Average Group Size Research Articles

When to go with the crowd: Modelling synchronization of all-or-nothing activity transitions in grouped animals

For groups of animals to keep together, the group members have to perform switches between staying in one place and moving to another place in synchrony. However, synchronization imposes a cost on individual animals, because they have to switch from one to the other behaviour at a communal time rather than at their ideal times. Here we model this situation analytically for groups in which the ideal times vary quasinormally and grouping benefit increases linearly with group size. Across the parameter space consisting of variation in the grouping benefit/cost ratio and variation in how costly it is to act too early and too late, the most common optimal solutions are full synchronization with the group staying together and zero synchronization with immediate dissolution of the group, if the group is too small for the given benefit/cost ratio. Partial synchronization, with animals at the tails of the distribution switching individually and the central core of the group in synchrony, occurs only at a narrow stripe of the space. Synchronization cost never causes splitting of the group into two as either zero, partial or full synchronization is always more advantageous. Stable solutions dictate lower degree of synchrony and lower net benefits than optimal solutions for a large range of the parameter values. If groups undergo repeated synchronization challenges, they stay together or quickly dissolve, unless the animals assort themselves into a smaller group with less variation in the ideal times. We conclude with arguing that synchronization cost is different from other types of grouping costs since it does not increase much with increasing group size. As a result, larger groups may be more stable than smaller groups. This results in the paradoxical prediction that when the grouping benefit/grouping cost ratio increases, the average group sizes might decrease, since smaller groups will be able to withstand synchronization challenges.

Predator‐mediated Allee effects in multi‐prey systems

Allee effects can have significant consequences for small populations and understanding the causal mechanisms for such effects is important for guiding conservation actions. One proposed mechanism is through predation, in which a type II functional response leads to increasing predation rates as prey numbers decline. However, models to support this mechanism have incorporated only a single declining prey species in the functional response, which is probably an oversimplification. We reevaluated the potential for predator-mediated Allee effects in a multi-prey system using Holling's disc equation. We also used empirical data on a large herbivore to examine how grouping behavior may influence the potential for predation-mediated Allee effects. Results based on a multi-prey expression of the functional response predict that Allee effects caused by predation on relatively rare secondary prey may not occur because handling time of the abundant prey dominates the functional response such that secondary prey are largely "bycatch". However, a predator-mediated Allee effect can occur if secondary prey live in groups and if, as the population declines, their average group size declines (a relationship seen in several species). In such a case, the rate at which the number of groups declines is less than the rate at which the population declines. Thus the rate at which a predator encounters a group remains relatively stable, but when a predator kills one animal from smaller groups, the predation rate increases. These results highlight the need to evaluate risks associated with potential changes in group size as populations decline.

Absence of sexual differences in aggregation behavior and association patterns in South American sea lion pups (Otaria flavescens) at Isla de Lobos, Uruguay

In the South American sea lion (Otaria flavescens), a polygynous species with a high degree of sexual dimorphism, pups form groups during maternal foraging trips. The aim of this study was to investigate the existence of sexual differences in aggregation behavior and association patterns in O. flavescens pups at Isla de Lobos, Uruguay, during the first 2 mo of life. Scan samplings of marked pups were conducted every hour from 0700 to 1800 to register behavior. Association patterns were analyzed in SOCPROG (using the Half-Weight Index of Association). We did not find significant sex differences in any variable (time spent in groups, average group size, time interacting with other pups, and time in groups at sea). Each pup showed preference for at least one partner. Associations between individuals of the same and different sex were not significantly different. As expected, during the first month, pups associated more strongly with pups born in the same zone than with those born in a different zone. This research provides new evidence on the development of social behavior in otariids and serves as a basis for future studies focusing on sexual differences in pup behavior and association patterns among individuals (e.g., related with kinship).

Sighting characteristics and photo-identification of Cuvier’s beaked whales (Ziphius cavirostris) near San Clemente Island, California: a key area for beaked whales and the military?

