Sexual segregation and flexible mating patterns in temperate bats

Angell et al. 2013 Sexual segregation and flexible mating patterns in temperate bats. PloS One. 8(1): e54194.  

Figure 3 from the paper. Posterior distributions for paternity probabilities at the group level. Posterior distributions for the probabilities that fathers (at the group level) came from roosts in the (blue) upper-elevation, (yellow) mid-elevation and (green) low-elevation, and from (red) swarming sites. For (A) low-elevation offspring (the inset graph shows the Wharfedale roost posterior distributions in greater detail), and (B) mid-elevation offspring. 

Following the last two discussions, this week’s paper was selected on the basis that it used non-lethal DNA collection techniques to determine how intra-specific niche separation influences mating patterns.

Matthew Guy

A large number of temperate bat species, including Myotis daubentonii, display sexual segregation along altitudinal gradients. In these species, mating usually occurs during autumn swarming events. However, at the upper limit of the female range, Senior et al. (2005), found evidence of summer mating within roosts where dominant males are tolerated by females. Using the same population of M. daubentonii, this paper extends this work to identify if this is the dominant mating strategy throughout the altitudinal range of the species and, if not, can the differences in mating strategy be explained by foraging habitat quality?

DNA was extracted from wing punches and a novel Bayesian approach was used to assign the probability of parentage of juveniles from low altitudinal roosts to males from different roosting sites and swarming sites. During our discussion, nobody had a lot of experience with the genetic methods used and we found the results section difficult to read. However, the figures clearly demonstrate that the probability that these juveniles are fathered by males from anywhere other than swarming sites is very low. We thought that this was a really nice example of how figures can be used to give a clear overall impression of complex data, especially for the lay person. This result was in contrast to that found by Senior et al. at mid-elevation roosts suggesting a flexible mating strategy over an altitudinal gradient.

Foraging habitat quality was assessed using bat activity, weight and temperature, all of which declined significantly with altitude. The paper surmises that by excluding males from roosts, pregnant and lactating females can reduce intra-specific competition for the high-quality foraging grounds. However, the carrying capacity at intermediate sites is lower and so supports fewer females. In these areas, the thermoregulatory benefits provided by males in the roost outweigh the costs incurred by additional competition. The paper pulls these results together qualitatively, stating that the mating strategy is adapted to the social structure, which, in turn has evolved in response to environmental conditions at a given altitude. However, we felt that an analysis of prevalent mating strategy (i.e. probability juveniles were fathered at swarming events) within individual roosts and local foraging habitat quality together would address the second part of the research question more directly.

Over all, we felt that the paper was well written and, in combination with the Senior et al. paper results, presented an interesting behavioural response. However, the scope of the paper is fairly limited, largely due to a combination of studying a single species and developing ideas of a previous single study. One potential way to widen the papers appeal could have been to incorporate a discussion on how the novel genetic technique developed in this study could be applied to other species populations.

The paper ends by posing the question: Is this flexible mating behaviour capable of dealing with changes in prey distribution and roost microclimate predicted by climate change? Our discussions came to the conclusion that climate change could cause a decrease in the success rate of mating during autumn swarming events, potentially reducing gene flow. An increase in temperature would drive prey species upstream, where the higher proportions of more turbulent water would reduce the quantity and quality of the forging grounds. This could lead to a reduction in females within local nursery roosts making them more reliant on males for roost thermoregulation, and hence, an increase in the prevalence of summer mating. We thought that actually addressing the question, at least to some extent, in the discussion would have made for interesting conclusion and again potentially widen the papers appeal.

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