Effects of vulture declines on facultative scavengers and potential implications for mammalian disease transmission

Vultures (Accipitridae and Cathartidae) are the only known obligate scavengers. They feed on rotting carcasses and are the most threatened avian functional group in the world. Possible effects of vulture declines include longer persistence of carcasses and increasing abundance of and contact between facultative scavengers at these carcasses. These changes could increase rates of transmission of infectious diseases, with carcasses serving as hubs of infection. To evaluate these possibilities, we conducted a series of observations and experimental tests of the effects of vulture extirpation on decomposition rates of livestock carcasses and mammalian scavengers in Kenya. We examined whether the absence of vultures changed carcass decomposition time, number of mammalian scavengers visiting carcasses, time spent by mammals at carcasses, and potential for disease transmission at carcasses (measured by changes in intraspecific contact rates). In the absence of vultures, mean carcass decomposition rates nearly tripled. Furthermore, the mean number of mammals at carcasses increased 3-fold (from 1.5 to 4.4 individuals/carcass), and the average time spent by mammals at carcasses increased almost 3-fold (from 55 min to 143 min). There was a nearly 3-fold increase in the mean number of contacts between mammalian scavengers at carcasses without vultures. These results highlight the role of vultures in carcass decomposition and level of contact among mammalian scavengers. In combination, our findings lead us to hypothesize that changes in vulture abundance may affect patterns of disease transmission among mammalian carnivores.     

Competition with flies promotes communal breeding in the burying beetle,Nicrophorus tomentosus

Communal breeding through nest-sharing may benefit cooperating individuals indirectly, in increased inclusive fitness, or directly, when environmental constraints reduce the fitness of solitary breeders. Burying beetles provide extensive parental care and can breed either in pairs or in larger groups of unrelated males and females. Parentage of communally-reared broods is usually shared but is skewed in favor of the individuals of each sex that provide longer care. Females provide care longer than males, and two females are more likely to remain together in the brood chamber than two males are. Flies and other burying beetles are the major competitors for carcasses and this study suggests that it is competition with flies that promotes communal breeding inNicrophorus tomentosus On medium-size carcasses (35–40 g) the presence or absence of oviposition by flies had a significant effect on the size of the brood reared, and on large carcasses (55–60 g) the number of beetles present, two or four, had a significant effect on brood size. On both medium and large carcasses, pairs rearing broods on flyblown carcasses had fewer young than pairs on clean carcasses or foursomes on flyblown carcasses. There was a strong trend for an interaction effect between number of beetles and competition with flies (Table 1). Duration of parental care was not affected by competition with flies except for that of the first male to depart, which provided care longer on flyblown carcasses (Table 2). Pairs and foursomes were equally able to defend the carcass and brood from conspecific intruders and from larger intrudingNicrophorus orbicollis (Table 3).     

Microbial decomposition of proteins and lipids in copepod versus rotifer carcasses

Zooplankton carcasses are common within aquatic systems and potentially serve as organic-rich substrates for bacteria. We compared the microbial decomposition of representative crustacean (copepod) and non-crustacean (rotifer) zooplankton carcasses and monitored changes in carcass protein and lipid contents. Our results showed that carcass decomposition was mainly driven by bacteria colonizing from the surrounding water. Carcass-associated bacteria displayed higher protease and lipase activities than free-living bacteria. Protein content of copepod carcasses decreased by 70% within the first 8 h and shifted from larger to smaller sized proteins, while protein loss in rotifer carcasses was insignificant. Carcass lipid content did not change significantly over 24 h in either zooplankton type, although polar branched fatty acids increased on copepod carcasses indicating an increase in viable microbial biomass. Our results suggest differential turnover of protein versus lipid within a zooplankton carcass and that carcasses from different zooplankton groups would affect water column microbial processes differently.     

Occurrence of large numbers of carcasses of the large,grazing copepod Calanus cristatus from the Japan Sea

Carcasses of Calanus cristatus were discovered in plankton samples collected from the Japan Sea throughout the year from 1970 to 1985. Many carcasses of copepodite Stages IV and V occurred in the layer between 15 and 300 m below a distinct thermocline. The number of copepodite Stage V carcasses also peaked in the layer between 1 500 and 2 000 m. The highest density of copepodite Stages IV and V carcasses was 169 individuals per 1 000 m³ and 1 573 individuals per 1 000 m³, respectively. Carcasses of adults occurred at depths below 500 m and numbers of males and females per 1 000 m³ were 1 to 16 and 1 to 42, respectively. Living males were larger in catch number than living females, but the relationship for carcasses was the opposite. Weight of carcasses was 15 to 25% of living C. cristatus. Carcasses contained about 51% carbon and 8% nitrogen by weight. Carcasses may have been drifting for more than one year in the epipelagic layer under the thermocline because of their slow decomposition rate.     

