Reproduction and development in rats chronologically exposed to 60-Hz electric fields

Previous studies have raised the possibility of reproductive and developmental changes in miniature swine chronically exposed to a strong 60-Hz electric field. Two replicate experiments on rats were performed to determine if similar changes could be detected in animals exposed under a comparable regime, which was based on average, induced-current densities and on the chronology of reproductive development, as dosimetrically and biologically scaled. Beginning at three months of age, female rats of the F0 generation and their subsequent offspring were chronically exposed to a 60-Hz electric field (100 kV/m unperturbed) for 19 h/day for the duration of experimentation. After four weeks of exposure, F0 female rats were mated to unexposed male rats during the field-off period. No significant developmental effects were detected in their litters, confirming our previous results with swine and rats. The F0 females were mated for a second time at 7.2 months of age, and the fetuses were evaluated shortly before term. In the first experiments, the incidence of intrauterine mortality was significantly less in exposed than in sham-exposed litters, and there was a tendency (P = .12) for an increased incidence of malformed fetuses in exposed litters. Neither end point was significantly affected in the second experiment. Copulatory behavior of the female F1 offspring, which were bred at three months of age, was not affected in either experiment. There was a statistically significant decrease in the fertility of F1 exposed females and a significant increase in the fraction of exposed litters with malformed fetuses in the first experiment; both end points were essentially the same in the sham and exposed groups of the second experiment. That the significant effects detected in the first experiment were not seen in the second may be attributed to random or biological variation. Alternatively, the finding may indicate that the response threshold for induction of malformations lies near 100 kV/m.     

Rats are not aversive when exposed to 60-Hz magnetic fields at 3.03 mT.

Thirty-two male rats were tested in two replicates of an experiment to determine whether body currents induced by 60-Hz magnetic fields might lead to avoidance behavior comparable to that which results from exposure to strong 60-Hz electric fields. The test apparatus was a two-compartment Plexiglas shuttlebox enclosed in a sound-attenuating plywood chamber, which in turn was encompassed by two copper bus bars that, when energized, served as a source of 60-Hz magnetic fields. Location of the rat, and traverse activity in the shuttlebox were monitored by nine infra-red photo detectors equally spaced along the length of the apparatus. Rats were divided into 2 groups: 1 group of rats (n = 8 per group per replicate) was sham exposed while rats in the other group (n = 8 per group per replicate) were exposed to a 3.03 mT (30.3 G), 60-Hz magnetic field whenever they traversed to or were located on the side (L or R) predetermined as the exposed side. To control artifact incident to side preference, the side exposed (L or R) was alternated over the exposed rats. Each rat was tested individually in a 1-h session. A 2-factor ANOVA (exposed vs. control, replicate 1 vs. replicate 2) failed to reveal any significant effects due to either factor or to an interaction between factors. These data demonstrate that rats do not avoid exposure to 60-Hz magnetic fields at a flux density of 3.03 mT and further imply that the avoidance by rats of high level 60-Hz electric fields is mediated by something other than the internal body currents induced by the exposure.     

Developmental studies of Hanford miniature swine exposed to 60-Hz electric fields

Evaluations of reproductive and developmental toxicology, including teratology, were included as part of a broad screening study in Hanford Miniature swine (HMS) to detect effects of exposure to electric fields. One group (E) was exposed to a uniform, vertical, 60-Hz, 30-kV/m electric field for 20 h/day, 7 days/week; sham-exposed (SE) swine were housed in a separate, environmentally equivalent building. The first generation (F0) gilts were bred after 4 months of study; some were killed for teratologic assays at 100 days of gestation (dg), and the others produced an F1 generation of offspring. The pooled incidence of terata in these litters (teratologic assays and live births) was similar in the E and SE groups. The F0 females, which produced the F1 generation, were bred again after 18 months of exposure and were killed at 100 dg. Malformation incidence in E litters (75%) was significantly greater than in SE litters (29%). No consistent differences in litter size, fetal mass, or mass of fetal organs were detected. The F1 gilts were bred at 18 months of age; defective offspring were found in significantly more of the E litters (71%) than in SE litters (33%). These F1 females were bred again 10 months later and teratologic assays were performed on their second litters at 100 dg. The percentage of litters with malformed fetuses was essentially identical in the E and SE groups (70% and 73%, respectively). There appears to be an association between chronic exposure to a strong electric field and developmental effects in swine, although the change in incidence of malformations between generations and between the first and second breedings makes it impossible to conclude unequivocally that there is a cause-and-effect relation.     

