monkeys. Reproductive inactivity could be due to delayed reproduction or long periods of lactation. Resource availability may also play a role. Slow reproduction and low birthrates in uakaris, relative other primates, could explain why uakaris have a patchy distribution and appear vulnerable to disturbance.

Life-history and activity shape catchability in a sedentary fish

Catchability, a key parameter in stock assessment, is often considered constant in time and space. However, when fishing with passive gears like traps or gillnets, fish behavior determines the odds of encounter with the fishers and thus catchability. Few studies have presented comprehensive empirical evidence of the link between behavior measured in the wild and catchability from a life-history strategy perspective. Here, a suite of different variables, including environmental cues, physiological states, fish activity, home range and catchability, were modeled using a sinusoidal function to describe their seasonality over the year and the degree of coupling among them in a sedentary coastal fish (Labrus bergylta). All the variables except the home range size showed a significant variation over the year, following a sinusoidal pattern. The models showed a tight match between the seasonality of catchability and fish activity, with high values of both variables occurring in late spring to early summer, when the highest levels of feeding and the period of reproductive inactivity occurred. Lower catchability values were predicted in late autumn to early winter, coinciding with the spawning season and the associated reduced activity. This integrative research shows that the spatio-temporal dimension of fish life-history strategy has a key role in shaping catchability even in highly sedentary species. Timevarying catchability needs to be incorporated into stock assessment models that aim to accurately describe fish population health and to estimate abundance indices.

Is the Santa Inês sheep a typical non-seasonal breeder in the Brazilian Southeast?

This study aimed to characterize the annual reproductive cycle of Santa Inês sheep in the Fluminense lowland region (latitude 22° 27' 45″ south, Rio de Janeiro, Brazil) between September 2011 and August 2012. Ten ewes were maintained in a semi-intensive system under natural photoperiods with access to pasture and shelter. Blood samples were collected every 2 weeks to determine plasma progesterone concentrations. The body condition score (BCS) was determined each month. There was no seasonal variation in the plasma progesterone concentration from the months of September to January, April, and May to August. In the months of February and April, the plasma hormone levels were higher than August to November. Seventy percent (7/10) of the sheep studied had short seasonal anestrus. The periods of anestrus were concentrated between the months of September and December (spring season) in 85.7 % (6/7) of the cases evaluated. In these cases, 57.1 % (4/7) also had short periods of reproductive inactivity during other months of the year. The progesterone values obtained in the spring corroborate the higher reproductive anestrus observed in this season. Higher plasma progesterone values were found in summer and autumn with reduction in the winter to lower values in the spring. No changes in the BCS during the study period were observed. Under the studied conditions, the Santa Inês sheep showed a low degree of reproductive seasonality. However, some individual ewes had seasonal anestrus during the spring. Further studies that include management techniques are needed to improve reproductive efficiency without hormone therapy in this breed under tropical conditions.

The effect of caste and reproductive state on the chemistry of the cephalic labial glands secretion of Bombus terrestris.

The cephalic labial glands are well developed in many bee species. In bumble bee males, they cover most of the head volume, and their secretion is used in marking reproductive territories and attracting virgin queens. In females, however, they are poorly studied. Here, we present chemical analyses of their secretion in queens and workers of Bombus terrestris under various social conditions. The secretion revealed a context-dependent composition with sterile females possessing large amounts of fatty acid dodecyl esters, ranging from dodecyl hexanoate to dodecyl oleate, compared to small amounts in fertile females. Significant reduction in the dodecyl esters also was found in queens at the competition phase, where worker reproduction, aggression, and gyne differentiation occur. The exclusive production of esters by sterile individuals also is typical of Dufour's gland secretion in this species, albeit in this case these are octyl rather than dodecyl esters, and the differences between sterile and fertile individuals are qualitative rather than quantitative. We propose that the labial gland esters provide yet another signal of reproductive inactivity. In virgin queens, it may signal that egg laying is deferred to the next season, while in workers it reinforces the message "I am sterile and out of the reproductive competition". Whether the reduction in dodecyl esters in fertile queens as a function of colony social development is involved in regulating gyne production and the onset of the competition phase is yet to be deciphered.

