Cousin Island Research Articles

Investigating the effectiveness of a well‐managed hatchery as a tool for hawksbill sea turtle (Eretmochelys imbricata) conservation

AbstractThe critically endangered hawksbill sea turtle (Eretmochelys imbricata) is of conservation concern worldwide. Conservation actions which act to reduce high levels of embryonic mortality aid to boost hatchling production. At Cousine Island, Seychelles, a mixed management method has been adopted to maximize the number of hatchlings entering the ocean. We examined the effectiveness of a hatchery at increasing hatching and emergence success from four seasons of data (2017–2021). Hatchery nests had significantly higher hatching and emergence success across all years relative to nests left in situ (i.e., natural), with inter‐annual variation observed. A greater percentage of in situ clutches was found to be influenced by both crab activity and predation as compared to relocated clutches. Overall, the mixed management approach increased hatching success (mean = 75.1%) relative to the various nest management techniques previously used (2004/2005–2016/2017; mean = 63.5%). By mitigating external influences such as tidal flooding, beach erosion, and crab activity/predation, this study provides evidence for the success of a hatchery in directly increasing hatchling recruitment. Strict and careful hatchery management as well as timely and efficient relocation procedures are needed to minimize potential negative effects of nest relocation.

Gut microbiome composition, not alpha diversity, is associated with survival in a natural vertebrate population

BackgroundThe vertebrate gut microbiome (GM) can vary substantially across individuals within the same natural population. Although there is evidence linking the GM to health in captive animals, very little is known about the consequences of GM variation for host fitness in the wild. Here, we explore the relationship between faecal microbiome diversity, body condition, and survival using data from the long-term study of a discrete natural population of the Seychelles warbler (Acrocephalus sechellensis) on Cousin Island. To our knowledge, this is the first time that GM differences associated with survival have been fully characterised for a natural vertebrate species, across multiple age groups and breeding seasons.ResultsWe identified substantial variation in GM community structure among sampled individuals, which was partially explained by breeding season (5% of the variance), and host age class (up to 1% of the variance). We also identified significant differences in GM community membership between adult birds that survived, versus those that had died by the following breeding season. Individuals that died carried increased abundances of taxa that are known to be opportunistic pathogens, including several ASVs in the genus Mycobacterium. However, there was no association between GM alpha diversity (the diversity of bacterial taxa within a sample) and survival to the next breeding season, or with individual body condition. Additionally, we found no association between GM community membership and individual body condition.ConclusionsThese results demonstrate that components of the vertebrate GM can be associated with host fitness in the wild. However, further research is needed to establish whether changes in bacterial abundance contribute to, or are only correlated with, differential survival; this will add to our understanding of the importance of the GM in the evolution of host species living in natural populations.

Time heals: Boosted breeding seabird populations on restored Cousine Island, Seychelles

AbstractHistorically, Cousine Island was used for agriculture and harvesting various seabird eggs, but following restoration efforts initiated in the 1990s, it has become an important nesting area for Seychelles seabirds. White‐tailed tropicbird (Phaethon lepturus), white tern (Gygis alba), and lesser (Anous tenuirostris) and brown noddy (Anous stolidus) populations were monitored on Cousine Island between 1997 and 2017 to determine the long‐term population responses to various environmental variables during the winter breeding season. White tern, brown noddy, and white‐tailed tropicbird populations are increasing, indicating that island conditions for breeding have improved for these species in the last 20 years. Lesser noddy populations are in decline, but not significantly, with winter air temperature significantly adding to this trend. Lesser noddies are true colonial nesters and are therefore potentially more affected by climatic conditions that result in nest failures and nearshore prey shortages. The relationship between numbers of seasonally breeding species (lesser and brown noddies) and those of year‐round breeders (white‐tailed tropicbirds and white terns) was also examined. Brown noddy population trends significantly predicted white‐tailed tropicbird and white tern winter breeding population trends, suggesting that these species populations respond similarly to local conditions. Data from this study establish a baseline for future population trend investigations and assessments of island conservation and management success.