Sea Otter Mortality Estimated from Carcasses Collected after the Exxon Valdez Oil Spill

Large numbers of sea otters (  Enhydra lutris) died following the 1989 Exxon Valdez oil spill. Two previous studies estimated spill-related mortality, one from the difference between counts of otters before and after the spill, and the other from the recovery rate of tagged carcasses that had been released at sea. I used a derivative of the second approach, but revised it to account for (1) moribund otters that had hauled out on shore; (2) carcasses that were collected at sea; and (3) differences in search effort in different areas of the spill—considerations that were not addressed previously. The mortality estimation procedure presented here, and applied to Prince William Sound, Alaska, had six input parameters, two with fixed values (the number of spill-related carcasses collected [391] and the number of otters taken alive that died in captivity [84]) and four that were assigned a range of values (the recovery rate for carcasses on the beach [60–90%], the proportion of recovered carcasses that were onshore [70–95%]), the proportion of offshore deaths recovered onshore (20–50%), and the proportion of moribund otters that hauled out (20–60%). Empirical estimates for these parameters were not available after the spill, but a range of plausible values for each was delimited using other available data. Randomly-selected combinations of values within these ranges produced mortality estimates for the sound ( x¯= 750, 5–95% quantiles ≈ 600–1000) that were lower than indicated by previous studies. These lower estimates do not diminish the tragic nature of the mortality, but instead highlight the significance of both the process and informational basis in producing estimates of catastrophic loss. Estimates of loss based on faulty or inadequate data may mislead investigations of population recovery and may jeopardize public trust in scientific assessments of future catastrophes.     

Brevetoxin exposure in bottlenose dolphins (Tursiops truncatus) associated with Karenia brevis blooms in Sarasota Bay, Florida

Bottlenose dolphins (Tursiops truncatus) face a variety of threats, including risk of exposure to brevetoxins produced by blooms of the harmful alga Karenia brevis. This study investigated brevetoxin exposure in a population of dolphins inhabiting Sarasota Bay, Florida, USA (27°N, 82°W), utilizing tissues from dolphins recovered between 1994 and 2003. Brevetoxin levels detected by ELISA in tissues, gastric samples and excreta from dolphin carcasses (n = 19) associated with K. brevis blooms were compared with with levels in carcasses (n = 16) associated with background K. brevis conditions. In the K. brevis-exposed set, 84% of dolphin carcasses recovered during K. brevis blooms had detectable brevetoxin levels, with values ranging between 7 and 2,896 ng PbTx-3 eq g⁻¹. Over 50% of dolphin carcasses recovered during non-bloom conditions also tested positive by ELISA for brevetoxins, with concentrations ranging from 6 to 44 ng PbTx-3 eq g⁻¹. Control samples from the east coast of Florida were negative by the ELISA. Results from this study establish baseline brevetoxin body burdens in a dolphin population frequently exposed to K. brevis blooms, and data indicate that dolphin carcasses not associated with large-scale mortality events can contain levels of brevetoxins comparable to carcasses stranding during such events.     

Effects of sex-associated competitive asymmetries on foraging group structure and despotic distribution in Andean condors

