Breeding versus survival:proximate causes of abrupt population decline under environmental change in a desert rodent,the midday gerbil(Meriones meridianus)

Studying abrupt ecological shifts under gradual environmental change caused,in particular,by human activi­ty is important for understanding the fundamental aspects and underlying mechanisms of ecological resilience.One of the rare well-documented examples of an abrupt ecological shift is the delayed step transition of the population of a desert rodent,the midday gerbil(Meriones meridianus),from high-abundance(1994-2002)to low-abundance(2003-2017)regimes.This was in response to landscape transformation from desert to steppe caused by the drastic reduction of livestock in the rangelands of southern Russia after the collapse of the USSR in the early 1990s.In this study,we tested whether demographic parameters were correlated with the observed abrupt downward population shift.We found that reproductive activity(the percentage of breeding females,the number of litters,fecundity and the number of young recruited per female)showed no trend over time and did not differ between periods of high and low abundance.In contrast,the adult sex ratio(SR=males:females)de­creased significantly with time and was as much as twice more female-biased for the low-abundance population regime.However,SR was not related to any reproductive parameter,including the percentage of breeding fe­males.We conclude that proximate reasons for an abrupt population decline in M.meridianus are not associat­ed with the changes in breeding patterns or mate limitation caused by the Allee effect but relate to the increased mortality as a result of the desert landscape being fragmented by steppezation.The mortality is expected to be higher for males as the mobile and dispersing sex.

Sociability and pair-bonding in gerbils: a comparative experimental study

In a study of gerbils with contrasting social and mating systems (group-living monogamous Mongolian gerbil Meriones unguiculatus, solitary nonterritorial promiscuous midday jird M. meridianus, and solitary territorial promiscuous pale gerbil Gerbillus perpallidus), we employed part ner pref ere nee tests (PPTs) to assess among-species variation in sociability and pair-bonding patterns and tested whether the nature of contact between individuals: direct contact (DC) versus nondirect contact (NDC) affected our results. We measured male pref ere nces as the time: 1) spe nt alone, 2) with familiar (partner), and 3) unfamiliar (stranger) female in the 3-chambered apparatus. Gerbil species differed strongly in sociability and male partner preferences. The time spent alone was a reliable indicator of species sociability indepe ndent of the nature of con tact, whereas the pattern and level of betwee n-species differe nces in male partner preferences depended on con tact type: DC PPTs, un like NDC-tests, discriminated well betwee n monogamous and promiscuous species. In the DC-tests, stranger-directed aggression and stranger avoidanee were observed both in the highly social monogamous M. unguiculatus and the solitary territorial promiscuous G. perpallidus, but not in the non territorial promiscuous M. meridianus. In M. unguiculatus, stranger avoidanee in the DC-tests increased the time spent with the partner, thus providing evidence of a partner preference that was not found in the NDC-tests, whereas in G. perpallidus, stran ger avoidance in creased the time spe nt alone. This first comparative experimental study of partner prefere nces in gerbils provides new in sights into the in terspecific variation in gerbil sociality and mating systems and sheds light on behavioral mechanisms un derlyi ng social fidelity and pair-b on ding.