The relationship between beaked whales and certain anthropogenic sounds remains poorly understood and of great interest. Although Cuvier’s beaked whales (Ziphius cavirostris) are widely distributed, little is known of their behavior and population structure throughout much of their range. We conducted a series of five combined visual-acoustic marine mammal surveys from 2006 to 2008 in the southern San Nicolas Basin, a site of frequent naval activity off the southern California coast, west of San Clemente Island. The study area was defined by a 1,800 km2 array of 88 bottom-mounted hydrophones at depths up to 1,850 m. The array was used to vector visual observers toward vocalizing marine mammal species. Thirty-seven groups of Cuvier’s beaked whales were encountered during the study period. The overall encounter rate was one group for every 21.0 h of survey effort, and was as high as one group per 10.2 h of effort during the October 2007 survey. Whales were encountered in the deepest portion of the study area, at a mean bottom depth of 1,580 m (SD 138). The average group size was 3.8 individuals (SD 2.4), which was higher than has been reported from other studies of this species. Twenty-four groups were observed over multiple surfacings (median = 4 surfacings, range 2–15). The mean encounter duration of extended sightings was 104 min (SD 98, range 12–466 min) and the mean distance moved over the course of sightings was 1.66 km (SD 1.56, range 0.08–6.65 km). Temporal surfacing patterns during extended encounters were similar to dive behavior described from Cuvier’s beaked whales carrying time-depth recording tags. Seventy-eight photographic identifications were made of 58 unique individuals, for an overall resighting rate of 0.26. Whales were sighted on up to 4 days, with duration from first to last sighting spanning 2–79 days. For those whales sighted on subsequent days, the mean distance between subsequent sightings was 8.6 km (SD 7.9). Individuals resighted over 2–3 days were usually in association with previous group members. Approximately one-third of groups contained more than one adult male, and many of the repeated associations involved adult males. These observations suggest the basin west of San Clemente Island may be an important region for Cuvier’s beaked whales, and also one which affords an unusual opportunity to collect detailed data on this species. Given its status as an active military range, it can also provide the ability to monitor the behavior of individuals in the presence of naval sonar, a critical step in the management of this and other beaked whale populations worldwide.

An Apparent Decrease in the Prevalence of "Ross Sea Killer Whales" in the Southern Ross Sea

Killer whales (Orcinus orca), both ecotype-B and -C, are important to the Ross Sea, Antarctic ecosystem. The ecotype-C is referred to as “Ross Sea [RS] killer whale.” Herein, we review data on occurrence patterns and diet of RS killer whales and present new information on numbers observed in the southwestern Ross Sea, 2002-2003 to 20082009 austral summers. These “resident” whales appear to feed principally on fish, including the large Antarctic toothfish (Dissostichus mawsoni). On the basis of sea watches from Cape Crozier, Ross Island, sighting frequency and average group size appears to have decreased; prevalence as indicated by casual observations from helicopter pilots flying over the area on a daily basis has also decreased in nearby McMurdo Sound. Consistent with a decrease in the catch-per-unit-effort of scientific fishing for toothfish in McMurdo Sound, we suggest and review evidence that the change in RS killer whale numbers in the southern Ross Sea is related to an industrial fishery-driven, densitydependent northward contraction of the toothfish stock and not to changes in the physical (and, in turn, biological) environment. We surmise that in this closely coupled foodweb, composed of very abundant penguin, seal, and whale components, loss of the toothfish option for RS killer whales would force more direct competition with other predators for capture of the smaller-fish prey. Therefore, we propose, the RS killer whales have opted to move elsewhere, in a scenario consistent with that of the Pacific coast of Canada, where numbers of resident killer whales have decreased following the loss of large fish as a prey choice.

Qualitative systemic review of randomized controlled trials on complementary and alternative medicine treatments in fibromyalgia.