Phenotype-limited interference models assume competitive asymmetries among conspecifics and unequal sharing of resources. Their main prediction is a correlation between dominance status and patch quality: dominant individuals should preferentially exploit better-quality habitats. We tested assumptions and predictions of the phenotype-limited interference model in Andean condors (Vultur gryphus), a New World vulture with strong sexual size dimorphism (males are 30–40% heavier than females). We recorded searching birds in habitats differing in quality: mountains and plains. We also observed scavenging behaviour at 20 sheep carcasses, and videotaped 5 of them. Intraspecific hierarchy at carcasses was based on size: males dominated females and, within each sex, older birds dominated younger ones. Adult males and juvenile females occupied extreme positions in the feeding hierarchy. Aggression was directed at those individuals belonging to lower hierarchical levels. In high-quality areas (mountains), more condors arrived at carcasses. Juvenile females were more often observed searching in low-quality areas (plains), far from breeding areas and main roost sites. GLM analyses of individual behaviour showed that the hierarchy did not influence time of arrival, but low-ranking individuals spent more time at carcasses, especially if the number of condors at arrival was high. Additionally, low-ranking condors spent less time feeding at carcasses when individuals of higher hierarchical levels were present. On the other hand, the number of condors present had a positive effect on feeding rates of dominant individuals, probably because of a reduction in individual vigilance. These results support most of the assumptions and predictions of the phenotype-limited distribution model, although a spatial truncated distribution between phenotypes was not observed. Asymmetric feeding pay-off, unequal parental roles and sexual selection constraints could favour sexual divergence in body size in Andean condors. Received: 6 April 1998 / Accepted after revision: 11 July 1998     

Community structure and vertical distribution of cyclopoid copepods in the Red Sea

The species abundance, vertical distribution and diurnal vertical migration of cyclopoid copepods was analyzed in the central Red Sea in October–November 1980. Samples were taken to a depth of 450 m with a multiple opening — closing plankton net with 0.1-mm mesh-size. Selected important species were allocated to five different groups according to their depth distributions during daytime. The greatest number of species (9) was found in the lower epipelagic zone (40 to 100 m), below the strong seasonal thermocline. The lowest number of species (1) occurred in the upper part of the upper mesopelagic zone (100 to 250 m), which is characterized by a strong dissolved oxygen gradient. Five species had a bimodal vertical distribution, with dual peak abundances in the epipelagic and upper mesopelagic zones. Distinct differences in distribution patterns were noted between sexes and/or developmental stages. The vertical range of diurnally migrating species was small, usually less than 50 to 100 m. Characteristic diurnal changes in the vertical succession of dominant species occur within the epipelagic zone (0 to 100 m). Species-specific vertical distribution patterns are compared with published data from other areas. A conspicuous difference in the proportion of carcasses was noted between species: small species (<0.5 mm in length) had a much higher proportion of carcasses, usually between 20 and 40% of the total standing stock, than larger ones (<5%). The potential causes of this phenomenon, which may be due to (1) methodological bias, (2) a lower sinking velocity of small carcasses, or (3) a higher mortality rate of small species, are discussed.     

Experimental tests of the information-center hypothesis with black vultures (Coragypsatratus) and turkey vultures (Cathartesaura)

In field tests of the information-center hypothesis (ICH) in south Texas with black vultures (Coragyps atratus) and turkey vultures (Cathartes aura), large carcasses were provided and kept under continuous observation. The use vultures made of these bait sites and their patterns of arrival were recorded to evaluate predictions derived from the ICH. Turkey vultures discovered most bait sites (30 of 31) first, but frequently were displaced from the food by later-arriving black vultures. This competitive exclusion by black vultures limited subsequent feeding opportunities for turkey vultures sufficiently that few (27%) returned on subsequent days to bait sites they had previously visited. I found no evidence that those turkey vultures that did return to bait sites acted as leaders for groups of naive birds and led them to bait sites – knowledgeable and naive turkey vultures did not arrive at bait sites together, and groups arriving at bait sites were not larger on subsequent days than on the first days carcasses were available. In contrast, a significantly larger percentage (47%) of knowledgeable black vultures returned to bait sites they had visited on previous days, and the first groups of black vultures arriving at bait sites on subsequent days were significantly larger than the equivalent groups on first days. Nine flocks of black vultures that arrived on subsequent days at bait sites before sunrise (which suggests the birds had commuted directly from a roost) contained knowledgeable birds, and two of these flocks contained both knowledgeable and naive individuals. Overall, 10 of 54 naive tagged black vultures (18.5%) arrived at bait sites under circumstances that suggested they had followed conspecifics to the food from a roost. However, most black vultures apparently found carcasses through independent search or by using local enhancement. Therefore, I conclude that while following from roosts to food sites is a strategy used by black vultures, at this study site it is one they use relatively infrequently. Received: 20 February 1997 / Accepted after revision: 28 June 1997     

