Fit females and fat polygynous males: seasonal body mass changes in the grey-headed flying fox

Abstract

When females and males differ in their timing of maximum reproductive effort, this can result in sex-specific seasonal cycles in body mass. Such cycles are undoubtedly under strong selection, particularly in bats, where they affect flying ability. Flying foxes (Old World fruit bats, Pteropus spp.) are the largest mammals that can sustain powered flight and therefore face critical trade-offs in managing body reserves for reproduction, yet little is known about body mass dynamics in this group. I investigated body mass changes in relation to reproductive behaviour in a large colony of grey-headed flying foxes (Pteropus poliocephalus). In this polygynous mammal, females were predicted to maximise reproductive effort during lactation and males during the breeding season. As predicted, female body condition declined during the nursing period, but did not vary in relation to sexual activity. By contrast, males accumulated body reserves prior to the breeding season, but subsequently lost over 20% of their body mass on territory defence and courtship, and lost foraging opportunities as they also defended their day roost territories at night. Males in better condition had larger testes, particularly during territory establishment, prior to maximum sexual activity. Thus, the seasonality of female mass reflected the high metabolic load that lactation imposes on mothers. However, male mass followed a pattern akin to the "fatted male phenomenon", which is commonly observed in large polygynous mammals with seasonal reproduction, but not in bats. This shows the importance of body reserves for reproduction in flying foxes, despite their severe constraints on body mass.

Relationship Between the Amount of Subcutaneous Fat, Testicular Morphometry, Epididymis and Some Sperm Parameters in Leptonycteris yerbabuenae Bat Before, During and After Mating

“…These results agree with those reported in a histological study in a population of L. yerbabuenae of Chamela, Jalisco [34]. Also, in this study we found that the testicular mass varied seasonally similar to body mass, indicating a close relationship between spermatogenesis and the individual condition, as in P. poliocephalus [10].…”

“…However, in L. yerbabuenae happens that while fat accumulates, it also increases the size of the testis and epididymis, as well as the quantity and quality of sperm stored in the epididymis, prior to the mating period that start in late May and through June [6]. This seasonal phenomenon of fat accumulation and testicular activity increase , in mammals, is not new, was described originally for primates [30], and is known as " fatted male phenomenon", has also been reported in Pteropus poliocephalus [10] as well as in large mammals like ungulates [31], pinnipeds [32] and ursids [33], among others. However, here is the first time that the ratio between fat and sperm parameters was studied.…”

“…Martínez-Coronel et al [6] found that in a resident colony of L. yerbabuenae of Chiapas, México, the fat accumulated by males is used for breeding and not for migration [1,3,6]. It was also found that compared to other species of bats that accumulate fat, in L. yerbabuenae, the increase of fat coincides with the increase in the size of the testis, and considering that increase testicular could be an indicator of spermatogenesis [7], then these observations are contradictory to those reported for most of other species of bats [8] but Pteropus poliocephalus and P. scapulatus [9,10].…”

Nest-building males trade off material collection costs with territory value

“…An alternative explanation is that more productive males sacrificed time they might have spent away from their territory for some other reason, such as for foraging. Male Grey-headed Flying-foxes (Pteropus poliocephalus) appear to sacrifice foraging time in favour of remaining on a territory, losing up to 20% of their body mass during the breeding season through territory defence and lost foraging opportunities (Welbergen 2011). Male Black Grouse (Tetrao tetrix) also lose mass while defending breeding territories on leks: more successful males, typically those with higher lek attendance, lose more mass than do less successful males at the lek periphery (Rintamaki et al 2001;Lebigre et al 2013).…”

Physiological biomarkers of Hendra virus infection in Australian Flying-foxes (Pteropus spp.)

“…The differing measures used to establish body condition demonstrated similar temporal trends. Fitness-maximising strategies of males and females typically differ, particularly with respect to timing of maximal reproductive effort, resulting in sex-specific seasonal cycles in body mass, body condition and fat deposition (Lindstedt and Boyce, 1985;Welbergen, 2011).…”