The objectives of the study were identification, quality evaluation and summary of RCTs on complementary and alternative medicine as defined by the National Institute of Health with the exception of dietary and nutritional supplements. A computerized search of databases from 1990 (year of publication of the ACR criteria for fibromyalgia) to July 2007 was performed. The RCTs were assessed by a methodological quality score. A total of 23 RCTs issued from 1992 to 2007 on acupuncture, balneotherapy, thermotherapy, magnetic therapy, homeopathy, manual manipulation, mind-body medicine, diet therapy and music therapy were identified. The RCTs had an average group size of 25 with the number of groups ranging from two to four. The quality score assessment of the RCTs yielded a mean score of 51 out of 100. The average methodological quality of the identified studies was fairly low. Best evidence was found for balneotherapy/hydrotherapy in multiple studies. Positive results were also noted for homeopathy and mild infrared hyperthermia in 1 RCT in each field. Mindfulness meditation showed mostly positive results in two trials and acupuncture mixed results in multiple trials with a tendency toward positive results. Tendencies for improvement were furthermore noted in single trials of the Mesendieck system, connective tissue massage and to some degree for osteopathy and magnet therapy. No positive evidence could be identified for Qi Gong, biofeedback, and body awareness therapy.

Monitoring important coastal sites for bottlenose dolphin in Cardigan Bay, UK

From 1994–2007 Ceredigion County Council and a network of shore-based, volunteer observers monitored levels of boat traffic and the occurrence of bottlenose dolphins Tursiops truncatus at seven sites on the coast of Cardigan Bay, Wales. We report high rates of site use and site occupancy by this species during the summer: at Mwnt for example, dolphins were recorded in >80% of 2 hours' observation periods; and at New Quay Harbour dolphins were present in >30% of all 15-minute intervals. At Mwnt and Aberporth there was a significant annual trend for increasing sighting rates; at Ynys Lochtyn a positive trend was only marginally non-significant; at New Quay Bird's Rock the trend was ambiguous but appeared stable over the seven most recent years. Although trends in site use may not reflect population trends in the wider region, these data were consistent with recent abundance estimates that indicate that the number of bottlenose dolphins using Cardigan Bay is stable or slightly increasing. Average group size at our study sites was small (<3 animals), which contrasts with observations of larger schools of the same population elsewhere in their range. The predominant behaviour in coastal Cardigan Bay is demersal foraging, often by solitary animals in shallow near-shore habitats. There was evidence that boat traffic suppressed site use by dolphins at New Quay Harbour, the busiest monitoring site: sighting rates fell when high numbers of boats were present and sighting rates were higher in 2007 than in previous years, when boat use was reduced due to poor weather during the main tourist season. This study demonstrates that networks of volunteer observers can provide a cost-effective, non-invasive means of gathering data on marine mammals for the purposes of coastal zone management.

The clinical effectiveness of different parenting programmes for children with conduct problems: a systematic review of randomised controlled trials

. Dretzke J., Davenport C., Frew E., Barlow J., Stewart‐Brown S., Bayliss S., Taylor R. S., Sandercock J. & Hyde C. ( 2009 ) , 3 , 7 . DOI: 10.1186/1753‐2000‐3‐7 . Background Conduct problems are common, disabling and costly. The prognosis for children with conduct problems is poor, with outcomes in adulthood including criminal behaviour, alcoholism, drug abuse, domestic violence, child abuse and a range of psychiatric disorders. There has been a rapid expansion of group‐based parent‐training programmes for the treatment of children with conduct problems in a number of countries over the past 10 years. Existing reviews of parent training have methodological limitations such as inclusion of non‐randomized studies, the absence of investigation for heterogeneity prior to meta‐analysis or failure to report confidence intervals. The objective of the current study was to systematically review randomized controlled trials (RCTs) of parenting programmes for the treatment of children with conduct problems. Methods Standard systematic review methods were followed including duplicate inclusion decisions, data extraction and quality assessment. Twenty electronic databases from the fields of medicine, psychology, social science and education were comprehensively searched for RCTs and systematic reviews to February 2006. Inclusion criteria were: RCT; of structured, repeatable parenting programmes; for parents/carers of children up to the age of 18 with a conduct problem; and at least one measure of child behaviour. Meta‐analysis and qualitative synthesis were used to summarize included studies. Results A total of 57 RCTs were included. Studies were small with an average group size of 21. Metaanalyses using both parent (SMD1−0.67; 95% CI: −0.91, −0.42) and independent (SMD −0.44; 95% CI: −0.66, −0.23) reports of outcome showed significant differences favouring the intervention group. There was insufficient evidence to determine the relative effectiveness of different approaches to delivering parenting programmes. Conclusion Parenting programmes are an effective treatment for children with conduct problems. The relative effectiveness of different parenting programmes requires further research.