Bushmeat Markets on Bioko Island as a Measure of Hunting Pressure

Abstract: Counts of the number of animal carcasses arriving at Malabo market, Bioko Island, Equatorial Guinea, were made during two, 8-month study periods in 1991 and 1996. Comparisons of the availability and abundance of individual species between years showed that more species and more carcasses appeared in 1996 than in 1991. In biomass terms, the increase was significantly less, only 12.5%, when compared with almost 60% more carcasses entering the market in 1996. A larger number of carcasses of the smaller-bodied species (i.e., rodents and the blue duiker [ Cephalophus monticola ] ) were recorded in 1996 than in 1991. Although an additional four species of birds and one squirrel were recorded in 1996, these were less important in terms of their contribution to biomass or carcass numbers. Concurrently, there was a dramatic reduction in the larger-bodied species, Ogilby's duiker ( C. ogilbyi ) and seven diurnal primates. We examined these changes, especially the drop in the number of larger animals. We considered the possible following explanations: (1) the number of hunters dropped either because of enforced legislation or scarcity of larger prey; (2) a shift in the use of hunting techniques occurred (   from shotguns to snares); or (3) consumer demand for primate and duiker meat dropped, which increased demand for smaller game. Our results suggest that the situation in Bioko may be alarmingly close to a catastrophe in which primate populations of international conservation significance are being hunted to dangerously low numbers. Although there is still a need for surveys of actual densities of prey populations throughout the island, working with the human population on Bioko to find alternatives to bushmeat is an urgent priority.     

美国和五大湖双边共同关注区域的白肚燕(双色树燕)雏鸟的有机污染状况

白肚燕,又名双色树燕,栖居于五大湖流域中27个双边共同关注区域(AOCs),从2010年到2014年对其进行了污染物暴露风险评估,以协助管理者和监管者评估五大湖AOCs的污染状况。本文比较了AOCs和附近非AOCs雏鸟体内污染物浓度的差异。AOCs白肚燕雏鸟尸体中多氯联苯(PCB)和多溴联苯醚的含量分别为30%和33%,低于非AOCs的平均浓度。AOCs白肚燕雏鸟胃含物中多环芳烃(PAH)浓度和血浆中全氟化合物浓度分别为67%和64%,也低于非AOCs的平均浓度。但是与已有生殖效应记载的高PCB污染区相比,有些AOCs雏鸟尸体内的PCBs浓度只是存在小幅升高。一些AOCs食物中PAHs的浓度足够高以致引起可测量的生理响应。在AOCs中,全氟化合物全氟辛烷磺酸在血浆中的最高浓度出现在Raisin河(密歇根州,美国;几何平均数330 ng/mL),但远低于预估毒性的参考值(1 700 ng/mL)。之前有研究报道雏鸟胃含物中PAH和PCB的浓度以及尸体中PCBs的含量与沉积物中污染物的含量显著相关,从而可加强白肚燕在评估沉积物污染生物有效性方面的应用。
精选自Thomas W. Custer, Christine M. Custer, Paul M. Dummer, Diana Goldberg, J. Christian Franson, Richard A. Erickson. Organic contamination in tree swallow (Tachycineta bicolor) nestlings at United States and binational Great Lakes areas of concern. Environmental Toxicology and Chemistry: Volume 36, Issue 3, pages 735–748, July 2017. DOI: 10.1002/etc.3598
详情请见http://onlinelibrary.wiley.com/wol1/doi/10.1002/etc.3598/full
     

Reproductive dominance and differential ovicide in the communally breeding burying beetle Nicrophorus tomentosus

Communal breeding can be characterized by the division of reproduction among cooperating individuals and, if the distribution of reproduction is inequitable, by the mechanisms for achieving skewed reproductive success. The burying beetle (Nicrophorus tomentosus) is a facultative communal breeder. Unrelated adults, especially females, provide extensive parental care to broods of mixed parentage. The frequency and degree of reproductive skew between two females were examined experimentally. On medium-size carcasses, the proportion of eggs attributed to each female was not significantly different from random in 42% of the broods, skewed in 42% and not shared in 16%. Although reproduction was usually skewed in favor of the larger female, the relative sizes of the two females did not predict the degree of skew. On large carcasses, the proportion of eggs attributed to each female was not different from random in 87% of the broods and weakly skewed in 13%. Several mechanisms for biasing reproductive success were investigated. Females increase the proportion of their offspring in the brood by committing differential ovicide. Secondly, burying and preparing a carcass cooperatively stimulates ovarian development of the larger female and slows it for the smaller female, reducing or delaying oviposition by the subordinate. Thirdly, larger females are more likely to be dominant and are more fecund than smaller females. Received: 20 July 1996 / Accepted after revision: 30 November 1996     

Social dominance in migrant and resident turkey vultures at carcasses: evidence for a despotic distribution?

