Social environment modulates investment in sex trait versus lifespan: red deer produce bigger antlers when facing more rivalry

Abstract

theory predicts that the plastic expression of sex-traits should be modulated not only by their production costs but also by the benefits derived from the presence of rivals and mates, yet there is a paucity of evidence for an adaptive response of sex-trait expression to social environment. We studied antler size, a costly and plastic sex trait, and tooth wear, a trait related to food intake and longevity, in over 4,000 male Iberian red deer (Cervus elaphus hispanicus) from 56 wild populations characterized by two contrasting management practices that affect male age structure and adult sex-ratio. As a consequence, these populations exhibit high and low levels of male-male competition for mating opportunities. We hypothesized that males under conditions of low intra-sexual competition would develop smaller antlers, after controlling for body size and age, than males under conditions of high intra-sexual competition, thus reducing energy demands (i.e. reducing intake and food comminution), and as a consequence, leading to less tooth wear and a concomitant longer potential lifespan. our results supported these predictions. to reject possible uncontrolled factors that may have occurred in the wild populations, we carried out an experimental design on red deer in captivity, placing males in separate plots with females or with rival males during the period of antler growth. Males living with rivals grew larger antlers than males living in a female environment, which corroborates the results found in the wild populations. As far as we know, these results show, for the first time, the modulation of a sexual trait and its costs on longevity conditional upon the level of intra-sexual competition. The expression of many sex-traits is plastic and responds to individual-and population-specific reaction norms 1-6. There is evidence for many species that environmental factors (e.g. resource availability) affect body condition and the expression of sex-traits, such as signals and weapons 1,7-11. Condition-dependence theory relies on the relative value of energy costs, because costs of producing traits are expected to be lower for individuals in good condition as compared to those in poor condition 12-15. But also, in a sexual selection context, rivals and potential mates in the social environment strongly influence the benefits associated with trait expression, so that trait investment should respond in a trade-off fashion depending on the costs but also on the benefits of trait development. One example of this in the inter-sexual selection context is that male zebra finches invest more in coloring their beaks when there are females to receive the signal 16. For intra-sexual competition, the challenge hypothesis 2 applied this idea to testosterone production relative to the probability of winning contests, which may be related

Pelt Biting as a Practical Indicator of Social and Environment Stress in Farmed Red Deer

“…This points out that pelt biting can be useful to identify individuals that are being bullied and apply corrective measures consequently (e.g., move the bullied animal to a new group structure; identify and remove the bully animal from the group). In deer hunting ranches, where density reaches up to 0.4 deer/ha and food supplementation takes place when graze is scarce, pelt biting is infrequent, based on observations over four thousand legally culled red deer to which we had access to take samples for research purposes over several years [ 26 , 47 ], and we never found a case in which pelt biting exceeded class 1.7. This is expected, as free-ranging conditions hardly create favourable conditions for bullying; population density is generally low, sufficient space precludes oversized groups, and graze is widely dispersed and therefore difficult to defend and of low energy reward value per food item [ 48 ], although there are exceptions [ 4 , 22 , 49 ].…”

“…Two groups of hypotheses were tested on the response of pelt biting, related to (i) the hierarchical rank of the deer and related body traits, which correlate positively with successful outcome in competition for resources, and (ii) the abiotic environmental stress where they live ( Table 1 ). It is hypothesised that males individuals (H4) [ 26 ], younger individuals (H3), lower hierarchical rank individuals (H1), smaller body size (i.e., body weight, H2) individuals, and those exposed to heat stress (H6) suffer higher rates of pelt biting ( Table 1 ).…”

The dark ventral patch: A bimodal flexible trait related to male competition in red deer

“…Both antler and body size are continuous traits, and the dark ventral patch may act as an amplifier for them [ 49 ] reducing the error in communication. Also, such a discrete nature of the ventral patch size suggests that trait expression not only provides information on male quality [ 27 ], but it may also reveal a behavioural strategy, like it has been shown for discrete signals in other species [ 66 – 68 ], of rival challenging, or willingness to be involved in competition for mates.…”

“…them [49] reducing the error in communication. Also, such a discrete nature of the ventral patch size suggests that trait expression not only provides information on male quality [27], but it may also reveal a behavioural strategy, like it has been shown for discrete signals in other species [66][67][68], of rival challenging, or willingness to be involved in competition for mates. Contrary to other morphological traits, the dark ventral patch can be modulated in the short term by urine spraying [28], which suggests a fine tune in communication during the mating season compared to body or antler size.…”

“…Antlers are renewed every year, so they are a rather flexible trait compared for instance with horns of bovid species, although they are produced several months before the starting of the rutting season. Apart from cues on their own condition to afford trait production, some information on the structure of the population, likely potential mates and number and quality of competitors, may also affect the growth of antlers [ 27 ]. However, males and females move from their normal ranges to mating areas during the rutting season, so short-term assessment during the rutting season may become valuable if the individual is able to modulate its competitive traits.…”