Beach profiling and ghost crab densities on a hawksbill turtle nesting beach in the Seychelles

Increasing beach sediment loss from erosion and high levels of crab Ocypode spp. predation are threatening turtle nests and nesting habitat. The 900 m long beach on Cousine Island, Seychelles, supports a nesting population of approximately 70–130 hawksbill turtle Eretmochelys imbricata nests each season. Seasonal and storm-related erosion and accretion cycles on Cousine Island have the potential of destroying 50% or more of all turtle egg clutches on the island in a single nesting season. Observed crab predation rates had reached 90–100% in preferred nesting beach zones in previous years. This has resulted in intensive management measures to minimise turtle egg and nest losses. We investigated the distribution and population density of ghost crabs and the morphology of the beach across the different beach area zones and across the turtle-nesting season during 2014–2015. Crab burrow numbers varied between beach zone areas and across the season and were highest on the backshore. Crab density correlated negatively with available beach area, and we found that crab density increased in the presence of turtle nests. When examining beach dynamics, we found them to be cyclical and found the nesting beach prone to higher levels of erosion than accretion with significant changes in beach width throughout the season. The mean vertical beach elevation drop on Cousine Island was higher than what hawksbill turtles have been reported to prefer. We suggest the continuation of beach elevation monitoring and management to use the beach morphology data to assist with hawksbill turtle nest translocations to minimise nest losses and maximise hatchling recruitment success.

Senescence of song revealed by a long-term study of the Seychelles warbler (Acrocephalus sechellensis)

Senescence is widespread in nature, often resulting in diminishing survival or reproduction with age, but its role in age-dependent variation in sexual traits is often poorly understood. One reason is that few studies of sexual traits consider non-linear relationships with age, or only consider a narrow range of years relative to the life span of the species. Birdsong has evolved to allow assessment of conspecific quality in numerous bird species. Whilst theory and empirical work suggests that song may become more elaborate with age, there are a paucity of long-term studies testing whether song is associated with age or longevity. In particular, the occurrence of song senescence has rarely been demonstrated. Using an exceptional long-term dataset for the Seychelles warbler (Acrocephalus sechellensis), we analysed relationships between male song, age, survival, and longevity. This species is a long-lived songbird with early life increases, followed by senescent declines, in survival and reproduction. The study population (Cousin Island, Seychelles) is a closed population, with no depredation of adults, providing an excellent opportunity to study senescence in free-living animals. We tested whether song traits were related to age at recording, future survival, longevity, and territory quality. We found age-dependent changes in five song traits (duration, maximum frequency, peak frequency of songs, and duration and frequency bandwidth of trills). Relationships with age were quadratic, indicating reversal in the expression of song coinciding with the onset of senescence in reproduction and survival in this species. One song trait (trill bandwidth) had a quadratic relationship with future survival, but no song traits were related to longevity, suggesting age-related patterns were not the result of selective disappearance. Our study provides one of the first examples of functional senescence in song, offering new insights into avian senescence. Late-life declines in avian song, and possibly other sexual traits, may be more common than currently known, and may play a fundamental role in age-dependent changes in reproductive success.

Effects of nest management methods on hatching success and predation rates of hawksbill turtles on Cousine Island, Seychelles

Sea turtle populations have declined globally and are of conservation concern. We investigated the effects of nest management methods on hatching success and egg/hatchling predation rates of hawksbill turtles Eretmochelys imbricata on Cousine Island, Seychelles. We determined the effectiveness of two different crab-barrier methods (netted and fenced) on hatching success and predation rates. We examined the relative influence of nest-site cover (full sun, partial sun or full shade) and location (zones of high risk or low risk of predation) on hatching success and predation rates. We also examined temperature disparity between netted and control nests to estimate potential effects on sex ratios and measured typical beach-substrate temperatures in the different nesting habitats. We compared data from 40 netted, 40 fenced and 40 control nests during the 2014/2015 breeding season. Overall, hawksbill turtle hatching success was not significantly affected by treatment or cover; however, netted nests and nests incubating in full sun had the highest mean hatching success. Predation rate was generally low but variable, and the rates differed significantly between the three treatments. Nest temperatures differed significantly between the netted and control nests, with netting used to protect nests having a substantial cooling effect. Hatchling sex ratios for natural nests were skewed towards females and those for netted nests skewed towards males. With climate change and increasing ambient temperatures imminent, a better understanding of how the spatial distribution of nests and variations in environmental factors influence hawksbill turtle hatching success and the levels of predation on eggs and hatchlings will assist future conservation measures.