We placed carcasses in three different vegetation types in the heterogeneous savannas of central Venezuela to investigate the role of social dominance in habitat use by flocking migrant and resident turkey vultures (Cathartes aura meridionalis and C. a. ruficollis). Migrants foraged primarily in savanna habitats while residents foraged almost exclusively in gallery forest. In the gallery forest residents discovered carrion first significantly more often than migrants, despite there being equal densities of residents and migrants foraging over this habitat. Because residents fed in smaller groups than migrants at carcasses they had higher feeding rates. There was also a negative relationship between group sizes of residents and migrants. The feeding rate of residents declined in response to increased group size of migrants, but group size of residents had no effect. Migrant group size also had a greater effect on resident feeding rates than king vulture presence or absence. When the effect of migrant and resident group size on feeding rates in migrants was compared, the most significant factor was migrant group size. A second analysis showed that both resident group size and presence or absence of king vultures had a significant effect on feeding rates in migrants. Rates of agonistic encounters in migrant and resident turkey vultures increased weakly in relation to group size. However, there was an increase in residents' encounter rate with migrants in relation to increased migrant group size; there was no difference in resident encounter rates with other residents in relation to resident group size. Migrants dominated residents in almost all agonistic interactions over carcasses. We suggest that savanna habitats were less attractive to residents for foraging because they held larger groups of migrants.     

Copepod carcasses in the upwelling region south of cap blanc,N.W. Africa

High numbers of copepod carcasses were found in zooplankton samples taken in the upwelling system off N.W. Africa. The validity of the measurements is discussed. It is suggested that rapid changes in the environment, caused by the pulsation of violent mesoscale upwelling at the shelf edge, result in an increased mortality in zooplankton. Considerations are made on the role of copepod detritus in the upwelling region.     

Female-coerced monogamy in burying beetles

The reproductive interests of the sexes often do not coincide, and this fundamental conflict is believed to underlie a variety of sex-specific behavioral adaptations. Sexual conflict in burying beetles arises when a male and female secure a carcass that can support more offspring than a single female can produce. In such a situation, any male attracting a second female sires more surviving offspring than he would by remaining monogamous, whereas the female's reproductive success decreases if a rival female is attracted to the carcass. Monogamously paired males on large carcasses do in fact attempt to attract additional females by means of pheromone emission, whereas males on small carcasses do not. Females physically interfere with male polygynous signaling using various behavioral tactics. We demonstrate that such interference leads to a significant decrease in the amount of time that males spend signaling, according females a means by which to impose monogamy on their mates.     

Correlates of bushmeat in markets and depletion of wildlife

We used data on number of carcasses of wildlife species sold in 79 bushmeat markets in a region of Nigeria and Cameroon to assess whether species composition of a market could be explained by anthropogenic pressures and environmental variables around each market. More than 45 mammal species from 9 orders were traded across all markets; mostly ungulates and rodents. For each market, we determined median body mass, species diversity (game diversity), and taxa that were principal contributors to the total number of carcasses for sale (game dominance). Human population density in surrounding areas was significantly and negatively related to the percentage ungulates and primates sold in markets and significantly and positively related to the proportion of rodents. The proportion of carnivores sold was higher in markets with high human population densities. Proportion of small‐bodied mammals (<1 kg) sold in markets increased as human population density increased, but proportion of large‐bodied mammals (>10 kg) decreased as human population density increased. We calculated an index of game depletion (GDI) for each market from the sum of the total number of carcasses traded per annum and species, weighted by the intrinsic rate of natural increase (r_max) of each species, divided by individuals traded in a market. The GDI of a market increased as the proportion of fast‐reproducing species (highest r_max) increased and as the representation of species with lowest r_max (slow‐reproducing) decreased. The best explanatory factor for a market's GDI was anthropogenic pressure—road density, human settlements with >3000 inhabitants, and nonforest vegetation. High and low GDI were significantly differentiated by human density and human settlements with >3000 inhabitants. Our results provided empirical evidence that human activity is correlated with more depleted bushmeat faunas and can be used as a proxy to determine areas in need of conservation action.     

