Research Areas‎ > ‎

Sexual Selection and Evolutionary Ecology

Most of my work on this area has focused on trying to understand the effects of secondary sexual traits (such as deer antlers) and sperm quality traits on fertility. I have used wild mammals, mainly ungulates and rodents as model taxonomic groups. Interestingly, this allows me to explore fecundity fitness related questions at the two extremes of the life history spectrum observed in mammals.

These are some of the questions I've tried to answer:

  • Is there variation in fertility rates in natural populations of mammals? 
  • What are the sperm quality traits that most influence male fertility? 
  • Does antler size and complexity advertise male fertility? 
  • Does the Trivers and Willard hypothesis apply also to males? 
  • Is testosterone responsible for the association observed between antler size and sperm quality? 

My work has pioneered the inclusion of sperm quality traits, together with sperm numbers to understand their relative contributions to male fertility in mammals. 

Diagram reflecting the relationships (arrows) between traits and fitness components addressed during my PhD.

Research highlights

Below I showcase some of my research on this area showing summaries of published articles and their main conclusions. The 

blue text is meant to be read by the non-specialist readers


The paper below successfully identified the drivers of sperm morphology. The shape  of the 

sperm head and the relative length of the flagellum drive speed! Explore the  figures below 

to discover the pattern. 

(# citations=150, 26th most cited paper in Biology Letters)

Sperm design and sperm function 

Malo AF, Gomendio M, Garde JJ, Lang-Lenton B, Soler AJ & Roldan ERS. 2006 

Biology Letters 2, 246–249

Sperm velocity is a key determinant of male fertilization success, but what determines sperm velocity is not known. Here, we examine the relationship between the size of several sperm components and sperm velocity in natural populations of red deer where selective pressures to enhance male reproductive success are expected to be strong. Results show that there is little within-male and considerable between-male variation in sperm dimensions. Spermatozoa with longer midpieces swim more slowly, a finding which does not support the hypothesis that the size of the midpiece determines the amount of energy which is translated into swimming speed. In contrast, spermatozoa with elongated heads, and those with longer relative length of the rest of the flagellum, swim faster. Thus, the hydrodynamic shape of the head and the forces generated by the relative size of the rest of the flagellum seem to be the key determinants of sperm swimming velocity.


The article below was the first one to show that fathers can 

significantly bias offspring sex ratio in natural population of 

mammals. Before this paper the assumption was that only 

females had that ability. Furthermore, this study  conducted 

on deer, an ungulate species, the same group that Trivers 

and Willard used to present their sex-ratio bias hypothesis

A excellent recent review on the topic by Edwards and 

Cameron "Forgotten Fathers" in TREE (2014) recognizes our 

paper as a "landmark study".

# citations=66

Male fertility and sex ratio at birth in red deer

Gomendio M, Malo AF, Soler AJ, Fernández-Santos MF, Esteso MC, García AJ, Roldan ERS, Garde J. 2006

Science 314, 1445-1447

Efforts to test sex ratio theory have focused mostly on females. However, when males possess traits that could enhance the reproductive success of sons, males would also benefit from the manipulation of the offspring sex ratio. We tested the prediction that more-fertile red deer males produce more sons. Our findings reveal that male fertility is positively related to the proportion of male offspring. We also show that there is a positive correlation between the percentage of morphologically normal spermatozoa (a main determinant of male fertility) and the proportion of male offspring. Thus, males may contribute significantly to biases in sex ratio at birth among mammals, creating the potential for conflicts of interest between males and females.


The next article tested several hypotheses concerning the 

effects of testosterone on important phenotypic traits that 

can influence reproduction and survival. 

# citations=56

What does testosterone do for red deer males? 