Lack of adverse effects in pregnant/lactating female rats and their offspring following pre- and postnatal exposure to ELF magnetic fields

We have recently reported that exposure of pregnant rats to 60 Hz at field strengths up to 0.5 mT during the entire period of pregnancy did not induce any biologically significant effects on both pregnant dams and embryo-fetal development. The present study was carried out to investigate the potential effects of gestational and lactational MF exposure on pregnancy, delivery, and lactation of dams and growth, behavior, and mating performance of their offspring in rats. Timed-pregnant female Sprague-Dawley (SD) rats (24/group) received continuous exposure to 60 Hz magnetic field (MF) at field strengths of 0 (sham control), 5 microT, 83.3 microT, or 0.5 mT. Dams received MF or sham exposures for 21 h/day from gestational day 6 through lactational day 21. Experimentally generated MF was monitored continuously throughout the study. No exposure-related changes in clinical signs, body weight, food consumption, pregnancy length, and necropsy findings were observed in dams. Parameters of growth, behavior, and reproductive performance of offspring showed no changes related to MF exposure. There were no adverse effects on embryo-fetal development of F2 offspring from dams exposed to MF. In conclusion, exposure of pregnant SD rats to 60 Hz at field strengths up to 0.5 mT from gestational day 6 to lactational day 21 did not produce biologically significant effects in dams, F1 offspring, or F2 fetuses.     

Developmental toxicity evaluation of ELF magnetic fields in Sprague-Dawley rats

To identify possible effects of horizontally polarized magnetic field (MF) exposure on maintenance of pregnancy and embryo-fetal development, an MF exposure system was designed and constructed and 96 time-mated female Sprague-Dawley (SD) rats (24/group) received continuous exposure to 60 Hz MF at field strengths of 0 (sham control) and 5, 83.3, or 500 microT (50, 833, or 5000 mG). Dams received MF or sham exposures for 22 h/day on gestational day 6-20. MF was monitored continuously throughout the study. There were no evidences of maternal toxicity or developmental toxicity in any MF exposed groups. Mean maternal body weight, organ weights, and hematological and serum biochemical parameters in groups exposed to MF did not differ from those in sham control. No exposure related differences in fetal deaths, fetal body weight, and placental weight were observed between MF exposed groups and sham control. External, visceral, and skeletal examination of fetuses demonstrated no significant differences in the incidence of fetal malformations between MF exposed and sham control groups. In conclusion, exposure of pregnant rats to 60 Hz at MF strengths up to 500 microT during gestation day 6-20 did not produce any biologically significant effect in either dams or fetuses.     

Chronic exposure of primates to 60-Hz electric and magnetic fields: III. Neurophysiologic effects

The neurophysiologic effects of combined 60-Hz electric (E) and magnetic (B) fields, of magnitudes comparable to those produced by high-voltage powerlines, were investigated in 10 monkeys (Macaca nemestrina). Six animals (experimental group) were each exposed to three different levels of E and B fields: 3 kV/m and 0.1 G, 10 kV/m and 0.3 G, and 30 kV/m and 0.9 G. Field exposures were preceded and followed by sham exposures, during which factors of field generation were present (e.g., heat, vibration, noise, etc.) without E and B fields. Each of the five segments (i.e., the three exposure segments and the initial and final sham exposure segments) lasted 3 weeks. Animals were exposed for 18 h/day (fields on at 1600 h, off at 1000 h). Four other animals (external control group) were given sham exposure for the entire 15-week period. Auditory, visual, and somatosensory evoked potentials were recorded twice a week, during the daily 6-h field-off period. E- and B-field exposure had no effect on the early or mid-latency evoked potential components, suggesting that exposure at these levels has no effect on peripheral or central sensory afferent pathways. However, there was a statistically significant decrease in the amplitudes of late components of the somatosensory evoked potential during the 10kV/m and 0.3 G, and 30 kV/m and 0.9 G exposure levels. This result is possibly related to the opiate antagonist effect of electromagnetic field exposure reported by others.     