Characteristics of a group of Hubei Golden Snub‐nosed Monkeys (Rhinopithecus roxellana hubeiensis) before and after major snow storms

Natural disasters can negatively affect primate population demography and social group structure. A clear understanding of these effects has important implications for wildlife conservation. The worst snow storms in nearly five decades hit portions of southern and central China between January 10 and February 6, 2008, presenting a unique opportunity to observe their immediate effects on a previously studied group of Hubei Golden Snub-nosed Monkeys (Rhinopithecus roxellana hubeiensis) in temperate forests in Shennongjia Nature Reserve, Hubei Province, China. We recorded social and demographic characteristics of the group before and after the snow storms. The average group size decreased from 270 individuals before the storms to 197 individuals after the storms, a reduction of 27.2%. Adult females (30.1%), juveniles (38.1%) and infants (55.4%) suffered higher mortality than did adult males (15.7%). Despite age and sex-based differences in mortality, the ratios of adult males to adult females, adults to immatures and adult females to immatures remained similar before and after the storms. However, higher mortality among females, juveniles and infants may reduce the group's long-term potential for growth.

The clinical effectiveness of different parenting programmes for children with conduct problems: a systematic review of randomised controlled trials.

BackgroundConduct problems are common, disabling and costly. The prognosis for children with conduct problems is poor, with outcomes in adulthood including criminal behaviour, alcoholism, drug abuse, domestic violence, child abuse and a range of psychiatric disorders.There has been a rapid expansion of group based parent-training programmes for the treatment of children with conduct problems in a number of countries over the past 10 years. Existing reviews of parent training have methodological limitations such as inclusion of non-randomised studies, the absence of investigation for heterogeneity prior to meta-analysis or failure to report confidence intervals.The objective of the current study was to systematically review randomised controlled trials of parenting programmes for the treatment of children with conduct problems.MethodsStandard systematic review methods were followed including duplicate inclusion decisions, data extraction and quality assessment. Twenty electronic databases from the fields of medicine, psychology, social science and education were comprehensively searched for RCTs and systematic reviews to February 2006.Inclusion criteria were: randomised controlled trial; of structured, repeatable parenting programmes; for parents/carers of children up to the age of 18 with a conduct problem; and at least one measure of child behaviour. Meta-analysis and qualitative synthesis were used to summarise included studies.Results57 RCTs were included. Studies were small with an average group size of 21. Meta-analyses using both parent (SMD -0.67; 95% CI: -0.91, -0.42) and independent (SMD -0.44; 95% CI: -0.66, -0.23) reports of outcome showed significant differences favouring the intervention group. There was insufficient evidence to determine the relative effectiveness of different approaches to delivering parenting programmes.ConclusionParenting programmes are an effective treatment for children with conduct problems. The relative effectiveness of different parenting programmes requires further research.

