A strong male-biased sex ratio in the Aesculapian Snake

 Aesculapian snake, Zamenis longissimus. Photo credit: 
Felix Reimann.
The adult sex ratio in all isolated populations of the Aesculapian Snake,  Zamenis longissimus at the northern limit of its distribution was found to be male-biased; this did not apply to the Austrian population, in the center of its distribution range, where the ratio was almost 1:1. A recently published article in Global Ecology and Conservation shows the sex ratio in the Polish population of the Aesculapian snake clearly deviates from 1:1. The ratio of 4:1 is one of the greatest, adult male-biased ratios when compared to other snake species, (published reports of the male-to-female ratio in this snake has previously not exceed 3:1). Kurek et al. (2019) found the proportion of females in the Bieszczady Mts. has declined in comparison with the data obtained from the same population two decades ago.  Those populations harbored no less than 30% of females, unlike the Polish population investigated in 2010-2012, which contained a mere 18% of females; this is the smallest percentage of female Aesculapian snakes yet recorded. One factor causing this may be the lower detection rate of females in field studies. However, the authors tried to minimize this factor through regular field visits and the use of artificial breeding sites (mounds). Even though the research efforts with regard to different populations are not fully comparable – only the Czech population was studied in a similar way.  
Population size is one of the major determinants of extinction risk. The authors hypothesize that the stronger environmental pressure on females in peripheral populations of the Aesculapian snake may be intensified by the high costs of reproduction due to the limited availability of suitable reproductive habitats. 

Kurek, K., Ćmiel, A., Bury, S., Zając, B., Najberek, K., Babiasz, R., Musilová, R., Baś, G. and Najbar, B., 2019. What has happened to the females? Population trends in the Aesculapian snake at its northern range limit. Global Ecology and Conservation, p.e00550.

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