Rats reproduce and rear litters during chronic exposure to 150-kV/m, 60-Hz electric fields

Mature female rats and their subsequent litters were exposed either to 112- or to 150-kV/m, 60-Hz electric fields or sham-exposed for 19 h daily through pre-breeding, breeding, and rearing periods of experimentation. Exposed females mated in equal percentages and reared litters of equal numbers, and mean body masses of pups were the same as those of sham-exposed animals. Thus, experiments to investigate electric-field effects on reproduction and development in rats are feasible at effective field strengths of 112 and 150 kV/m.     

A 60 Hz magnetic field does not affect the incidence of squamous cell carcinomas in SENCAR mice

Two groups of SENCAR mice were treated with a single dose of carcinogen and then, for 23 weeks, with a chemical tumor promoter to induce skin tumors. During this period, one group was coexposed to a 2 mT power frequency (60 Hz) magnetic field, while the other was exposed to sham conditions. Application of the tumor promoter ceased after 23 weeks, but the exposure to sham conditions or magnetic fields continued for an additional 29 weeks. No difference was found between the two groups of mice in terms of the incidence of total tumors (P =.297) or squamous cell carcinomas (SSC) (P =.501). In summary, there was no evidence to support the hypotheses that 60 Hz magnetic fields (MF) can influence the development of either papillomas or SSC under our defined experimental conditions. The overall results add to previous animal studies that find no association between exposure to 60 Hz MF and the incidence of benign or malignant tumors.     

Effects of 50-Hz magnetic field exposure on superoxide radical anion formation and HSP70 induction in human K562 cells

Epidemiological studies suggest a correlation between exposure to low-level extremely low-frequency (ELF) magnetic fields (MF) and certain cancers and neurodegenerative diseases. Experimental studies have not provided any mechanism for such effects, although at flux density levels significantly higher than the ones encountered in epidemiological studies, radical homoeostasis and levels of stress response proteins can be affected. Here, we report on the influence of MF exposure (50-Hz sine wave; 1 h; 0.025–0.10 mT; vertical or horizontal MF exposure direction) on different cellular parameters (proliferation, cell cycle distribution, superoxide radical anion, and HSP70 protein levels) in the human leukaemia cell line K562. The positive control heat treatment (42°C, 1 h) did not affect either cell proliferation or superoxide radical anion production but caused accumulation of cells in the G2 phase and increased the stress protein HSP70. MF exposure (0.10 mT, 1 h) did not affect either cell cycle kinetics or proliferation. Both vertical and horizontal MF exposures for 1 h caused significantly and transiently increased HSP70 levels (>twofold), at several flux densities, compared to sham controls and also compared to heat treatment. This exposure also increased (30–40%) the levels of the superoxide radical anion, comparable to the positive control PMA. Addition of free radical scavengers (melatonin or 1,10-phenantroline) inhibited the MF-induced increase in HSP70. In conclusion, an early response to ELF MF in K562 cells seems to be an increased amount of oxygen radicals, leading to HSP70 induction. Furthermore, the results suggest that there is a flux density threshold where 50-Hz MF exerts its effects on K562 cells, at or below 0.025 mT, and also that it is the MF, and not the induced electric field, which is the active parameter.     

Extremely low frequency magnetic field effects on premorbid behaviors produced by cocaine in the mouse

We investigated the premorbid behavioral changes produced by the administration of cocaine and acute exposure to extremely low frequency (ELF) magnetic field (MF) in the mouse. ICR mice received intraperitoneal injections of cocaine at two doses (65 and 70 mg/kg) and were subsequently exposed to one of eight ELF-MF fields (2, 3, 4, 8, 10, 15, 25, or 60 Hz) of about 20 G (2 mT) intensity immediately after injection. Twelve mice were used for each of applied cocaine dose and ELF-MF level. For a given dose of cocaine, the applied MF frequencies were randomly ordered, and blind tests were carried out in which the behavior observer did not know the frequencies of MF. The premorbid behaviors were defined in the ICR mice and their changes were observed over the exposure of various ELF-MFs. Our data show that the onset times of stop rearing and tonic-clonic seizure in the 4 Hz MF exposure group are significantly different from those of the sham group.     