Reproductive Strategy in a Littoral Population of the Cockle CerastodermaGlaucumfrom the Gulf of Gabès Area (Southeastern Tunisia)

Gametogenic activity of the cockle Cerastoderma glaucum was studied monthly in a littoral population collected from the Gulf of Gabes area. The analysis of oocyte diameter and histological examination of gonads indicates that C. glaucum showed a defined pattern of seasonal gametogenic development. An intense reproductive activity was observed throughout the year, mainly during the autumn and winter seasons. Spawning occurred most of the year, without prolonged periods of reproductive inactivity. Histological observations showed strong interindividual variations of gonad maturity stages over the same sampling period. The study also revealed spawning asynchrony in this species and the presence of various stages of maturity within the same individual. This strategy allows for a semicontinuous production of larvae in a way that some of the larvae will encounter favorable environmental condition over time, which favors their subsequent development. The current study also reveals an occasional manifestation of hermaphroditism.

The sexual segment of hemidactylus turcicus and the evolution of sexual segment location in squamata

The kidneys of the Mediterranean Gecko, Hemidactylus turcicus (Gekkonidae), were investigated using light and electron microscopy with the primary focus placed on morphology of the sexual segment of the kidney. The nephrons of male H. turcicus are composed of five distinct regions: 1) a renal corpuscle and glomerulus, 2) a proximal convoluted tubule, 3) an intermediate segment, 4) a distal convoluted tubule, and 5) the sexual segment of the kidney/collecting duct. Female H. turcicus is similar but lack a sexual segment of the kidney. The sexual segment of the kidney is hypertrophied during the months of March through August, which corroborates previous reports of reproductive activity. During inactive months, the sexual segment of the kidney is nondiscernable from the collecting ducts. The sexual segment consists of tall columnar epithelial cells with basally positioned nuclei. Perinuclear Golgi complexes and rough endoplasmic reticulum are present. Secretory granules, which fill the apices of the epithelial cells, are electron dense and released into the lumen by a merocrine secretory process. Narrow intercellular canaliculi separate each epithelial cell and are sealed by tight junctions at the luminal aspect. Basally, leukoctyes are observed within the intercellular canaliculi and outside the basal lamina. Mast cells can be found just outside the basal lamina in close association with renal capillaries. The sexual segment of the kidney of H. turcicus is similar to that of three unrelated lizards for which ultrastructure was investigated with secretion mode being the major difference Also, H. turcicus is similar to most other lizards in that complete regression occurs during reproductive inactivity, but differs in this trait from the skink, Scincella lateralis, and most snakes which display a hypertrophied sexual segment of the kidney throughout the entire year. Although some unique similarities appear during the optimization, no direct patterns or directions are observed, and only the molecular based phylogeny resolves the ancestral condition of the Squamata as the sexual segment of the kidney being observed in the distal convoluted tubule, collecting duct, and ureter.

Testicular melanization has evolved in birds with high mtDNA mutation rates

Melanin is mainly found in the integument of animals, but it also appears in several extracutaneous tissues. The presence of melanin in testes has been anecdotally reported in all vertebrate groups, but the causes and functions of this melanin remain unknown. Similar to other extracutaneous melanins, testicular melanin may protect male germ cells from oxidative stress. Given the high respiratory activity of spermatozoa, oxidative stress generated by mitochondrial dysfunction as a consequence of mtDNA mutations directly affects sperm viability. Thus, natural selection may favour testicular melanization in males of species with high historical mutation rates in the mitochondrial genome. Here, we tested this hypothesis using information on occurrence of testicular melanization and mutation accumulation as reflected by cytochrome b mtDNA base pair substitution rates in a large set of 134 species of birds, controlling for the confounding effects of body mass, reproductive activity and phylogeny. We found that testicular melanization has evolved in species with high rates of accumulated mitochondrial mutations and propose that this is an adaptive response related to the protective capacity of melanin against oxidative stress. In support of this hypothesis, testicular melanization was more frequently observed during the breeding season of birds (i.e. when spermatogenesis is likely to occur) than during reproductive inactivity. In contrast to other extracutaneous melanins whose abundance seems to reflect skin and coat colour, we did not find a correlation between the proportion of plumage coloured by melanins and occurrence of testicular melanization. Whereas future experimental studies should test these hypotheses, our study highlights for the first time that melanization patterns in animals may evolve as a response to historical mutation rates.