Demography of the California Condor: Implications for Reestablishment

Abstract: The remnant wild population of California Condors ( Gymnogyps californianus ) of the 1980s exhibited a rapid population decline caused by high mortality rates among adult and immature birds. The most prominent mortality factor was lead poisoning resulting from ingestion of bullet fragments in carcasses. Successful captive breeding has allowed many birds to be released to the wild since 1992, based originally on an assumption that exposure to lead could be prevented by food subsidy. The mortality of released birds, however, has generally exceeded levels needed for population stability calculated from simple population models. Collision with overhead wires was the most frequent cause of death in releases before 1994. Lead poisoning again surfaced as a problem starting in 1997 as older birds began feeding on carcasses outside the subsidy program. Although poisonings have been treated successfully by chelation therapy in recaptured birds, food subsidy is proving an ineffective solution to lead exposure. The best long-term solution appears to be either the creation of large reserves where hunting is prohibited or the restriction of hunting to nontoxic ammunition in release areas. Until sources of lead contamination are effectively countered, releases cannot be expected to result in viable populations. In addition, problems involving human-oriented behavior have resulted in the permanent removal of many released birds from the wild. The most promising reduction in human-oriented behavior has been achieved in one release of aversively conditioned, parent-reared birds. Rigorous evaluation of the factors reducing attraction to humans and human structures has been hampered by confounding of techniques in releases. Behavioral problems could be more quickly overcome by adoption of a comprehensive experimental approach.     

Turning up the heat: temperature influences the relative importance of top-down and bottom-up effects

Understanding how communities respond to changes in temperature is a major challenge for community ecology. Temperature influences the relative degree to which top-down and bottom-up forces structure ecological communities. In greenhouse experiments using the aquatic community found in pitcher plants (Sarracenia purpurea), I tested how temperature affected the relative importance of top-down (mosquito predation) and bottom-up (ant carcasses) forces on protozoa and bacteria populations. While bottom-up effects did not vary consistently with temperature, the top-down effects of predators on protozoa increased at higher temperatures. These results suggest that temperature could change the relative importance of top-down and bottom-up effects in ecological communities. Specifically, higher temperature may increase the strength of top-down effects by raising predator metabolic rate and concomitant processes (e.g., activity, foraging, digestion, growth) relative to cooler temperatures. These findings apply broadly to an understanding of trophic interactions in a variable environment and are especially relevant in the context of ongoing climate change.     

Reproductive benefits of infanticide in a biparental burying beetle Nicrophorus orbicollis

Summary Single females and pairs of Nicrophorus orbicollis Say were established as residents on carcasses to investigate interactions with intruders under natural conditions. When carcasses were exhumed 10–12 days later, one or more intruders were found on 19% of nests. Intruders expelled one or both of the residents and sometimes paired with a resident or another intruder of the opposite sex. An intruder tended to be larger than the same-sex adult it replaced on a carcass. A male intruder that joined a single female, on the other hand, was not necessarily larger than the resident female. There was indirect evident that intruders committed infanticide and then started their own reproductive attempt on the carcass. Follow-up experiments in the laboratory indicated that interactions between intruders and residents were agonistic and that infanticide regularly occurred as a consequence of a takeover. Use of a genetic marker demonstrated that male and female intruders obtained reproductive benefits from infanticide. However, intruders sometimes cared for mixed broods consisting of their own young and young of a prior resident.     

Joint breeding in female burying beetles

Summary Burying beetles (Nicrophorus) exhibit advanced parental care, by feeding and guarding their offspring on buried vertebrate carrion. Till now, interactions between two conspecific females on a carcass have been thought to be mostly competitive, and parental care was thought to be provided by single females or male-female pairs exclusively. Here we demonstrate that cooperative brood care occurs in this species, and that its incidence is contingent on carcass size. Small carcasses are usually monopolized by one female; typically the larger of two female combatants secures the carcass for her offspring (Figs. 1 and 2). On large carcasses fights still occur, but in most cases both females stay on the carcass long enough to provide care for the brood. The use of genetic markers revealed that the maternity of offspring is shared evenly among joint breeders (Figs. 3, 4). We hypothesize that cooperative breeding is an adaptive response to a situation that arises partly as a consequence of a physical constraint. Correspondence to: J.K. Müller