Malo AF, Gomendio M, Garde JJ, Soler AJ, Vicente J, Gortazar C & Roldan ERS. 2009 

Proceedings of the Royal Society London B 276, 971–80

Testosterone has been proposed to have a dual effect, enhancing sexual traits while depressing parasite resistance in males. We test this hypothesis in red deer, examining males from captive populations during the whole annual cycle and males from natural populations during the breeding season. Our results show that in captive populations seasonal changes in testosterone levels were mirrored by changes in testes size, and that during the rut there was a strong correlation between both. In natural populations, males with higher testosterone levels had larger testes, improved sperm quality, stronger antlers, higher haematocrit levels, and increased nematode parasite load. By contrast, no significant relationship was found between testosterone and spleen size or tick parasite load. We showed that testosterone (i) improves males' reproductive investment and physical stamina, (ii) improves antler strength but reduces burr diameter, and (iii) imposes a cost in terms of depressed parasite resistance.


The article below tested whether antler size is an indicator 

of male fertility. We found evidence that males with longer 

antlers (as measured by a comprehensive index of antler 

size) have faster sperm. This was the first test in mammals 

of Ben Sheldon's Phenotype-linked fertility hypothesis. This 

work opened the possibility that in the red deer mating 

system hinds can visually inspect the antlers in the 

mating arena to collect information on male fertility and 

then decide what males they allow/promote to be 

inseminated by. This worked opened the possibility that 

females present a much more active role in the red deer 

mating system and generating a diverse landscape of 

mating strategies and interactions. It also allows the 

possibility that rival stags use the information conveyed by 

the antlers to allocate sperm effectively. 

# citations=147

Antlers Honestly Advertise Ejaculate Competitiveness
Malo AF, Roldan ERS, Garde JJ, Soler AJ & Gomendio M. 2005 

Proceedings of the Royal Society London B 272, 149–157

Evolutionary theory proposes that exaggerated male traits have evolved via sexual selection, either through female mate choice or male–male competition. Among mammals sexual characters are commonly regarded as weapons whose main function is to enhance male competitiveness in agonistic encounters. One particularly controversial hypothesis to explain the function of male sexual characters proposes that they advertise male fertility. We test this hypothesis in red deer. We find that antler size is associated with relative testes size and sperm velocity. Our results exclude the possibility that condition dependence, age or time of culling, drive these associations. Red deer antlers could signal male fertility to females, the ability to avoid sperm depletion throughout the reproductive season and/or the competitive ability of ejaculates. By contrast, male antlers could also signal to other males not only their competitive ability at the behavioural level (fighting ability) but also at the physiological level (sperm competition).


Effects of genetic captive breeding protocols on sperm quality and fertility 

Malo AF, Martinez-Pastor F, Alaks G, Dubach J, Lacy R. 2010 

Biology of Reproduction 83, 540-548

The genetic effects of artificial selection for docility and inbreeding had never been considered jointly in captive breeding programs. To determine if genetic effects negatively affecting sperm traits were responsible for this extinction, we explored the sperm determinants of fertility, and we compared fertility and sperm traits between the remaining docility replicate and the two other breeding protocols. Genetic drift among and between replicates might have reduced our ability to document the effects of inbreeding or selection; however, we were able to document important differential effects in the breeding protocols. The DOC males performed worse than both RAN males (with the highest accumulated inbreeding levels) and MK males, suggesting that selection for docility has more deleterious effects than inbreeding at the f levels considered. This may result from the fact that selection for docility would be expected to have cumulative effects on fitness if an association between the two exists, whereas inbreeding would be expected to show lower association with fitness if inbreeding accumulation is slow enough for purging to operate. Overall, our results showed that 1) different fertility components are affected by different sperm traits; 2) mice selected for docility (DOC) have the lowest levels of fertility, and the driver for the extinction of a DOC population in generation 8 seems to be poor sperm quality; 3) sperm morphology traits impacted the probability of siring litters; 4) random bred mice (RAN) and mice bred minimizing mean kinship (MK) show no difference in average reproductive fitness after 10 generations; 5) negative effects of inbreeding on sperm traits are found within protocols; and 6) inbreeding translates into decreased fecundity by reducing the size of second litters.