The Time for Insects Is Now

The Time for Insects Is Now

Nesting ecology and hatching success of the hawksbill turtle (2004–2014) on Cousine Island, Seychelles

Populations of the Critically Endangered hawksbill turtle Eretmochelys imbricata in the Seychelles showed significant declines in the past. They have since recovered and nesting numbers are increasing because of increased protection and management intervention. Data on their nesting ecology on Cousine Island were collected and analysed for the breeding seasons from 2004/2005 to 2013/2014 with the aim of improving management methods. The findings indicate that the number of nesting individuals increased during the study period (from x̅ = 34.3 nesters between 2004 and 2009, to x̅ = 55.4 nesters between 2010 and 2014); however, the clutch frequency was relatively low compared with other study areas. Night-time nesting increased, most likely because of increased disturbances across the study period. Clutch sizes decreased significantly across the nesting season, and there was a significant drop in hatching success in clutch sizes exceeding 200 eggs. The predation rate and infertility rate each showed an increasing trend over time. Nests incubating in full sun showed the highest mean hatching success rate and had the shortest mean incubation duration. The present study provides insight into the species’ nesting ecology on Cousine Island and will better inform management decisions aimed at increasing hatching success and minimising threats such as beach erosion and predation.

Plastic pollution in paradise: Daily accumulation rates of marine litter on Cousine Island, Seychelles

The daily accumulation rates, composition, sizes and potential sources of marine litter collected on a remote island within the Western Indian Ocean were investigated. In total, 9119 items of marine litter were collected during 40 surveys, which equated to 0.0082 items·m−1·d−1. Between 2003 and 2019 there was a significant increase in the amount of litter deposited, with the highest daily accumulation rate recorded in 2019 (0.0255 items·m−1·year−1). All specific litter types increased over time and also differed significantly in their accumulation rates, with polystyrene fragments/pieces (0.00249 items·m−1·d−1), plastic items (0.00135 items·m−1·d−1) and plastic bottles (0.0011 items·m−1·d−1) being the most commonly encountered during this study. The majority of the litter found was ≤5 cm in size. Nearly all (>80%) litter collected was made of or contained some form of plastic. Recommendations for improved management of litter and the importance of establishing regular beach clean-ups within the Seychelles are briefly discussed.

First record of the marine turtle leech (Ozobranchus margoi) on hawksbill turtles (Eretmochelys imbricata) in the inner granitic Seychelles.

Ozobranchus spp. are leeches that feed solely on turtle blood. They are common ectoparasites found on a range of marine turtle species, with some species of the leech being implicated as vectors of fibropapilloma-associated turtle herpesvirus (FPTHV). Green (Chelonia mydas) and hawksbill (Eretmochelys imbricata) turtles are the two commonly occurring species in the inner granitic islands of the Seychelles. Routine monitoring of nesting turtles on Cousine Island, Seychelles, allowed for opportunistic sightings of leeches on two hawksbill females. In both cases infestation was low, with three leeches collected off one female turtle and five off the other. No obvious signs of papillomas secondary to infection of FPTHV were seen. All of the turtle leeches collected were determined to be Ozobranchus margoi as they had five pairs of lateral digiform branchiae. The specimens were deposited in the Seychelles Natural History Museum on Mahé. To the best of our knowledge this is the first record of Ozobranchus margoi recorded in the inner granitic Seychelles on hawksbill turtles.

Flat Wasp Fauna from Cousine Island, Seychelles, with Description of a New Species of Holepyris (Hymenoptera, Bethylidae)

The species richness of the flat wasp fauna from Cousine Island, Seychelles, is reported for the first time. Three species are represented on this island in the Seychelles archipelago. The male of the previously described Seychelles species, Goniozus kiefferi Gordh is redescribed and its genitalia described and illustrated for the first time; the Oriental species Megasprosternum cleonarovorum Gupta and Azevedo, previously known from India, is recorded for the first time from the Seychelles and the Afrotropical region; and Holepyris gaigherae sp. nov. is described and illustrated. The Seychelles flat wasp fauna is now represented by nine species in total recorded from Cousine, Mahe and Silhouette islands.

