Alternative tactics in male African ground squirrels: the impact of variable rainfall on condition and physiology

Abstract

Alternative reproductive tactics occur when conspecifics of the same sex use different behavioral processes to maximize reproductive success. Resource availability can have significant impacts on the proximate factors impacting the success and persistence of alternative tactics, such as an individual’s physiology, immunity, and body condition. Male Cape ground squirrels (Xerus inauris) display two alternative tactics, with some mature males dispersing from their natal groups and joining bands of nonrelated males (band males), whereas others do not disperse and remain philopatric in their natal groups past sexual maturation (natal males). Despite differences in metabolism, home ranges, and behavior, there is no difference in reproductive success between the two tactics, allowing us to test if rainfall (as a measure of primary productivity) influences body condition and physiological indices in tactics when fitness is equal. Over a period of 6 years, rainfall was positively related to body condition (mass–spine length residuals) in band males (N = 76), but not in natal males (N = 75). When we followed 18 males prior to and after dispersal, we also found a positive relationship between body condition and age following dispersal, but not prior to dispersal, and natal males maintained a better body condition in low rainfall years. During 2 years of low rainfall, band males (N = 39) had higher total ectoparasite abundance, and white blood cell differentials, implying they are less able to tolerate parasitic infection than natal males (N = 31). We found no difference in bacterial growth inhibition between tactics, suggesting that band and natal males both are equally resistant to parasites. The poorer body condition of band males during lower rainfall suggests they are less tolerant to parasites in these years. Dispersal during years of low rainfall thus may increase the costs to body condition and parasite infection, but the reproductive benefit of overlapping with more females may contribute to maintaining equal fitness between tactics.

Intraspecific variation in male mating strategies in an African ground squirrel (Xerus inauris)

“…In striped mice, females who maintain smaller home ranges also experience delayed reproduction and dispersal (Schradin & Pillay, 2005 ). While increased investment in reproductive anatomy may be attributed to higher resource availability, male X. inauris also have different dispersal tactics that are impacted by rainfall (O'Brien et al, 2021 ). Sexually mature males either disperse at reproductive maturity and join a male band (Waterman, 1995 ), or delay dispersal and remain with their natal group (O'Brien et al, 2021 ; Waterman, 1995 ; Waterman, 1997 ).…”

“…While increased investment in reproductive anatomy may be attributed to higher resource availability, male X. inauris also have different dispersal tactics that are impacted by rainfall (O'Brien et al, 2021 ). Sexually mature males either disperse at reproductive maturity and join a male band (Waterman, 1995 ), or delay dispersal and remain with their natal group (O'Brien et al, 2021 ; Waterman, 1995 ; Waterman, 1997 ). Dispersal tactics result in similar reproductive success (Manjerovic & Waterman, 2015 ), but physiological and behavioral differences are affected by resources (O'Brien et al, 2021 ; Scantlebury et al, 2008 ).…”

“…Sexually mature males either disperse at reproductive maturity and join a male band (Waterman, 1995 ), or delay dispersal and remain with their natal group (O'Brien et al, 2021 ; Waterman, 1995 ; Waterman, 1997 ). Dispersal tactics result in similar reproductive success (Manjerovic & Waterman, 2015 ), but physiological and behavioral differences are affected by resources (O'Brien et al, 2021 ; Scantlebury et al, 2008 ). Band males are more mobile, with higher resting metabolic rates and larger home ranges and consequently spend less time feeding (Manjerovic & Waterman, 2015 ; Scantlebury et al, 2008 ).…”

“…We collected field data between 2002 and 2006 at two sites with known differences in both quantity and variability of rainfall (O'Brien et al, 2021 ; Pettitt et al, 2008 ) and presumably, resource availability as there tends to be a strong correlation between rainfall and primary productivity in arid and semi‐arid environments (Happold & Happold, 1992 ). Our “high resource site,” located in central South Africa (27°35′S, 25°35′E), receives an average of 502 mm annual rainfall and has a contiguous distribution of Eragrostis spp.…”

“…We used t‐tests to compare the number of breeding females per group and the number of females and males per hectare, calculated as the total number of squirrels in the study site divided by the size of the study area in hectares. Adult males either disperse at reproductive maturity or delay dispersal and remain with their natal group (O'Brien et al, 2021 ; Waterman, 1995 , 1997 ). Both dispersal tactics result in reproductive success (Manjerovic & Waterman, 2015 ); thus all adult males present were included, regardless of dispersal tactic, in our calculations for the number of males per hectare.…”