Perinatal exposure to 60-Hz electric fields: Effects on the development of the visual-evoked response in rats

Two independent series of experiments were performed on 114 male Sprague-Dawley derived, albino rat pups, which represented 61 litters in experimental series I and 53 litters in experimental series II. Animals were exposed for 20 h/day from conception to testing (postnatal days 11–20) to a vertical, 65-kV/m, 60-Hz electric field or sham-exposed. Recordings of the visual-evoked response (VER) were obtained using a small silver ball electrode placed epidurally over the visual cortex. Visual stimuli consisted of 10-μS light flashes delivered at 0.2 Hz. Computer-averaged VERs were obtained and power spectral analyses (fast Fourier transform) were performed on the tapered (split cosine-bell window), averaged VERs. The expected age-related changes were clearly evident; however, a detailed analysis of VER component latencies, peak-to-peak amplitude, and power spectra failed to reveal any consistent, statistically significant effect of exposure to 60-Hz electric fields.     

Effects of low-frequency magnetic fields on fetal development in rats

We studied effects of alternating magnetic fields on the embryonic and fetal development of rats. Mated females of the Han:Wistar-strain were sham exposed or exposed continuously to a 50-Hz field or to a 20,000 pulse-per-second (pps) sawtooth magnetic field from day 0 to day 20 of pregnancy for 24 h/day until necropsied on day 20. The respective peak-to-peak intensities of the fields were 35.6 μT (sinewave) and 15.0 μT (sawtooth). Each treatment group contained 72 bred females. Control animals were kept under the same conditions without the magnetic field. No adverse effects were seen in the dams. The mean numbers of implantations and living fetuses per litter were statistically significantly increased in the 50-Hz group. There were, however, three total resorptions of litters in dams of the control group, which contributed to the difference in the number of living fetuses. The corrected body-mass gains (gains without uterine content) of dams were similar in all groups. Pregnancy rates, incidences of resorptions. late fetal deaths, and fetal body masses were similar in all groups. The incidence of fetuses with minor skeletal anomalies was statistically significantly increased in both exposed groups. Only one serious malformation (anophthalmia, sawtooth-exposed group) and a few minor visceral malformations were found. In conclusion, the magnetic fields used in this study did not increase the incidence of major malformations or resorptions in Wistar rats. The increased number of skeletal anomalies and implantations we observed indicates, however, that some developmental effects in rats may attend exposure to time-varying magnetic fields. © 1993 Wiley-Liss. Inc.     

Lack of a co‐promotion effect of 60 Hz circularly polarized magnetic fields on spontaneous development of lymphoma in AKR mice

The present study was conducted to investigate the possible effect of 60 Hz circularly polarized magnetic fields (MFs) as promoters of genetically initiated lymphoma in AKR mice. One hundred sixty female animals were divided into four different groups. They were exposed to four different intensities of circularly polarized MFs. Animals received exposure to 60 Hz circularly polarized MF at field strengths (rms‐value) of 0 µT (sham control, T1, Group I), 5 µT(T2, Group II), 83.3 µT (T3, Group III), or 500 µT(T4, Group IV), for 21 h/day from the age of 4–6 weeks to the age of 44–46 weeks. There were no exposure‐related changes in mean survival time, clinical signs, body weights, hematological values, micronucleus assay, gene expression arrays, analysis of apoptosis, and necropsy findings. At histopathological examination, lymphoma was seen in all the groups. The tumor incidence was 31/40(78%), 30/40(75%), 32/40(80%), and 31/40(78%) in sham control, 5, 83.3, and 500 µT groups, respectively. However, there were no differences in the tumor incidence between the sham control (T1) and circularly polarized MF exposure groups (T2–T4). In conclusion, there was no evidence that exposure to 60 Hz circularly polarized MF strengths up to 500 µT promoted lymphoma in AKR mice. Bioelectromagnetics 31:130–139, 2010. © 2009 Wiley‐Liss, Inc.     