Peripheral and behavioral plasticity of pheromone response and its hormonal control in a long-lived moth

Reproductive success in many animals depends on the efficient production of and response to sexual signals. In insects, plasticity in sexual communication is predicted in species that experience periods of reproductive inactivity when environmental conditions are unsuitable for reproduction. Here, we study a long-lived moth Caloptilia fraxinella (Ely) (Lepidoptera: Gracillariidae) that is reproductively inactive from eclosion in summer until the following spring. Male sex pheromone responsiveness is plastic and corresponds with female receptivity. Pheromone response plasticity has not been studied in a moth with an extended period of reproductive inactivity. In this study, we ask whether male antennal response and flight behavior are plastic during different stages of reproductive inactivity and whether these responses are regulated by juvenile hormone. Antennal response to the pheromone blend is significantly reduced in reproductively inactive males tested in the summer and autumn as compared with reproductively active males tested in the spring. Reproductively inactive autumn but not summer males show lower antennal responses to individual pheromone components compared with spring males. Treatment with methoprene enhances antennal response of autumn but not summer males to high doses of the pheromone blend. Behavioral response is induced by methoprene treatment in males treated in the autumn but not in the summer. Plasticity of pheromone response in C. fraxinella is regulated, at least in part, by the peripheral nervous system. Antennal and behavioral response to pheromone differed in reproductively active and inactive males and increased with methoprene treatment of inactive males.

Plasticity of male response to sex pheromone depends on physiological state in a long-lived moth

Plasticity of response to sexual signals is predicted to occur in long-lived animals that have periods of reproductive inactivity. Response should be adaptive to avoid wasted energy mate searching when conditions are unfavourable. Here we test that male response to female-produced sex pheromone in a long-lived moth species, Caloptilia fraxinella, is plastic and depends on male physiological state. Electrophysiological and behavioural studies indicated that the sex pheromone is composed of two compounds: (Z)-11-hexadecenal and (Z)-11-hexadecen-1-ol. Experiments were conducted to examine how males that differed in physiological state responded to signals of varying quality and quantity produced by manipulating synthetic pheromone blends. Reproductively active males had a lower pheromone response threshold and showed an upper threshold compared to males in reproductive diapause, which responded only to high pheromone doses with no upper threshold. The nature of pheromone response also varied depending on the male's physiological state. Reproductively active males showed a more narrowly tuned response than males in diapause. However, reproductively active males did not discriminate among pheromones with (Z)-11-hexadecenal and 1–50% of (Z)-11-hexadecen-1-ol. Males in diapause were not attracted to pheromone composed of (Z)-11-hexadecenal and 1% (Z)-11-hexadecen-1-ol, showing reduced signal perception in moths in diapause. Taken together, these results indicate that male C. fraxinella pheromone response is plastic with respect to threshold and specificity and depends on physiological state of the responder.

Reproductive Inactivity and Prolonged Developmental Time Induced by Seasonal Decline in Host Plant Quality in the Willow Leaf BeetlePlagiodera versicolora(Coleoptera: Chrysomelidae)

Plagiodera versicolora Laicharting (Coleoptera: Chrysomelidae) stops reproduction in mid-August at Ishikari, Hokkaido, Japan, despite the fact that the degree-days required for complete development predict an additional generation late in the season. We examined whether the life cycle of P. versicolora was constrained by the seasonal decline in quality of its host plant, Salix sachalinensis Fr. Schmidt (Salicaciae). P. versicolora was maintained for three successive generations throughout a season under constant laboratory conditions and fed host leaves obtained from the field site. Larval and adult performance declined in the generations that were produced later in the season. Developmental time in the last generation was significantly longer than in the first generation, although larval survivorship did not differ significantly between generations. Preoviposition periods were significantly longer and the number of eggs laid from the first oviposition to the 10th day of oviposition was significantly fewer in the last two generations compared with the first generation. In addition, reproductively inactive females that laid no eggs were significantly more frequent in the last two generations. This reproductive inactivity in female adults in late generations may be caused by diapause, which can prevent P. versicolora from producing an additional generation on poor-quality host plants. However, the physiological condition of the reproductively inactive females was different from that induced by short photoperiod in diapausing females because feeding was observed in the former females but not in the latter ones.