Turkish Visitors to Italy: Notes on the Cultural Package Tourist

This research analyzes characteristics and economic aspects of culture-oriented package tourism focusing on Turkish visitors traveling to Italy. Data references the package tour arrangements covering 2,278 visitors traveling in 82 package groups. This information was gathered via random sampling in 2003. Although the number of travelers in each group ranged from 8 to 54, the average group size was 27.78. Seven- and four-night packages were the standard packages having 57.95% and 31.96% shares, respectively, followed by three-night packages at 4.08%. Results show that the mean total cost of holiday per person was €260.00 (US$308.20) and mean total cost per person per night was €45.37 (US$53.78). Study results also indicate that if the number of nights stayed increased, the mean cost of package per person per night decreased. The overall findings contribute toward a better understanding of the Turkish package tourism characteristics and show that package tourism generates economies of scale that keep the prices relatively low and allow more people—especially from developing countries, in this case being Turkey—to be involved in international tourism. Finally, the paper suggests future research.

Predicting global abundance of a threatened species from its occurrence: implications for conservation planning

AbstractAim Global abundance is an important characteristic of a species that is correlated with geographical distribution and body size. Despite its importance these estimates are not available since reliable field estimates are either expensive or difficult to obtain. Based on the relationship between a species’ local abundance and distribution, some authors propose that abundance can be obtained through spatial distribution data from maps plotted at different scales. This has never been tested over the entire geographical range of a species. Thus, the aim of this study was to estimate global abundance of the Neotropical primateBrachyteles hypoxanthus(northern muriqui) and compare the results with available field estimates.Location From southern Bahia to Minas Gerais and Espírito Santo states, in the Brazilian Atlantic rain forest.Methods We compiled 25 recent occurrence localities ofB. hypoxanthusand plotted them in grid cells of five different sizes (1, 25, 50, 75 and 100 km per side) to evaluate the performance and accuracy of abundance estimates over a wide range of scales. The abundance estimates were obtained by the negative binomial distribution (NBD) method and corrected by average group size to take into account primate social habits. To assess the accuracy of the method, the predicted abundances were then compared to recent independent field estimates.Results The NBD estimates were quite accurate in predictingB. hypoxanthusglobal abundance, once the gregarious habits of this species are taken into account. The predicted abundance estimates were not statistically different from those obtained from field estimates.Main conclusions The NBD method seems to be a quick and reliable approach to estimate species abundance once several limiting factors are taken into account, and can greatly impact conservation planning, but further applications in macroecological and ecological theory testing needs improvement of the method.

Population Status and Conservation of Capped Langurs (Trachypithecus pileatus) in and Around Pakke Wildlife Sanctuary, Arunachal Pradesh, India

A survey of the distribution and population status of the capped langur (Trachypithecus pileatus) was conducted in and around the Pakke Wildlife Sanctuary in Arunachal Pradesh, India. From 2001–2003, we surveyed ten selected localities, recording 26 groups and a total of 195 individuals. The data was obtained using line transect surveys and total count methods. Capped langurs were found in a number of different habitats: tropical semi-evergreen forest and moist deciduous forests. Of the 195 individuals registered, 14% were adult males, 52% adult females, 2% unidentified adults, 7% sub-adults, 11% juveniles and 15%) infants. The smallest group numbered 3, and the largest 13, with an average group size of 7.5 individuals. The male-female ratio was 1:3.6. The most common size class of the group was of 7–9 individuals.

Relationship between group size and feeding success of cattle egrets (Bubulcus ibis) in the central Free State

Regardless of habitat or time of day, cattle egrets feeding independently of hosts generally occurring larger groups than those feeding in close association with ungulates. The average group size of three individuals feeding in association with hosts stays remarkably constant with regard to divergent situations. Based on the type of habitat and the grazing speed of the host concerned, cattle egrets achieve the highest feeding success (number of prey items) with the least energy inputs (number of paces) in association with cattle, and to a lesser extent with the closely related buffalo. Compared to solitary birds, cattle egrets feeding in groups experience without exceptional higher feeding success. Results of feeding experiments, as well as the exceptional occurrence of so-called feeding lines, confirm the phenomenon that the feeding success of cattle egrets correlates closely with the size of the feeding group.