Two generation reproduction study of ethylbenzene by inhalation in Crl-CD rats

BACKGROUND: This study was conducted to evaluate the potential adverse effects of ethylbenzene (EB) on reproductive capability from whole-body inhalation exposure of F0 and F1 parental animals. METHODS: Four groups of Crl:CD(SD)IGS BR rats (30/sex/group for F0 and 25/sex/group for F1) were exposed to 0, 25, 100, and 500 ppm EB for 6 hr/day for at least 70 consecutive days before mating. Inhalation exposure for the F0 and F1 females continued throughout mating, gestation through gestation day (GD) 20, and lactation days (LD) 5-21. On LD 1-4, females received EB in corn oil via oral gavage at dose levels of 26, 90, and 342 mg/kg/day (divided into three equal doses, approximately 2 hr apart), as calculated from a physiologically-based pharmacokinetic (PBPK) model to provide similar maternal blood area-under-concentration (AUC) as provided by inhalation. Pups were weaned on postnatal day (PND) 21 and exposure of the F1 generation started on PND 22. Estimates of internal exposure were determined by measuring EB concentrations in blood collected from F1 dams (4/group) and their culled pups 1 hr after the last gavage dose on PND 4. On PND 22, blood was collected from these same F1 dams and their weanlings for EB analysis 1 hr after a 6-hr inhalation exposure. The remainder of the F2 generation was not directly exposed. RESULTS: EB exposure did not affect survival or clinical observations. Male rats in the 500 ppm group in both generations gained weight more slowly than the controls. There were no indications of adverse effects on reproductive performance in either generation. Male and female mating and fertility indices, pre-coital intervals, spermatogenic endpoints, ovarian follicle counts, reproductive organ weights, lengths of estrous cycle and gestation, live litter size, pup weights, developmental landmarks, and postnatal survival were unaffected. No adverse exposure-related macroscopic pathology was noted at any level. CONCLUSIONS: Increased liver weights were found in the animals exposed to 500 ppm. F1 maternal whole blood EB concentrations of 0.49, 3.51, or 18.28 mg/L were found 1 hr after administration of a composite oral dose of 26, 90, or 342 mg/kg/day, respectively, but no detectable EB was found in blood samples of their F2 PND 4 culled pups. F1 maternal mean whole blood EB levels 1 hr after a 6-hr inhalation exposure on postpartum day (PPD) 22 was 0.11 mg/L (25 ppm), 0.56 mg/L (100 ppm), and 11 mg/L (500 ppm). For the offspring exposed with their dams on PND 22, F2 pup blood EB concentrations ranged from 0.017-0.039 mg/L (25 ppm), 0.165-0.465 mg/L (100 ppm), and 8.82-15.74 mg/L (500 ppm). Because decreased weight gain in the 500 ppm males was transient and no histopathological changes were associated with the increased liver weights in the 500 ppm male and female groups, these changes were not considered adverse. Therefore, for parental systemic toxicity, 100 ppm was considered a NOEL and 500 ppm a NOAEL in this study. The 500 ppm exposure concentration was considered a NOAEL for F0 and F1 reproductive toxicity and offspring developmental endpoints.     

Effect of ambient levels of power-line-frequency electric fields on a developing vertebrate

Fertilized eggs of Gallus domesticus were exposed continuously during their 21-day incubation period to either 50- or 60-Hz sinusoidal electric fields at an average intensity of 10 Vrms/m. The exposure apparatus was housed in an environmental room maintained at 37 degrees C and 55-60% relative humidity (RH). Within 1.5 days after hatching, the chickens were removed from the apparatus and tested. The test consisted of examining the effect of 50- or 60-Hz electromagnetic fields at 15.9 Vrms/m and 73 nTrms (in a local geomagnetic field of 38 microT, 85 degrees N) on efflux of calcium ions from the chicken brain. For eggs exposed to 60-Hz electric fields during incubation, the chicken brains demonstrated a significant response to 50-Hz fields but not to 60-Hz fields, in agreement with the results from commercially incubated eggs [Blackman et al., 1985a]. In contrast, the brains from chicks exposed during incubation to 50-Hz fields were not affected by either 50- or 60-Hz fields. These results demonstrate that exposure of a developing organism to ambient power-line-frequency electric fields at levels typically found inside buildings can alter the response of brain tissue to field-induced calcium-ion efflux. The physiological significance of this finding has yet to be established.     

Progression of arteriovenous bypass restenosis in mice exposed to a 50 Hz magnetic field

The controversy over whether magnetic fields (MF) produced by electrical wiring and appliances contribute to diseases such as cancer has been debated in the literature for more than 2 decades. These extremely low frequency fields at 50 or 60 Hz are omnipresent in the industrialized world and have been linked to various forms of cancer by epidemiological studies. Little has been published investigating any possible role of MF and cardiovascular disease, and this is the first study looking specifically at the effect of exposure to high-intensity MF on the development and progression of restenosis. A mouse arteriovenous bypass model was used, and mice were exposed to MF for periods of 1, 2, or 3 weeks. Neointima formation, infiltration of mononuclear cells, and heat shock protein 60 expression were all studied at the conclusion of the exposure regimen. Animals exposed to the MF for 1 week showed significantly smaller neointima formation compared with control mice exposed to a null field, although this difference was not observed in mice exposed for 2 or 3 weeks. No difference was found between mice exposed to MF and controls in any of the other parameters investigated.     