Extra-pair parentage and personality in a cooperatively breeding bird

Why so much variation in extra-pair parentage occurs within and among populations remains unclear. Often the fitness costs and benefits of extra-pair parentage are hypothesised to explain its occurrence; therefore, linking extra-pair parentage with traits such as personality (behavioural traits that can be heritable and affect reproductive behaviour) may help our understanding. Here, we investigate whether reproductive outcomes and success are associated with exploratory behaviour in a natural population of cooperatively breeding Seychelles warblers (Acrocephalus sechellensis) on Cousin Island. Exploratory behaviour correlates positively with traits such as risk-taking behaviour and activity in other wild bird species and might promote extra-pair mating by increasing the rate at which potential extra-pair partners are encountered. We therefore predicted that fast-exploring individuals would have more extra-pair offspring. There is also a potential trade-off between pursuing extra-pair parentage and mate guarding in males. We therefore also predicted that fast-exploring males would be more likely to pursue extra-pair parentage and that this would increase the propensity of their mate to gain extra-pair parentage. We found that neither the total number of offspring nor the number of extra-pair offspring were associated with a male’s or female’s exploratory behaviour. However, there was a small but significant propensity for females to have extra-pair fertilisations in pairs that were behaviourally disassortative. Overall, we conclude that, due to the small effect size, the association between exploratory behaviour and extra-pair paternity is unlikely to be biologically relevant.Significance statementTrue genetic monogamy is rare, even in socially monogamous systems, and multiple factors, such as behaviour, social structure, morphology and physiology, determined by the biological system can cause variation in extra-pair parentage (EPP). Therefore, investigating the inherent differences in these factors among individuals could be informative. We investigated whether reproductive outcomes/success are associated with differences in the propensity to explore novel environments/objects in a promiscuous, island-dwelling cooperatively breeding bird, the Seychelles warbler. Our results showed that exploratory behaviour was not associated with the number of offspring produced by an individual, and thus the long-term fitness consequences of different exploratory tendencies did not differ. We also found that the propensity to engage in EPP in females was higher in dissimilar behavioural pairs, but due to the small effect size, we hesitate to conclude that there are personality-dependent mating outcomes in the population.

The importance of post-translocation monitoring of habitat use and population growth: insights from a Seychelles Warbler (Acrocephalus sechellensis) translocation

Translocations are a valuable tool within conservation, and when performed successfully can rescue species from extinction. However, to label a translocation a success, extensive post-translocation monitoring is required, ensuring the population is growing at the expected rate. In 2011, a habitat assessment identified Frégate Island as a suitable island to host a Seychelles Warbler (Acrocephalus sechellensis) population. Later that year, 59 birds were translocated from Cousin Island to Frégate Island. Here, we determine Seychelles Warbler habitat use and population growth on Frégate Island, assessing the status of the translocation and identifying any interventions that may be required. We found that territory quality, an important predictor of fledgling production on Cousin Island, was a poor predictor of bird presence on Frégate Island. Instead, tree diversity, middle-storey vegetation density, and broad-leafed vegetation density all predicted bird presence positively. A habitat suitability map based on these results suggests most of Frégate Island contains either a suitable or a moderately suitable habitat, with patches of unsuitable overgrown coconut plantation. To achieve the maximum potential Seychelles Warbler population size on Frégate Island, we recommend habitat regeneration, such that the highly diverse subset of broad-leafed trees and a dense middle storey should be protected and replace the unsuitable coconut. Frégate Island’s Seychelles Warbler population has grown to 141 birds since the release, the slowest growth rate of all Seychelles Warbler translocations; the cause of this is unclear. This study highlights the value of post-translocation monitoring, identifying habitat use and areas requiring restoration, and ultimately ensuring that the population is growing.

Cryptic biodiversity and phylogeographic patterns of Seychellois Ligia isopods.

Ligia isopods are conspicuous inhabitants of rocky intertidal habitats exhibiting several biological traits that severely limit their dispersal potential. Their presence in patchy habitats and low vagility may lead to long term isolation, allopatric isolation and possible cryptic speciation. Indeed, various species of Ligia have been suggested to represent instead cryptic species complexes. Past studies; however, have largely focused in Eastern Pacific and Atlantic species of Ligia, leaving in doubt whether cryptic diversity occurs in other highly biodiverse areas. The Seychelles consists of 115 islands of different ages and geological origins spread across the western Indian Ocean. They are well known for their rich biodiversity with recent reports of cryptic species in terrestrial Seychellois organisms. Despite these studies, it is unclear whether coastal invertebrates from the Seychelles harbor any cryptic diversity. In this study, we examined patterns of genetic diversity and isolation within Ligia isopods across the Seychelles archipelago by characterizing individuals from locations across both inner and outer islands of the Seychelles using mitochondrial and nuclear markers. We report the presence of highly divergent lineages of independent origin. At Aldabra Atoll, we uncovered a lineage closely related to the Ligia vitiensis cryptic species complex. Within the inner islands of Cousine, Silhouette, and Mahé we detected the presence of two moderately divergent and geographically disjunct lineages most closely related to Ligia dentipes. Our findings suggest that the Seychelles may harbor at least three novel species of Ligia in need of description and that these species may have originated independently.

