The effect of asymmetries on stock index return value-at-risk estimates

It is widely accepted that equity return volatility increases more following negative shocks rather than positive shocks. However, much of value-at-risk (VaR) analysis relies on the assumption that returns are normally distributed (a symmetric distribution). This article considers the effect of asymmetries on the evaluation and accuracy of VaR by comparing estimates based on various models

Higher cortisol:cortisone ratios in the preovulatory follicle of completely unstimulated IVF cycles indicate oocytes with increased pregnancy potential

BACKGROUND: Conception following gonadotrophin-stimulated IVF and embryo transfer has been associated with a higher intrafollicular cortisol:cortisone ratio and decreased metabolism of cortisol to cortisone. The role of glucocorticoids in human oocyte maturation is not fully understood, but active glucocorticoid (cortisol) may be important. This study relates intrafollicular cortisol and cortisone concentrations to oocyte fertilization and embryo implantation in unstimulated cycles. METHODS: Patients aged <40 years with favourable sperm underwent unstimulated IVF–embryo transfer. Study 1 related intrafollicular cortisol levels to oocyte and IVF outcome: (i) fertilized, pregnant (n = 9); (ii) fertilized, not pregnant (n = 21); and (iii) unfertilized (n = 12). Study 2 was a case–control study of 27 patients (same outcome groups of equal size) which measured intrafollicular cortisol, cortisone and the cortisol:cortisone ratio. RESULTS: Conception cycles demonstrated higher cortisol concentrations compared with the fertilized group (study 1) [median (95% confidence interval): 299 (249–330) versus 227 nmol/l (185–261); P < 0.05] and higher cortisol:cortisone ratios when compared with the unfertilized group (study 2) [7.38 (5.23–9.19) versus 3.56 (1.75–7.46) respectively; P = 0.02]. Of the women with cortisol:cortisone ratios greater than the outcome independent mean of 5.90, 58% conceived compared with only 13% with ratios <5.90 (P < 0.02). CONCLUSION: Higher cortisol:cortisone ratios in conception cycles suggest that active glucocorticoid may be important for final oocyte maturation and embryo implantation in unstimulated cycles

Total cortisol levels are reduced in the periovulatory follicle of infertile women with minimal-mild endometriosis

PROBLEM: To measure and compare concentrations of total and free glucocorticoids with oocyte fertilizing capacity in the follicular fluid (FF) of women with minimal–mild endometriosis and tubal damage.METHOD OF STUDY: Follicular fluid was collected from individual periovulatory follicles during oocyte retrieval for in vitro fertilization (IVF) in natural cycles. Total and free levels of cortisol and cortisone were measured using specific radioimmunoassays after chloroform extraction.RESULTS: Cortisol concentrations in women with minimal–mild endometriosis were significantly lower compared with controls (women with tubal infective damage) (258 versus 328 nmol/L, P < 0.02). There was no correlation between total or free concentrations of cortisol or cortisone and the fertilization capacity of the oocyte.CONCLUSIONS: Total cortisol levels are lower in the follicles of women with endometriosis. Our findings provide further evidence of follicular dysfunction contributing to the subfertility associated with minimal–mild endometriosis

A Case of Natural Queen Succession in a Captive Colony of Naked Mole-Rats, Heterocephalus glaber

Naked mole-rats occur in large colonies where usually a single queen monopolizes reproduction. Queen succession occurs from within usually as a result of aggressive encounters with subordinate females that queue for reproductive succession following colony instability, which inevitably results in death of either the queen or the challenging conspecific. We monitored a queen succession following the death of the breeding male in a colony of the naked mole-rat, Heterocephalus glaber, prior to, during as well as after replacement of the original breeding female. The response of the pituitary luteotrophs was investigated in the non-reproductive females during this period of instability by the administration of endogenous gonadotrophin releasing hormone (GnRH) and evaluating the subsequent luteinising hormone (LH) response in the blood. Larger and older non-breeding females engaged in aggressive encounters that culminated in death. The new breeding successor which arose from within the colony was a large female who continued to procreate. The six non-breeding females that were killed during reproductive takeover were larger and older females which exhibited elevated basal circulating LH concentrations as well as increased pituitary sensitivity as measured by the amount of releasable LH to an exogenous GnRH challenge. By contrast, non-breeding females that survived the succession were smaller and younger animals with reduced basal and GnRH challenged LH concentrations. Likewise, five non-breeding males which were heavier and older than those non-breeding males which survived were killed. These animals did not, however, show elevated basal or exogenous GnRH challenged LH concentrations when compared to the surviving males. The non-breeding animals of both sexes which survived the reproductive takeover event represented individuals which posed a minimal threat to the new successor and hence promoted the continuation of the marked reproductive skew that is prevalent in this highly inbred colonial subterranean hystricomorph.The authors thank R.P. Millar, Department of Chemical Pathology, University of Cape Town, for donating the mammalian GnRH. We also thank the National Institute of Biological Standards and Control, Hertfordshire, England, for the LH pituitary preparation (2nd International Standard 1988, code 80/552). The work was supported by research grants from the then Foundation for Research Development (to N.C.B. and J.U.M.J.), the University of Pretoria (to N.C.B.) and the University of Cape Town (to J.U.M.J). This study was approved by the Animal Ethics Committee of the University of Cape Townhttp://africanzoology.journals.ac.za/am2013ab201