No influence of magnetic fields on cell cycle progression using conditions relevant for patients during MRI

The purpose of this study was to examine whether exposure to magnetic fields (MFs) relevant for magnetic resonance imaging (MRI) in clinical routine influences cell cycle progression in two tumor cell lines in vitro. HL60 and EA2 cells were exposed to four types of MFs: (i) static MF of 1.5 and 7.05 T, (ii) extremely low frequency magnetic gradient fields (ELFMGFs) with +/- 10 mT/m and 100 Hz, as well as +/- 100 mT/m and 100 Hz, (iii) pulsed high frequency MF in the radiofrequency (RF) range (63.6 MHz, 5.8 microT), and (iv) a combination of (i-iii). Exposure periods ranged from 1 to 24 h. Cell cycle distribution (G(0)/G(1), S, and G(2)/M phases) was analyzed by flow cytometry. Cell cycle analysis did not reveal differences between the exposed and the control cells. As expected, positive controls with irradiated (8 Gy) HL60 and EA2 cells showed a strong G(2)/M arrest. Using conditions that are relevant for patients during MRI, no influence of MFs on cell cycle progression was observed in these cell lines. Care was taken to control secondary parameters of influence, such as vibration by the MR scanner or temperature to avoid false positive results.     

Effect of feeding fungal endophyte (Acremonium coenophialum )-infected tall fescue seed on reproductive performance in CD-1 mice through continuous breeding

This study was designed to assess the effect of endophyte-infected (Acremonium coenophialum ) tall fescue (KY-31) seed (80% infected) on reproductive performance in CD-1 mice by continuous breeding. Twenty-four pairs of 70-d-old CD-1 mice were randomly allocated to four diets: 1) mouse chow ad libitum; 2) 40% infected fescue seed and 60% chow (w/w); 3) reduced intake (100% chow) similar to the intake, adjusted daily, in Diet 2; and 4) 60% infected fescue seed and 40% chow. Males and females were randomly paired (six pairs/treatment) and placed on the above diets. The mice were fed the corresponding diets for 80 d, although the pairs were separated on Day 60 (prior to the birth of the 3rd litter) and the females were monitored for one additional gestation period (20 d). The pregnancy data (litters produced) among the four treatments were 100.0 (18), 77.8 (14), 100.0 (18) and 80.0% (12) respectively. Similarly, the average number of pups born per litter among the four treatments was 11.8, 9.3, 10.1, and 9.8. When the chow treatment (1 and 3) and the fescue treatments (2 and 4) were pooled and compared, the percent pregnancy was 100.0 (n = 36) and 78.8 (n = 26), and the pups born per litter (means +/- SEM) were 11.0 +/- 0.5 and 9.5 +/- 0.6, respectively. Also the intervals between the three litters born during the 60-d cohabitation period were 21.6 +/- 1.1 and 24.5 +/- 0.9 d for the chow and fescue treatments, respectively. The results point out that 40 and 60% infected fescue seed in the diet of mice does influence (P < 0.05) their reproductive capacity as measured by percent pregnancy and litter size.     

Sex and the Single Vole: Effects of Social Grouping on Prairie Vole Reproductive Success

Prairie voles (Microtus ochrogaster) have a mating system that is primarily monogamous with paired males and females together defending breeding space against intruders of either sex. Breeding success may be affected when other adults intrude on the territorial space of pairs. We conducted an experiment to determine the impact of additional members of either sex on reproductive success of pairs. In laboratory arenas, we formed pairs (1F:1M) and two kinds of triad (2F:1M, 1F:2M). Females in pairs had the highest conception rates, litter sizes and survival of litters. Females in 1F:2M groups had slightly reduced litter sizes and reduced numbers of weanlings, and some females had litters sired by both males. Females in 2F:1M groups had low conception rates and the smallest litters, and >35% of their litters suffered infanticide; in no case did both females become pregnant. Throughout the trials, individuals of the sex doubly represented in triads were more likely to die than were individuals of the sex singly represented. We conclude that there may be fitness costs associated with the presence of unrelated supernumerary adults during gestation and lactation.