Repeatable and heritable behavioural variation in a wild cooperative breeder.

Quantifying consistent differences in behaviour among individuals is vital to understanding the ecological and evolutionary significance of animal personality. To quantify personality, the phenotypic variation of a behavioural trait is partitioned to assess how it varies among individuals, which is also known as repeatability. If pedigree data are available, the phenotypic variation can then be further partitioned to estimate the additive genetic variance and heritability. Assessing the repeatability and heritability of personality traits therefore allows for a better understanding of what natural selection can act upon, enabling evolution. In a natural population of facultative cooperatively breeding Seychelles warbler (Acrocephalus sechellensis) on Cousin Island, a lack of breeding vacancies forces individuals into different life-history strategies, and these differences in reproductive state could generate behavioural differences among individuals in the population. We used this population to estimate the repeatability of 4 behavioural traits (novel environment exploration, novel object exploration, obstinacy/struggle rate, and escape response), and narrow-sense heritability (of behavior, h2B; behavior minus observer variance; and personality), and evolvability, of the repeatable behavioural traits. We also tested for an among-individual correlation between the repeatable traits. We found that, compared to estimates in other study species, the exploratory behaviours were moderately repeatable (0.23-0.37), there was a positive among-individual correlation (0.51) between novel environment and novel object exploration, and that novel environment exploration was moderately heritable (0.17; h2B was low as it includes observer variance). This study further clarifies the additive genetic variance available for selection to act upon in this cooperatively breeding bird.

Large-scale coral reef restoration could assist natural recovery in Seychelles, Indian Ocean

The aim of ecological restoration is to establish self-sustaining and resilient systems. In coral reef restoration, transplantation of nursery-grown corals is seen as a potential method to mitigate reef degradation and enhance recovery. The transplanted reef should be capable of recruiting new juvenile corals to ensure long-term resilience. Here, we quantified how coral transplantation influenced natural coral recruitment at a large-scale coral reef restoration site in Seychelles, Indian Ocean. Between November 2011 and June 2014 a total of 24,431 nursery-grown coral colonies from 10 different coral species were transplanted in 5,225 m2 (0.52 ha) of degraded reef at the no-take marine reserve of Cousin Island Special Reserve in an attempt to assist in natural reef recovery. We present the results of research and monitoring conducted before and after coral transplantation to evaluate the positive effect that the project had on coral recruitment and reef recovery at the restored site. We quantified the density of coral recruits (spat <1 cm) and juveniles (colonies 1-5 cm) at the transplanted site, a degraded control site and a healthy control site at the marine reserve. We used ceramic tiles to estimate coral settlement and visual surveys with 1 m2 quadrats to estimate coral recruitment. Six months after tile deployment, total spat density at the transplanted site (123.4 ± 13.3 spat m-2) was 1.8 times higher than at healthy site (68.4 ± 7.8 spat m-2) and 1.6 times higher than at degraded site (78.2 ± 7.17 spat m-2). Two years after first transplantation, the total recruit density was highest at healthy site (4.8 ± 0.4 recruits m-2), intermediate at transplanted site (2.7 ± 0.4 recruits m-2), and lowest at degraded site (1.7 ± 0.3 recruits m-2). The results suggest that large-scale coral restoration may have a positive influence on coral recruitment and juveniles. The effect of key project techniques on the results are discussed. This study supports the application of large-scale, science-based coral reef restoration projects with at least a 3-year time scale to assist the recovery of damaged reefs.

