Aims and methods
We examine the earlier proposed exceptional all-offspring natal dispersal in R. naso as well as its social structure and mating system to evaluate proposed selective pressures in the evolution of natal dispersal patterns and mating systems.
To achieve this, we use an integrated approach of exhaustive long-term behavioural observations of individually banded R. naso in multiple social groups and genetic paternity and kinship analysis.
The evolution of natal dispersal patterns
Natal dispersal – leaving the group of birth prior to sexual maturity – is a behaviour found among virtually all animals affecting the demography and genetic structure of populations as well as the evolution of social and reproductive behaviour. As a key life history trait, natal dispersal patterns have been studied for over 40 years and their evolution is still subject to much debate.
Exceptional natal dispersal patterns – a valuable opportunity to study the proposed evolutionary pressures
Natal dispersal is usually sex-biased with female-biased natal dispersal prevailing in the majority of bird species while male-biased natal dispersal prevails in mammals. The main discussed evolutionary forces shaping natal dispersal patterns include the avoidance of inbreeding, the avoidance of local mate competition, the avoidance of local resource competition and kin cooperation. Though, there is still much disagreement about the relative importance of these ultimate causes for sex-biased natal dispersal. Studying exceptions to general patterns of natal dispersal, e.g. female-biased natal dispersal in mammals as we found in Rhynchonycteris naso, provides a valuable opportunity to test the validity and relative importance of proposed evolutionary pressures.
The evolution of mating systems and male reproductive tactics
The majority of different types of polygyny in mammals is defined by the means males use to monopolize access to females – either classical, by direct female defence (female-defence polygyny) or, uncommon in mammals, by defending a resource critical for females (resource-defence polygyny). Which tactic males use is assumed to depend on the economic defensibility of females or resources and the male’s role in parental care. Although the potential for intraspecific transitions between all four main classes of mating systems (monogamy, polygyny, polyandry and promiscuity) has been shown in several species, studies on intraspecific transitions between male reproductive tactic (i.e. from resource defence to female defence or vice versa) are rare. They offer a valuable opportunity to gain new insights into ecological causes of male reproductive tactics evolution.
Intraspecific transitions between male reproductive tactics give important insights into ecological causes of male reproductive tactic evolution
Regarding its roosting habits, R. naso form an extreme by occupying sites which are usually completely exposed to daylight (e.g. tree trunks, vines or rocks). This is accompanied by morphological and behavioural adaptations to remain cryptic in exposed day roosts. Our results reveal nocturnal male territoriality – a tactic which most closely resembles a resource-defence polygyny that is frequent also in other tropical bats. Its contrasting clumped social dispersion during the day (see foto above) is likely to be the result of strong selection for crypsis in exposed roosts and is accompanied by direct female defence in addition to male territoriality. To the best of our knowledge, such contrasting male mating strategies within a single day-night cycle have not been described in a vertebrate species so far and illustrate a possible evolutionary trajectory from resource-defence to female-defence tactic by small ecologically driven evolutionary steps.
< The videos on the left show (top) a territorial male trying to copulate, but being rejected by the female, (middle) a territorial male performing a female-defence against another male that is trying to copulate, (bottom) a successful copulation.
For details please see these project related publications:
Günther L, Knörnschild M, Nagy M+ & Mayer F+ (2017) The evolution of a rare mammalian trait – benefits and costs of male philopatry in proboscis bats. Scientific Reports, 7, 15632. (+joint senior authors)
Günther L, Lopez MD, Knörnschild M, Reid K, Nagy M+ & Mayer F+ (2016) From resource to female defence: the impact of roosting ecology on a bat’s mating strategy. Royal Society Open Science, 3, 160503. (+joint senior authors)