Estimating animal density using camera traps without the need for individual recognition

Summary 1Density estimation is of fundamental importance in wildlife management. The use of camera traps to estimate animal density has so far been restricted to capture–recapture analysis of species with individually identifiable markings. This study developed a method that eliminates the requirement for individual recognition of animals by modelling the underlying process of contact between animals and cameras. 2The model provides a factor that linearly scales trapping rate with density, depending on two key biological variables (average animal group size and day range) and two characteristics of the camera sensor (distance and angle within which it detects animals). 3We tested the approach in an enclosed animal park with known abundances of four species, obtaining accurate estimates in three out of four cases. Inaccuracy in the fourth species was because of biased placement of cameras with respect to the distribution of this species. 4Synthesis and applications. Subject to unbiased camera placement and accurate measurement of model parameters, this method opens the possibility of reduced labour costs for estimating wildlife density and may make estimation possible where it has not been previously. We provide guidelines on the trapping effort required to obtain reasonably precise estimates.

Population demography and social structure changes in Eulemur fulvus rufus from 1988 to 2003

Eulemur fulvus rufus has been described as having stable multi-male/multi-female groups, a male-biased sex ratio, and female philopatry. However, in a 16-year study of this subspecies we documented a great deal of demographic change as several groups permanently fissioned, some groups disappeared, and new groups formed. We split the dataset into two periods, 1988 to 1993 and 1994 to 2003, which coincided with the first disappearance of a study group (in August 1994) and the first permanent group fission (in December 1994). The average group size decreased by nearly half between the study periods (10.5-5.6), while the frequency of group membership changes increased (2.0-8.3 times/year), and the birth rate decreased (0.56-0.38). Females, as well as males, immigrated into study groups and transferred between groups, something that has been rarely seen in this subspecies. We also found a significant decline in the amount of fruit from the earliest part of the study to the latter part of the study. Study groups did not switch to other types of foods during periods of fruit shortage, but traveled outside of their home range areas more often over the study period. Finally, the density E. f. rufus decreased in the study area while the densities of their main food competitors, Varecia variegata and Eulemur rubriventer, increased. Although few primate populations are numerically stable over time, we suggest that female behavioral responses to decreases in fruit availability may have influenced some of the demographic changes we witnessed in this study.

Primate Group Size and Abundance in the Caatinga Dry Forest, Northeastern Brazil

The Caatinga dry forest poses a series of ecological challenges for mammals in general and primates in particular. The erratic rainfall pattern impacts on plant diversity and phenological patterns; from year to year there is marked variability in fruit production and failure to fruit is common. The harshness apparently accounts for the impoverished mammalian fauna. However, data on primate abundance, distribution, and possible environmental effects on primate density are lacking in this type of dry forest. I censused the primate community in 3 habitats of the Serra da Capivara National Park, Piaui, NE Brazil, over a total distance of 318 km. Overall, the abundance of primates in the Caatinga dry forest is very low as a consequence of low abundance of food resources both in space and time. Alouatta caraya (predominantly folivorous) occurs at extremely low density, and during the dry season are apparently confined to canyon areas, where trees retain their leaves. Callithrix jacchus has morphological feeding specializations for gum-eating, and gum is an important resource during food bottleneck periods. Nonetheless, Callithrix jacchus occurs at comparatively low densities. Group sizes for howlers and marmosets in the Caatinga are significantly smaller than in other forest types. Contrarily, Cebus apella libidinosus had an average group size within the range reported for Amazonian and Atlantic forests. Researchers consider the generalized diet of capuchins as the explanation for their similar abundance in different habitats, indicating relative independence from ecological constraints. However, I suggest that capuchin foraging style and cognitive abilities are important factors accounting for their unreduced group size and density even under extreme conditions.