A helping hand: artificial nest site provisioning increases breeding success of a tropical seabird

Common white terns Gygis alba lay a single egg balanced on rocks or branches and consequently are at risk of low nesting success. A novel technique of hollowing out coconut husks and providing artificial nest sites was developed on Cousine Island, Seychelles. Our study aimed to critically assess whether common white tern nesting success differed between artificial and natural nest sites. For natural nest sites, nesting success and the stage of nest failure were also compared between seasons, nest heights, and tree species. Finally, we compared results from our study to those of a study conducted 10 years ago on the same island. Nesting success differed significantly between artificial nests and natural nests, with the success of artificial nests nearly double that of natural nests. Hatching success was similar for artificial and natural nests (65% and 52% respectively), suggesting coconut husks do not reduce egg losses, but rather provide secure perches for young chicks. Most natural nest failures occurred during the egg phase (70%), followed by the youngest chick phase (21%). Of the eggs that hatched in natural nests, 61% fledged successfully. Tree species and season significantly influenced nesting success for natural nests and nest height varied significantly between tree species. Higher nesting success was observed during the mixed seasonal period (50%) followed by the cooler, windier and drier South-East Monsoon season (33%). The hotter and more humid North-West Monsoon period had the lowest nesting success (20%). Comparisons of nesting success at natural and artificial nest sites with the earlier study did not differ significantly, despite a smaller population of common white terns being present on the island at that stage. Our results support the use of artificial nest sites to improve nesting success for this species and serve as a model for similar species currently under threat.

The impact of conservation-driven translocations on blood parasite prevalence in the Seychelles warbler.

Introduced populations often lose the parasites they carried in their native range, but little is known about which processes may cause parasite loss during host movement. Conservation-driven translocations could provide an opportunity to identify the mechanisms involved. Using 3,888 blood samples collected over 22 years, we investigated parasite prevalence in populations of Seychelles warblers (Acrocephalus sechellensis) after individuals were translocated from Cousin Island to four new islands (Aride, Cousine, Denis and Frégate). Only a single parasite (Haemoproteus nucleocondensus) was detected on Cousin (prevalence = 52%). This parasite persisted on Cousine (prevalence = 41%), but no infection was found in individuals hatched on Aride, Denis or Frégate. It is not known whether the parasite ever arrived on Aride, but it has not been detected there despite 20 years of post-translocation sampling. We confirmed that individuals translocated to Denis and Frégate were infected, with initial prevalence similar to Cousin. Over time, prevalence decreased on Denis and Frégate until the parasite was not found on Denis two years after translocation, and was approaching zero prevalence on Frégate. The loss (Denis) or decline (Frégate) of H. nucleocondensus, despite successful establishment of infected hosts, must be due to factors affecting parasite transmission on these islands.

The history, status and trends of the Endangered Seychelles Magpie-robinCopsychus sechellarum

SummaryThe once ‘Critically Endangered’ Seychelles Magpie-robin was down to just 12 individuals in 1960 on one island of the granitic Seychelles. In 2015, due to intensive long-term management the population stands at around 280 birds on five islands, marking a significant success for this species. Translocations to the islands of Cousin and Cousine have led to population saturation and stability, a translocation to Denis Island has resulted in a continuing population increase and the founder population on Frégate Island is likewise increasing. The latest translocation to Aride Island in 2002 resulted in population increase then stability but is now showing a steep decline throughout 2014 into 2015. Reasons for this decline are yet unknown though disease, lack of recruitment, the impacts of social conflict and the possibility of genetic issues are discussed. This report summarises the history of management for this species, compiling all available published and unpublished information, to provide a comprehensive account of the Seychelles Magpie-robin recovery.

Longevity and survival of the Endangered Seychelles Magpie Robin Copsychus sechellarum

The Seychelles Magpie Robin Copsychus sechellarum was once one of the most threatened birds in the world, but was downgraded from Critically Endangered to Endangered after a long-term recovery programme was success- fully implemented. Comprehensive long-term monitoring of this species was conducted on the islands of Cousin and Cousine over an 18-year period. We report here on the species longevity and annual survival at these two sites. The oldest recorded individual was a male who died on Cousine Island on 28 September 2000 at just under 16 years old. This individual was recorded to have hatched on Frégate Island on 3 January 1985, before being translocated to Cousine Island in 1995. Mean annual survival rates over an 18-year period were 81.6% on Cousin and 77.9% on Cousine islands. A decrease in annual survival was noted with increasing population size on both islands (Cousin: t = −3.09, p < 0.05; Cousine: t = −2.71, p < 0.05), which is a likely consequence of increased territory disputes and competition for food.