Modelling survival from fieldwork data: a case study on the social bat Molossus molossus
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XIIIth European Bat Research Symposium, Sibenik (Croatia) 1-5 September 2014.
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Modelling survival from fieldwork data: a case study on the social bat Molossus molossus
- 1. Yann Gager Modelling survival from fieldwork data: a case study on the social bat Molossus molossus
- 2. Acknowledgements Dina Dechmann Teague O’Mara Olivier Gimenez Rachel Page + many fieldwork assistants
- 3. Many animals are social…but why?
- 4. Many animals are social…but why? Balance of costs and benefits of group living
- 5. Sociality in bats Tropics Group hunting? Temperate zones (summer) Social thermoregulation Group hunting
- 6. Social foraging in insectivorous bats [Safi & Kerth 2007] - Energetically limited (small size, flight + echolocation) - Extreme specialization (narrow wings + ephemeral insects) - Acoustic information transfer on site between roost members
- 7. How is survival related to group size ? Group size Survival - Survival benefits/costs associated with group living?
- 8. [Dechmann et al. 2010]
- 9. Collection of survival data 1350 animals transpondered during regular captures 7 roosts with automated readers 1980 « days » Capture-recapture 14 sites – 77 captures
- 10. Roost variation Monthly variation 1a. Size of groups: spatial variation
- 11. 2. Survival analyses (capture) - Data: capture-recapture (285 adult females) - Analysis: Cox-proportional hazard regression - Results: Effect of marking year Variable p Marking year 0.00966 *** Initial group size 0.33938 Mean group size 0.97945 Site 0.81306 GLOBAL 0.12201
- 12. 3. Survival analyses (transponder-reader) - Data: 4 sites followed over 15 months (59 adult females) - Aim: Model monthly survival - Analysis: Model comparison (Multi-state Mark-Recapture) - Results: Group size as the best explanatory variable Model QAICc ΔQAICc No. Of parameters Group size 1546.1 0.0 20 Month 1583.6 19.5 28 Marking year 1773.6 209.5 8 . 1786.8 222.7 4 Site 1791.2 227.1 7
- 13. 3. Survival analyses (transponder-reader)
- 14. To conclude Conclusions 1- Survival benefits with increased group size 2- Constraints to medium groups Perspectives: - Effect of group size on foraging efficiency - Relatedness between group members
- 15. Thank you!
Editor's Notes
- 1
- Sociality: benefits of work in group
- Numerous species living in groups: social insects like ants and termites, school of fish, flocks & colonies of birds and social mammals (colonies to foraging groups) 2. Encountered in various habitats and displaying various social behaviours (roost construction, cluster formation)
- 1. Probably multiple pathways for sociality and convergent evolution of group living – from ancestral solitary state – in various taxa 2. For sociality to evolve, more benefits than costs! e.g. Costs: higher parasite transmission, resource competition and inbreeding risk e.g. Benefits: predator-avoidance and foraging benefits.
- 1. Animals social seasonally in temperate zones Primary explanation of sociality in temperate zones: a) Females in temperate zones: costly pregnancy and lactation b) Males in temperate zones: linked to increasing amounts of ephemeral insects in their diet and a morphological adaptation (i.e. wing) to the open space. 2. Animals social-year round in tropical zones, they can form long-term stable groups. No disruptive event like hibernation or migration, interesting for comparative analysis across climatic zones. Energetic limitation, dependance on ephemeral food resources and correlated information transfer among roost members.
- Narrow wings: fast! But high wing load that is more expensive for manoeuvring
- Give example from other taxa…Maybe not colonies! Bat species with an extremely narrow ecological ecological (energetical costs of flight, echolocation + small size) Benefit of foraging effiency ? With longer-term consequences on survival and reproductive success? A crucial factor of the environment: the group size, potentially numbers of animals Involved in social foraging. A) The less the merrier B) The more the merrier C) Merrier in medium groups D) Merry whatever happens
- Use of animals exiting the roost as a proxy for group size Harem structure: one male with many females One limitation: certainty in determinig group size, limited number of roots studied
- - Group size increase during the summer (juveniles) - Comparable average group size between capture and transponder-readers dataset Capture : Medians between 5 and 18 but mean = 9.6 ± 6.7 Transponder-readers : Medians between 8 and 12 but mean = 10.3 ± 3
- Cox-proportional hazard regression: - multiple regression methods - calculates the relative risk of an event to occur Use of capture data on adult females: - Use of survival times & regression methods - Many individuals not recaptured - relatively constant recapture rate No significant effect of group size or site Significant effect of marking year: lower survival with earlier marking year
- - Importance of group size
- Results - Slight increase of survival with group size - Potential difference in lifespan of ~200 days - Higher transition rates to 9-10 and 11-12
- - Info on range of group size: only up to 20 individuals where it can sum up millions of individuals in a species from the same family Constrained group size?
- Social groups Only female adults kept for analyses Batli + capture data Add photo house