Rich Pickings Near Large Communal Roosts Favor ‘Gang’ Foraging by Juvenile Common Ravens, Corvus corax

Ravens (Corvus corax) feed primarily on rich but ephemeral carcasses of large animals, which are usually defended by territorial pairs of adults. Non-breeding juveniles forage socially and aggregate in communal winter roosts, and these appear to function as ‘information centers’ regarding the location of the rare food bonanzas: individuals search independently of one another and pool their effort by recruiting each other at roosts. However, at a large raven roost in Newborough on Anglesey, North Wales, some juveniles have been observed recently to forage in ‘gangs’ and to roost separately from other birds. Here we adapt a general model of juvenile common raven foraging behavior where, in addition to the typical co-operative foraging strategy, such gang foraging behavior could be evolutionarily stable near winter raven roosts. We refocus the model on the conditions under which this newly documented, yet theoretically anticipated, gang-based foraging has been observed. In the process, we show formally how the trade off between search efficiency and social opportunity can account for the existence of the alternative social foraging tactics that have been observed in this species. This work serves to highlight a number of fruitful avenues for future research, both from a theoretical and empirical perspective

Unrelated Helpers in a Primitively Eusocial Wasp: Is Helping Tailored Towards Direct Fitness?

The paper wasp Polistes dominulus is unique among the social insects in that nearly one-third of co-foundresses are completely unrelated to the dominant individual whose offspring they help to rear and yet reproductive skew is high. These unrelated subordinates stand to gain direct fitness through nest inheritance, raising the question of whether their behaviour is adaptively tailored towards maximizing inheritance prospects. Unusually, in this species, a wealth of theory and empirical data allows us to predict how unrelated subordinates should behave. Based on these predictions, here we compare helping in subordinates that are unrelated or related to the dominant wasp across an extensive range of field-based behavioural contexts. We find no differences in foraging effort, defense behaviour, aggression or inheritance rank between unrelated helpers and their related counterparts. Our study provides no evidence, across a number of behavioural scenarios, that the behaviour of unrelated subordinates is adaptively modified to promote direct fitness interests

Helpful Female Subordinate Cichlids Are More Likely to Reproduce

BACKGROUND: In many cooperatively breeding vertebrates, subordinates assist a dominant pair to raise the dominants' offspring. Previously, it has been suggested that subordinates may help in payment for continued residency on the territory (the 'pay-to-stay hypothesis'), but payment might also be reciprocated or might allow subordinates access to reproductive opportunities. METHODOLOGY/PRINCIPAL FINDINGS: We measured dominant and subordinate female alloparental brood care and reproductive success in four separate experiments and show that unrelated female dominant and subordinate cichlid fish care for each other's broods (alloparental brood care), but that there is no evidence for reciprocal 'altruism' (no correlation between alloparental care received and given). Instead, subordinate females appear to pay with alloparental care for own direct reproduction. CONCLUSIONS/SIGNIFICANCE: Our results suggest subordinate females pay with alloparental care to ensure access to the breeding substrate and thereby increase their opportunities to lay their own clutches. Subordinates' eggs are laid, on average, five days after the dominant female has produced her first brood. We suggest that immediate reproductive benefits need to be considered in tests of the pay-to-stay hypothesis

Geographic variation in social organization of Galápagos mockingbirds: ecological correlates of group territoriality and cooperative breeding

To investigate ecological influences on cooperative social organization, I studied the four allopatric species of mockingbirds ( Nesomimus spp.) endemic to the Galápagos archipelago on four islands. On three small, low and arid islands (Genovesa, Champion and Española), mockingbird territories filled all terrestrial habitat, mean group size varied from 4.5 to 14.2 adults, maximum group size ranged from seven to 24 birds, and 70–100% of groups contained more than two birds. San Cristóbal is larger and higher, and it supports a broader range of habitats. At one highland and two coastal sites on this island, mockingbirds did not hold territories in all available habitats, group size averaged 2.2 adults, only 25% of groups were larger than two, and none included more than three adults. Adults dispersed into vacant habitat to establish new territories only on San Cristóbal. Helping behavior has not yet been observed on San Cristóbal, but it occurs on the other three islands. These results support the hypothesis that social groups and cooperative breeding are maintained where limited availability of preferred habitat constrains dispersal. The mechanism relaxing habitat saturation on San Cristóbal, however, remains undetermined. Predation by introduced rats and cats may reduce survival and indirectly reduce group size; these predators are absent from Genovesa, Champion and Española. Differences in food supplies could also affect interand intra-island variation in population density. Variation in social organization among arid coastal sites on the four islands, and similarity between climatically different sites on San Cristóbal, suggest that climatic conditions are less important as determinants of dispersal and breeding. Skews in adult sex ratios also fail to account for inter-island variation in sociality. Although they live in a climatically variable environment, territorial behavior and the physical limits of suitable habitat have an overriding influence on cooperative social organization in Galápagos mockingbirds.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46894/1/265_2004_Article_BF00302932.pd

A comparative perspective on the evolution of tamarin and marmoset social systems

Tamarins and marmosets (callitrichids) present an unusual opportunity for study of the determinants of primate social systems, because both the mating and infant care patterns of callitrichids are variable, even within individual populations. In this paper, I briefly describe three characteristics of callitrichid social systems that distinguish them from most other primates: extensive male parental care, helping by nonreproductive individuals, and variable mating patterns. I then discuss the evolution of these characteristics and of the frequent twinning exhibited by callitrichids. I suggest that an ancestor of modern callitrichids gave birth to a single offspring at a time, mated monogamously, and had significant paternal care. The idea that males of this ancestral form must have provided paternal care, even though only single infants were born, derives from a comparison of litter/mother weight ratios in modern primate species. Twinning perhaps then evolved because of a combination of dwarfing in the callitrichid lineage, leading to higher litter/mother weight ratios, and a high infant mortality rate, and because the extensive paternal care already present facilitated the raising of twins. I propose that the helping behavior of older offspring may have coevolved with twinning, because helpers would have increased the chances of survival of twins, and the presence of twins would have increased the benefits of helping. Finally, the high costs of raising twins and the variability of group compositions, especially the fact that some groups would not have had older offspring to serve as helpers, may have selected for facultative polyandry in saddle-back tamarins ( Saguinus fuscicollis ) and perhaps in other callitrichid species. Both helping and cooperative polyandry have been extensively studied in bird species, and I apply some of the conclusions of these studies to the discussion of the evolution of callitrichid social systems.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44556/1/10764_2005_Article_BF02193696.pd