Social and Play Behavior in a Wild Eastern Coyote, <em>Canis latrans</em>, Pack

I had close and consistent observations of a wild eastern Coyote pack (Canis latrans) from January 2000 to August 2007. During this time, I obtained 3156 radio-locations on a specific radio-collared breeding male (“Sill”) and observed him and/or members of his pack on 375 occasions. The average group size = 3.0 ± 2.3 (SD) Coyotes with 1.9 ± 1.2 (SD) being adults and 1.1 ± 1.9 being pups. Maximal group size involved 12 Coyotes (9 pups, 3 adults). During these observations, Coyotes most often behaved in a friendly manner toward each other as indicated by 80 of my observations involving play between pups, and 15 involving play among adult Coyotes. On the evening of 6 July 2007 I observed the breeding male (>8 yr old), his mate (>5 yr old), one of their full-sized probable yearlings, and five pups playing intensely for 33 minutes. This paper details social and play behavior from this pack, especially from the 6 July 2007 observation.

Survey of Helminth Parasites in Populations of Alouatta palliata mexicana and A. pigra in Continuous and in Fragmented Habitat in Southern Mexico

The 2 howler species that occur in southern Mexico, Alouatta palliata mexicana and Alouatta pigra are endangered, mainly as a result of habitat loss and fragmentation from human activity. Little is known about the gastrointestinal parasite communities affecting their populations, and lack of baseline information for populations of howler species in continuous forest habitats, makes evaluations of gastrointestinal parasite prevalence in populations in fragmented landscapes difficult. We report the results of a one-time broad survey of gastrointestinal parasites in fecal samples of individuals from several demographically stable populations of Alouatta palliata mexicana and A. pigra existing in continuous and/or protected forests. We further report similar data for populations of both species in human-fragmented landscapes. We detected 6 parasites for each howler monkey species, but only 3 of them (Trematode I, Controrchis biliophilus, Trypanoxyuris sp.) were common to both species. While parasitic prevalence in populations of both howler species was, in general, higher in the fragmented habitat than in continuous and/or protected forests. The difference is only marginally significant in Alouatta pigra. Some parasites (Coccidia and Strongylid) only appeared in populations in fragmented landscapes. Preliminary data suggest that adult males tended to have higher parasite prevalence values than those of adult females in both howler species. Parasite prevalence is associated to average group size, but not to population density in Alouatta pigra.

Non-additive coupling enables stable propagation of synchronous spiking in purely random networks

Precise timing of spikes and synchronization in the millisecond range has been experimentally observed in different neuronal systems. Their occurrence correlates with external stimuli and is thus considered a key feature of neural computation. The dynamical origin of precise and coordinated spike timing, however, is not well understood. Here we show in a modeling study that synchronous spiking activity of subgroups can persist and propagate in purely random networks if we take into account the non-additive nature of dendritic input integration that was recently uncovered experimentally. We find a transition to stable propagation of synchronized spiking: For additive coupling and at low strength of non-additivity, synchronous spiking dies out; above a critical strength, stable propagation of a group of synchronized spikes becomes possible. We derive a map giving the average future size of a synchronous group in terms of the current group size. For networks with homogeneous parameters the map can be obtained analytically and reveals that the discontinuous transition found is due to a tangent bifurcation at a critical strength of non-additivity. We discuss the mechanism underlying this transition and its consequences for networks with inhomogeneous parameters and additional external noise. Prominent 'synfire-chain' models for the stable propagation of synchronous activity in cortical networks require the existence of feed-forward structures that are superimposed on otherwise randomly connected local cortical circuits. It is unclear, however, whether feed-forward structures actually exist in such local circuits. Our study suggests that additional structural features of the network connectivity may not be required for the propagation of synchronous spiking activity if synaptic interactions exhibit non-additive dendritic integration.