Variación de los índices gonádico, de rendimiento muscular y de la glándula digestiva de Spondylus princeps (Gray, 1825) (Mollusca: Bivalvia) en Isla Cedros y Punta Eugenia, México

The variation of the gonadal, muscular yield and digestive gland indexes was studied in Spondylus princeps from Punta Eugenia and Isla Cedros, Mexico. Specimens were collected from March to July 1997 and from August to December 2003. Gonadal indexes from 2% to 4% suggest reproductive inactivity from March to May and from October to December. The sustained increase in the index from May to July suggests gonadal maturity (10% of the soft tissues weight in July); the index decreased from 10% to 2% from July to October, corresponding with the spawning period. The muscular yield index showed high values (45-50%) from March to June and diminished sustainedly until August, when the lowest value (37%) was attained. Muscular yield index recovered during September and October with values of 45% staying around 40% during November and December. The muscular yield suggests that during the gonadal maturity, the energy stored in the abductor muscle is not used until the spawning season. The digestive gland index showed the highest value in March and decreased lightly until June, indicating that the energetic substances stored in the digestive gland sustain together with the external food, the gonadal development from July up to September when a fast decrease of the index occurs, suggesting that reserve substances in the digestive gland and in the muscle are used during the spawning period.

Reproductive cycle of Argopecten purpuratus (Bivalvia: Pectinidae) in La Rinconada marine reserve (Antofagasta, Chile): Response to environmental effects of El Niño and La Niña

Gonadosomatic index (GSI) and ovarian histology were used to determine the seasonality, amplitude, and magnitude of spawnings of the Argopecten purpuratus population within the marine reserve at La Rinconada, Chile, between December 1995 and February 2000. In general, spawnings occurred most of the year without prolonged periods of reproductive inactivity. Studies of this population throughout warm and cold oceanographic phenomena such as the El Niño-Southern Oscillation (ENSO) event of 1997–1998 and the La Niña event of 1998–2000 showed that the reproduction of this population occurred mainly between September and April, declining in the winter months from June through August, during which a period of sexual quiescence was observed in some individuals. An intense reproductive activity in the population, following the 1997–1998 warmer period, extended over the whole year. Histological analysis of the female portion of the gonad confirmed the GSI results and demonstrated the presence both of acini which appeared to be mature and other acini with various degrees of atresia and/or oocyte lysis. The study also revealed spawning asynchrony in this species and the presence of various stages of maturity within the same individual. Periods of high reproductive activity were correlated with strong diurnal temperature fluctuations and rises in phaeopigment concentration and phytoplankton cell counts, suggesting that these factors could be triggers for Argopecten spawning. These new insights on the reproductive cycle and conditioning of A. purpuratus would facilitate future management practices on the timing of artificial collectors and spat collection.

Reproductive status influences the survival of new cells in the dentate gyrus of adult male meadow voles

Reproductive status influences cell proliferation and the survival of new cells in the dentate gyrus of adult laboratory-reared and wild female meadow voles; reproductively inactive (RI) females have more proliferating cells and more labeled cells that survive 5 weeks vs. 2 h than reproductively active (RA) females. However, the effect of season has only been studied in a wild sample of male meadow voles and factors such as age and experience that have been shown to influence neurogenesis in the dentate gyrus of adult mammals cannot be controlled in a wild sample. Therefore, we investigated whether reproductive status regulates neurogenesis (cell proliferation and/or the survival of new cells) in the dentate gyrus of laboratory-reared adult male meadow voles so that confounding variables could be controlled. Males were acclimated to a short- or a long-photoperiod to simulate the non-breeding or breeding season, respectively, and reproductive status was verified by testes mass. The density of labeled cells was similar in RI and RA males ( P=0.20) 2 h after an injection of bromodeoxyuridine (50 mg/kg) but the density of labeled cells was elevated in the RA males relative to the RI males 5 weeks after an injection of [ 3H]thymidine (5 μCi/g; P≤0.04). The results suggest that reproductive status regulates the survival of new cells within the dentate gyrus of adult male meadow voles but not cell proliferation.

Non-invasive assessment of oestrous cycles and evaluation of reproductive seasonality in the female wild black rhinoceros (Diceros bicornis minor).

Female wild black rhinoceroses in Zimbabwe were monitored non-invasively using faecal progesterone metabolite analysis and observation of reproductive behaviour. A postpartum period of reproductive inactivity of at least 4 months, followed by a period of 4-7 months of oestrous cyclicity, was detected in six multiparous females. Three-quarters of the oestrous cycles (n = 21) had a total duration (mean +/- SEM) of 26.8 +/- 1 days. Other types of cycle were characterized either by an extended luteal phase, lasting on average twice as long as the normal cycle, or by an extended follicular phase. These extended cycles may have resulted from early embryo loss and heat stress. Female rhinoceroses did not conceive before 8 months after giving birth and some females (n = 2) most likely aborted after 3.0-3.5 months of gestation. The detected period of cyclic oestrus occurred between May and March in females (n = 9), and there was a 3 month extended interoestrous interval in nulliparous females during the period of decreasing daylengths that can be presumed to be the period of poorest fertility for the black rhinoceros under tropical latitudes. In contrast, the period of optimum fertility in the Southern hemisphere coincided with the late spring and early summer, and corresponded to the early rainy season. As a result, a higher incidence of births was detected in the late rainy season, providing the lactating female with the most suitable environment in terms of nutritional requirements.

