Elevation clines in reproductive allocation and parental care have crucial consequences for the expression of costly sexual characters in passerines with
bi-parental care (Badyaev and Ghalambor 2001). Males from upland grounds have
to shift rapidly from sexual to parental behaviours, which requires that testosterone
be maintained at low levels, or to rapidly decline, to avoid an undermining of
reproductive success by testosterone-driven aggressiveness (Apfelbeck and
Goymann 2011). These conditions should tilt the balance between parental and
mating effort of males towards the former, considering that opportunities for
additional mating and extra-pair fertilizations may decline with elevation, because
of low densities or synchronic reproduction. As a matter of evidence, Badyaev
(1997b) and Snell-Rood and Badyaev (2008) found reduced plumage dimorphism
and shorter and simpler songs in high elevation Cardueline species, while
Apfelbeck and Goymann (2011) and Bastianelli et al. (2015) highlighted weaker
male territorial aggressiveness in Phoenicurus and Anthus species.
11.4.6 Mammals
Literature is limited in mammals compared to other taxonomic groups. One possible explanation is that life-history strategies are constrained by aspects of the
ecological niche (e.g. aquatic, aerial or terrestrial life; diurnal vs. nocturnal habits)
with a poor relationship with elevation (Fisher et al. 2001; Bielby et al. 2007; Sibly
and Brown 2007). Mammal growth and reproduction tend to be highly plastic
(Hansen and Boonstra 2000), and temporal patterns are often more divergent than
spatial ones. Seasonality is a strong driver of life-history diversification. Thus,
elevational clines should be envisioned, but high latitudes have instead been the
favourite scenario for analysing major temperature and photoperiod influences.
Polyestrous rodents represent one of the best examples of seasonal diversification:
spring-born young grow fast, mature early and reproduce in the year of birth, while
those born later grow slowly, overwinter as immature and reproduce the next year
(Bronson 1989).
When analysing variation across elevation, Bronson (1979), Zammuto and
Millar (1985) and Yoccoz and Ims (1999) found that highland populations of
ground squirrel (Spermophilus columbianus) and voles (Chionomys nivalis) have
longer lifespans, lower litter sizes and later ages at reproduction than those from
lowlands. Lower litter size in ground squirrels depends on reduced ovulation rates
because embryonic mortality is low and decreases with elevation (Bronson 1979).
A reduction in litter size with altitude has also been recorded in the deer mice
Peromyscus maniculalus (Dunmire 1960; Fleming and Rauscher 1978) and holds at
the interspecific level among species of this genus (Smith and McGinnis 1968). In
the alpine collared pika Ochotona collaris, adult survival is the trait that contributes
most to population growth rate, and fecundity is less variable than in other lagomorphs (Morrison and Hik 2007). As previously mentioned, buffering of survival
and bet hedging are thought to secure persistence in alpine environments.
11 Life-History Responses to the Altitudinal Gradient
265
bi-parental care (Badyaev and Ghalambor 2001). Males from upland grounds have
to shift rapidly from sexual to parental behaviours, which requires that testosterone
be maintained at low levels, or to rapidly decline, to avoid an undermining of
reproductive success by testosterone-driven aggressiveness (Apfelbeck and
Goymann 2011). These conditions should tilt the balance between parental and
mating effort of males towards the former, considering that opportunities for
additional mating and extra-pair fertilizations may decline with elevation, because
of low densities or synchronic reproduction. As a matter of evidence, Badyaev
(1997b) and Snell-Rood and Badyaev (2008) found reduced plumage dimorphism
and shorter and simpler songs in high elevation Cardueline species, while
Apfelbeck and Goymann (2011) and Bastianelli et al. (2015) highlighted weaker
male territorial aggressiveness in Phoenicurus and Anthus species.
11.4.6 Mammals
Literature is limited in mammals compared to other taxonomic groups. One possible explanation is that life-history strategies are constrained by aspects of the
ecological niche (e.g. aquatic, aerial or terrestrial life; diurnal vs. nocturnal habits)
with a poor relationship with elevation (Fisher et al. 2001; Bielby et al. 2007; Sibly
and Brown 2007). Mammal growth and reproduction tend to be highly plastic
(Hansen and Boonstra 2000), and temporal patterns are often more divergent than
spatial ones. Seasonality is a strong driver of life-history diversification. Thus,
elevational clines should be envisioned, but high latitudes have instead been the
favourite scenario for analysing major temperature and photoperiod influences.
Polyestrous rodents represent one of the best examples of seasonal diversification:
spring-born young grow fast, mature early and reproduce in the year of birth, while
those born later grow slowly, overwinter as immature and reproduce the next year
(Bronson 1989).
When analysing variation across elevation, Bronson (1979), Zammuto and
Millar (1985) and Yoccoz and Ims (1999) found that highland populations of
ground squirrel (Spermophilus columbianus) and voles (Chionomys nivalis) have
longer lifespans, lower litter sizes and later ages at reproduction than those from
lowlands. Lower litter size in ground squirrels depends on reduced ovulation rates
because embryonic mortality is low and decreases with elevation (Bronson 1979).
A reduction in litter size with altitude has also been recorded in the deer mice
Peromyscus maniculalus (Dunmire 1960; Fleming and Rauscher 1978) and holds at
the interspecific level among species of this genus (Smith and McGinnis 1968). In
the alpine collared pika Ochotona collaris, adult survival is the trait that contributes
most to population growth rate, and fecundity is less variable than in other lagomorphs (Morrison and Hik 2007). As previously mentioned, buffering of survival
and bet hedging are thought to secure persistence in alpine environments.
11 Life-History Responses to the Altitudinal Gradient
265