Reproductive ageing and the menopause.

This brief review describes early work initiated by Anne McLaren and John Biggers, in which they repeated on mice a very early experiment carried out by John Hunter on pigs, to test the effect of unilateral ovariectomy on subsequent breeding performance. This and subsequent experiments led to the conclusion that reproductive ageing in the female mouse was largely due to ageing changes in the uterus. As a result of these changes fewer implanted blastocysts are carried to term in the older females, with the result that the size of litters produced gradually drops and ceases altogether well before the expected time of death, thus leading to a period of reproductive inactivity at the end of life. Other organs undergo ageing changes but it appears to be those in the uterus which limit reproductive performance in the female. The somatic organs concerned in bringing the male gametes into the environment are still able to function effectively almost until the time of death so that males have a very short period of reproductive inactivity at the end of their lives. Due to the prenatal onset of meiosis in the germ cells, female mammals and some, but not all, other vertebrates are born with a finite crop of oocytes in the ovary, which cannot be increased after birth. Nevertheless, with the exception of women, female mammals appear to be able to produce ova well into old age, and have them fertilized. When examined after death the ovaries still contain oocytes so this is not a limiting factor in reproduction in old females. In women the situation is completely different. They also have an extended period of reproductive quiescence in middle and old age, the menopause, but, unlike other female mammals, this is not due to failure of the uterus but is caused by the ovary becoming depleted of oocytes in middle age. The reason women run out of oocytes before the end of life, whereas the other mammals which have been studied do not, is associated with the greatly extended lifespan of humans compared to other mammals of equivalent size. There is a linear relationship between longevity and body weight in mammals, small mammals have much shorter lives than large ones. This is probably associated with the increased production of free radical oxygen necessary to maintain body temperature in smaller animals. Heat is lost through the body surface which becomes relatively less as the animal increases in weight, so the smaller animal has to metabolise and thus produces more free radical oxygen to maintain body temperature. For reasons unknown this seems not to apply to humans. The menopause has thus evolved as a consequence of two adaptations: the prenatal onset of meiosis, common to all mammals and many other vertebrates and the greatly increased longevity of all humans, both male and female. In view of this dual origin it is unlikely to have evolved in response to an adaptive need to have grandmothers to help rear the young, as has been suggested!

Progesterone and testosterone concentrations during oestrous cycle and pregnancy in the common vole ( Microtus arvalis Pallas)

Serum progesterone and testosterone concentrations were measured during different stages of oestrous and pregnancy in paired and unpaired female common voles ( Microtus arvalis). Hormone concentrations were measured by ELISA, and cycle stages were determined by vaginal smears. Paired females usually had serum progesterone concentrations of more than 10 ng/ml in the oestrous cycle. A significant maximum was detected in prooestrous (51.70±7.84 ng/ml, mean±S.D.). Serum progesterone concentrations increased from about 40 ng/ml at the beginning of pregnancy to about 70 ng/ml on days 15 and 16. The last 2 days before parturition (days 19 and 20) were characterised by a decrease of progesterone concentrations to ca. 30 ng/ml. The maximum concentration of testosterone was found in prooestrous (1.58±0.31 ng/ml). Concentrations during pregnancy varied between 1.5 and 2.1 ng/ml. In two of three cases unpaired females exhibited progesterone values below 10 ng/ml, but with varying vaginal smear patterns. The combination of progesterone concentrations and vaginal smear patterns was found to be regular in only 23.8% of the cases. The most frequent cycle stage found was the oestrous (44.2%). Mean concentrations of progesterone (10.43±13.81 ng/ml) and testosterone (0.85±1.11 ng/ml) in unpaired females were significantly lower than in paired females, thereby denoting reproductive inactivity in the former. The study presents basic data for several parameters of the reproductive biology in the common vole and confirms the importance of combining hormone assays and vaginal smear monitoring in reproductive research.